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Brood size, duckling survival and parental care in canvasbacks Sobrino, Cristina N. de 1995

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Brood size, duckling survival and parental care in Canvasbacks by CRISTINA N . D E SOBRTNO  B . A . , The University of California, Santa Cruz, 1989  A THESIS S U B M I T T E D I N P A R T I A L F U L F I L L M E N T O F T H E REQUIREMENTS FOR T H E D E G R E E O F MASTER OF SCIENCE in T H E F A C U L T Y O F G R A D U A T E STUDIES  (Department of Zoology)  We accept this thesis as conforming to the required standard  T H E U N I V E R S I T Y O F BRITISH C O L U M B I A  February 1995  © Cristina N . de Sobrino  In  presenting this  degree at the  thesis  in  University of  partial  fulfilment  of  of  department  this or  publication of  thesis for by  his  or  her  The University of British Columbia Vancouver, Canada  DE-6 (2/88)  It  this thesis for financial gain shall not  Department of  2  representatives.  rWOi  for  an advanced  Library shall make it  agree that permission for extensive  scholarly purposes may be granted  permission.  Date  requirements  British Columbia, I agree that the  freely available for reference and study. I further copying  the  is  by the  understood  that  head of copying  my or  be allowed without my written  ABSTRACT  Clutch size in precocial birds is generally thought to be limited by factors acting during egg-laying and incubation.  However, hypotheses  advanced to explain limits on clutch size are not sufficient to explain observed clutch sizes. I tested if factors acting after hatching can limit clutch size in precocial Canvasbacks, (Aythya valisineria). Clutch size may be limited by post-hatching factors if large broods have higher mortality, or if large broods require more parental care. To examine the effect of brood size on duckling mortality I compared duckling survival in experimentally enlarged and reduced broods. While duckling survival was inversely related to brood size, the relationship was not significant. By modifying an existing model of clutch size determination (Arnold et al. 1987), I demonstrate that even a slight brood size-dependent decline in duckling survival is sufficient to reduce optimal clutch size. To determine if there was a potential for costs of reproduction to vary with brood size, I collected time-activity budgets of females rearing experimentally manipulated broods. If costs of reproduction vary with brood size, females rearing enlarged broods would be expected to spend more time in parental care activities than those rearing small broods. Brood-rearing females did not alter their behaviour with respect to brood size, nest initiation date, or body condition. Duckling survival was unaffected by the amount of time that females spent in parental care or self maintenance activities, suggesting that offspring survival is dependent only on the presence or absence of a brood-rearing hen. The level of parental care that females provide is unlikely have brood size dependent costs, and is thus unlikely to affect optimal clutch size of Canvasbacks.  ii  TABLE OF CONTENTS Page Abstract  ii  Table of Contents  iii  List of Tables  iv  List of Figures  v  Acknowledgements  vii  General Introduction  1  Chapter I.  4  Duckling Survival  Introduction  4  Methods  6  Brood Size Manipulations  6  Brood Monitoring  8  Analyses  9  Results  10  Discussion  11  Chapter II.  The effect of brood size on parental behaviour  29  Introduction  29  Methods  31  Behavioural Observations  31  Analyses  32  Results  '.  33  Discussion  35  General Conclusions  50  Literature Cited  53  iii  LIST OF TABLES Chapter I.  Page  Table 1.  Mayfield estimates of Canvasback nest success  20  Table 2.  Fates of manipulated Canvasback clutches  21  Chapter II. Table 3.  The effect of brood size on parental behaviour, body mass and survival in waterfowl  iv  38  LIST OF FIGURES Chapter I. Figure 1.  Page Canvasback duckling survival in relation to brood size  22  Figure 2.  Duckling survival in relation to brood size and year  23  Figure 3.  Duckling survival in relation to nest initiation date  24  Figure 4.  Duckling survival as a function of late-incubation body mass of brood-tending females  25  Figure 5.  Duckling survival as a function of the weather during the first 7 days after hatching  Figure 6.  26  The average number of fledged ducklings per clutch in relation to predation risk and egg viability, adapted from Arnold et al. (1987)  Chapter II. Figure 7.  Page Time-activity budgets of brood-rearing Canvasbacks in relation to brood size  Figure 8.  41  Time-activity budgets of brood-rearing Canvasbacks in relation to nest initiation date  Figure 9.  28  42  Time-activity budgets of brood-rearing Canvasbacks in relation to duckling age  v  43  LIST OF FIGURES Chapter II. (continued) Figure 10.  The distribution of distances of brood-rearing females from their brood  Figure 11.  44  The relationship between duckling survival and behaviour of brood-tending females  Figure 12  Page  46  The relationship between the behaviour of females and their late-incubation body mass  48  Acknowledgements  I gratefully acknowledge the Delta Waterfowl and Wetlands Research Station for providing funding for this study. Thanks to my advisor, Kim Cheng, for his patience, support and advice during this study. I want to thank Frank Rohwer for providing many helpful suggestions throughout the course of the study. His energy and zeal were inspiring. Thanks also to Jamie Smith for his careful editing and to Martin Adamson and Lee Gass for their suggestions.  Mike Anderson was kind  enough to share his knowledge of Canvasbacks with me. I thank all of the people at Minnedosa who helped with data collection, particularly Laura Flynn whose undaunting enthusiasm and suggestions were invaluable. Pam Garrettson helped me keep my spirits up when things went wrong and taught me about the merits of marshmallows. Thanks also to my buddies at UBC, especially Troy Day who provided desperately needed statistical advice. Todd Arnold helped in many ways; by finding nests and supplying homemade beer in the field, as well as by providing numerous comments and suggestions which greatly improved this thesis. Without the kind permission of numerous landowners in the Minnedosa area, this study would not have been possible.  vii  General Introduction  Avian clutch size may breeding period. history, as may  be affected by factors acting throughout the  The elements that influence clutch size vary with life  the point in the cycle where these factors act. Altricial birds  have small clutches, short incubation periods and relatively long broodrearing periods. In these species, egg-formation costs are low (Ricklefs 1977), and the period of greatest energetic demand occurs while parents are feeding nestlings. Lack (1947,1967,1968) proposed that clutch size in birds is limited by food availability during the period of greatest energetic demand, e.g. in altricial birds, during the brood-rearing period. However, subsequent brood enlargement experiments have demonstrated that in many altricial species, parents can rear more young than they attempt to produce (review in VanderWerf 1992). Clutch sizes may  be smaller than the most productive  brood size if large broods are more costly to parents (Williams 1966, Pianka and Parker 1975); although most evidence suggests that costs of reproduction are of minor importance for altricial birds (De Steven 1980, Boyce and Perrins 1987, Pettifor 1993; but see Gustafsson  and  Sutherland  hypotheses have proposed that pre-hatching factors may  1988).  Other  limit clutch size in  altricial birds (Hogstedt 1980, Lundberg 1985, Lima 1987). Nevertheless, these ideas have received relatively little attention and despite the paucity of supporting evidence, post-hatching factors are still thought to limit clutch size (Ydenberg and Bertram 1989, VanderWerf 1992). Precocial birds have large clutches, long incubation periods, and are self-feeding at hatch. Laying and incubation are the most energetically costly periods of the reproductive cycle for these species (Ricklefs 1977). Thus, Lack (1967, 1968) suggested that clutch size in precocial species is limited by food  1  availability during laying and  incubation.  Refinements to Lack's  egg  production hypothesis have focused mainly on waterfowl and propose that clutch size is limited by nutrient reserves of laying females (Ryder  1970,  Drobney and  Fredrickson  This  hypothesis may  be sufficient to explain clutch size in birds that rely entirely  1985,  Ankney and  Alisauskas 1991).  on stored nutrients during laying and incubation (e.g. some arctic nesting geese and  eiders: Ankney and  Maclnnes 1978,  Parker and  Holm 1990),  although it is also possible that these birds store enough reserves to produce a clutch size that is determined by other factors. At any rate, the nutrient limitation hypothesis does not explain clutch sizes of temperate nesting waterfowl (Arnold and Rohwer 1991).  Other mechanisms acting prior to  hatch, such as nest predation and declining egg viability (Arnold et al. 1987, Milonoff 1991)  may  determine clutch size in precocial birds.  However,  hypotheses proposing pre-hatch limits on clutch size do not appear sufficient to explain observed clutch size in waterfowl (Ankney et al. 1991, Arnold and Rohwer 1991). Factors acting after hatch could determine clutch size if young in large broods suffer higher mortality than those in small broods (Safriel 1975). Alternatively, larger broods might require levels of parental care that reduce the future fecundity or survival of parents, thereby reducing their lifetime reproductive success (Williams 1966, Pianka and Parker 1975). Even if broodrearing costs alone do not limit clutch size, if parents or offspring of enlarged broods have even slightly reduced survival, such a selection differential will act in conjunction with other factors to reduce optimal clutch size. My  objectives in this thesis are 1) to determine whether survival of  precocial young are affected by brood size, and 2) to determine whether there is a potential for brood size to decrease parental  2  survival or  future  reproduction. I chose Canvasbacks (Aythya valisineria) for this work because they are visible during the brood-rearing period, and their breeding biology is well studied.  3  Chapter I: Duckling Survival  Introduction  Clutch size in waterfowl is thought to be limited by factors acting during egg-laying and incubation (Lack 1967, 1968, Ryder 1970, Ankney and Alisauskas 1991, Arnold and Rohwer 1991). The alternative hypothesis that factors acting during the brood-rearing  period determine clutch size has  received little attention (Rohwer 1992). To date, no single hypothesis has been advanced that describes observed clutch sizes of waterfowl (Ankney et al. 1991, Arnold and Rohwer 1991). However, clutch size may not be determined by any single factor, but rather, by several factors acting during the egg-laying and brood-rearing  periods (Erikstad et al. 1993). Offspring survival  may  constrain clutch size if ducklings in large broods suffer disproportionate mortality. Reduced survival in large broods could act alone or in conjunction with any  pre-hatching constraints to determine clutch size.  The  most  important causes of mortality for ducklings are predation and bad weather (Johnson et al. 1992); if brood size affects offspring survival it may  do so via  brood-size dependent variation in susceptibility to these factors. Some evidence suggests that predation risk may  vary with brood size.  Safriel (1975) found that experimentally enlarged broods of Semipalmated Sandpipers (Calidrus pusilla) had higher mortality rates. He attributed these to greater predation losses. Safriel suggested that larger broods were easier for predators to detect, and that once detected, these broods suffered almost complete mortality. Ducklings cannot thermoregulate until they are 1-2 weeks old (Afton and Paulus 1992). Until that time, they depend on brooding by their mothers  4  for warmth and shelter. Ducklings in large broods may be more susceptible to bad weather if females are incapable of brooding large numbers of young (Seymour 1982, Talent et al. 1983). Vulnerability to predation or chilling may  be increased when  individual ducklings are separated from the brood (Seymour 1982). broods may  be more difficult  ducklings in large broods may mortality.  for parents to monitor.  Large  Consequently,  be more susceptible to separation, and  Brood size might also affect duckling survival if competition  among ducklings for food increases with brood size; resulting in lower growth rates or poorer fledging success in large broods (Rohwer 1992). Data regarding how post-hatching costs affect brood size in precocial birds are equivocal (reviewed by Rohwer 1992). Some studies have found no relationship between brood size and survival (Clawson et al. 1979, Dow  and  Fredga 1984, Rohwer 1985, Milonoff and Paananen 1993), while others have noted lower survival in large broods (Safriel 1975, Andersson and Eriksson 1982, Lessells 1986, Leblanc 1987, Rockwell et al. 1987). Most of these studies were descriptive and examined survival of offspring from broods enlarged by intraspecific nest parasitism (Clawson et al. 1979, Dow  and Fredga 1984,  Rockwell et al. 1987). Thus, these studies may not adequately separate the relationship between parental ability and  clutch size.  Experimental  manipulations are necessary to decouple any relationship between parental ability  and  Canvasbacks  parasitism. I experimentally manipulated brood to determine how  size in  brood size affects duckling survival.  5  Methods  My  study was  conducted during the summers of 1992-1993 in the  prairie pothole region of southern Manitoba. My  58"km study area 2  was  located approximately 10 km southeast of the town of Minnedosa (50°10'N, 99°47'W).  Wetlands in the region range from small (< 0.5 ha) seasonally  flooded basins to large permanent wetlands (26 ha), and wetland density in the area averages 26.3 basins/km (Stoudt 1982). Wetlands lie within an 2  agricultural landscape that is dominated by small grain farming. Detailed descriptions of the area have been provided by Kiel et al. (1972) and Stoudt (1982). Canvasbacks pair during spring migration and arrive on the breeding grounds during mid to late April. Nesting begins in late April and early May. Nests are built in the emergent vegetation (primarily cattail, Typha latifolia, and bullrush, Scirpus acuta) of semipermanent and permanent wetlands (Stoudt 1982). I conducted nest searches between 5 May and 15 June by wading through vegetation in likely nesting ponds. When a nest was  found, the  number and incubation stage of eggs were recorded (Westerkov 1950). Parasitic Redhead and Canvasback eggs were identified by variation in color, texture and incubation stage.  Brood Size Manipulations Brood sizes were experimentally manipulated to control for variation in parental ability. I manipulated nests to contain either 3 or 8 ducklings. Mean brood size at hatching in this population was 5.5 (Anderson et al. Unpubl. MS); my manipulated brood sizes were 2.5 ducklings smaller and 2.5 ducklings larger than the long-term average. Both of these values are well  6  within the range of naturally occurring brood sizes. Ideally, I would have enlarged broods by more than 50%, however, I had a limited number of eggs available for manipulations and larger brood sizes would have reduced  my  sample of enlarged broods. To control for seasonal effects, enlargements and reductions were paired with respect to hatching date. Eggs were collected approximately halfway through the twenty-six day incubation period and put in an incubator (Petersime Model 4). Natural clutches were replaced with an equal number of hard-boiled brown chicken eggs, which were accepted and incubated by the nesting female. Eggs from the incubator provided a source for manipulations and minimized egg loss to predators. Anderson et al. (Unpubl. MS) showed that experimental manipulation of Canvasback nests did not significantly increase predation rates or nest abandonment. Approximately 1-2 days before the predicted hatching date of a female's original clutch, the chicken eggs were replaced with pipping Canvasback eggs. Replacement clutches contained eggs from at least two different hens to control for potential variation in egg quality.  At the time of clutch  replacement, females were nest trapped with drop-door traps (Blums et al. 1983), marked with nylon nasal pieces (Lokemoen and Sharp 1985) and fitted with aluminum leg bands. I determined body mass (±10g) with a springloaded Pesola scale. Lipid stores in birds are frequently correlated with body mass, and Johnson et al. (1985) observed that adjusting mass by a measure of structural size may improve this relationship. However, I used body mass as an index of body condition because Sparling et al. (1992) found that such an adjustment explains only 1-3% of the variation in lipid stores of Canvasbacks. Plumage characteristics were used to distinguish between yearling and adult females (Serie et al. 1982).  7  Manipulated nests were checked for hatching 2-3 days after trapping. I considered that a clutch had hatched if at least one duckling had hatched and left the area of the nest. Abandoned nests were those in which all eggs or ducklings could be accounted for in, or near, the nest. I considered a nest depredated if there were fragments of eggs or ducklings present, or if the nest platform was disturbed and eggs or ducklings were missing. In some cases it was difficult to determine whether a nest had been abandoned before being depredated, 3 uncertain cases were scored as abandonments.  Brood Monitoring If eggs hatched, the area surrounding the nesting pond was searched until the female and her brood were located. If the brood was not found after 2 weeks, intensive searches were stopped and the brood was censored from analysis. Censored broods may have included females that lost their entire brood and moved off of the study area to molt (Devries 1993). However, Canvasback broods may move up to 4.4 km from their nest pond (Austin and Serie 1991), so some censored broods could have survived without being located. I tried to observe females with broods every 4-5 days, however, some females were not found after overland movements, and some were not located until later in the brood rearing period. Survival estimates for broods that were observed for only a portion of the brood rearing period are calculated only for the period that they were under observation (Pollock et al. 1990).  I concluded that ducklings had died if the number observed  smaller than on the previous observation.  Total brood mortality  was was  assumed when a marked female was observed three or more times without ducklings (Leonard 1990). In no case was a female that fit these criteria seen with a brood later.  I estimated survival from hatch to 42 days because  8  Canvasback hens often stop attending broods before ducklings can fly (56 days, Dzubin 1959), making estimates of fledging success difficult. Leonard (1990) found that 42-day survival estimates approximated fledging success well. At  3 nests, 1 or 2 eggs failed to hatch. In these broods, survival  estimates were based on the number of eggs that hatched. Survival estimates for these broods were included in whichever treatment group the brood was originally assigned to. To examine effects of weather on survival, I compared duckling survival during the first two weeks with the average minimum daily temperature and average daily precipitation during the first week after hatching. I was not able relocate broods frequently enough to determine survival for most broods during the seven days immediately after hatching. However, Leonard (1990) found that most duckling mortality occurred during the first week, suggesting that if weather affects survival, it is most likely to do so when ducklings are young  and inefficient at thermoregulation  (Koskimies and Lahti 1964). Weather data for the town of Minnedosa were obtained from the Atmospheric Environment Service of Environment Canada.  Analyses Rohwer (1985) found that the survival of ducklings within a brood is not independent, so I calculated duckling survival rates for each brood. I compared mean survival rates of ducklings in large and small broods during each of three 14-day periods from day 0 (hatch) to day 42. Since all broods were not observed during each period, sample sizes are not equal. Means are reported ± 1 standard error. In 1992 I was only able to monitor survival of 5 broods. Data from the two years were combined for all analyses. Data were  9  not normally distributed and were therefore analyzed with Mann-Whitney U-tests (SYSTAT 1991). Categorical data were analyzed using the G-test for goodness-of-fit (SYSTAT 1991) with William's correction for small sample size (Sokal and Rohlf 1987). The effects of nest initiation date, female body mass, mean minimum temperature and mean precipitation on duckling survival were analyzed using least squares regression (SYSTAT 1991). Estimates of nest success were calculated using the Mayfield-40% method (Johnson 1979), which controls for uneven exposure periods among nests.  Results  I found 197 nests; however, due to nest predation, abandonment, and failed trapping attempts, only 66 nests were manipulated (Table 1, Table 2). Not all manipulated nests hatched; 11 (17%) were depredated and 13 (20%) were abandoned leaving only 42 hatched nests (Table 2).  Ten of the  remaining 42 broods were never located and have been censored from the analyses (Table 2). Thus, I was able to monitor survival of ducklings in 32 broods. Total brood mortality was detected in 1 brood in 1992 and in 7 broods in 1993. Most duckling mortality (65%) occurred during the first 2 weeks after hatching (Fig. 1). Survival of ducklings to 42 days old was 0.48 ± 0.07. Brood size manipulations had no effect on whether or not clutches hatched (Table 2; G  a d j  = 3.33, d.f. =1, P > 0.05), or experienced total brood  mortality (G j = 0.63, d.f. =1, P > 0.05). The observed level of total brood ad  mortality may be biased low if most censored broods experienced total brood loss. However, even if all censored broods are assumed to have experienced total brood loss, the incidence of total brood mortality remains independent of brood size (G j = 0.93, d.f. = 1, P > 0.05). Duckling survival to 42 days was ad  10  0.39 ± 0.10 in large broods and 0.56 ± 0.11 in small broods (Fig. 1), however, this difference was not significant (U = 111.5, P = 0.16). Ducklings in smaller broods had a significantly higher survival probability in the third and  fourth  weeks after hatching (Fig. 1: U = 80.00, P = 0.005), yet this difference did not persist to the end of the brood-rearing period. Enlarged broods produced almost twice as many ducklings as reduced broods (2.85 ± 0.72 vs. 1.61 ± 0.31), however, this difference was not detected statistically (U = 59.00, P = 0.18). In 1992,  duckling density on the study area was  average (Anderson et al. Unpubl. MS). duckling survival and conditions.  Any  much lower than  potential relationship between  brood size might be affected by  poor breeding  However, I obtained survival rates for only 5 broods in 1992,  and  there was no indication of an interaction between year and brood size (Fig. 2). Duckling survival was not correlated with nest initiation date (Fig. 3; r = 0.01,  P = 0.64,  n = 26).  There was  2  a significant, but weak, positive  relationship between female body mass and duckling survival (Fig. 4; r = 2  0.19, P = 0.03, n = 25). Neither minimum daily temperature nor the amount of precipitation during the 7 days after hatching affected duckling survival during the first 14 days (Fig. 5a; r = 0.01, P = 0.65, n = 29 and Fig. 5b; r = 0.01, P 2  2  = 0.65, n = 29).  Discussion  Canvasback duckling survival appeared to be independent of brood size in this study. However, my sample size was only big enough to allow me to detect a fairly large difference in survival between the 2 brood sizes. Small brood-size dependent variation in survival can  11  impact clutch size, as  demonstrated by modification of a model developed by. Arnold et al. (1987). Duckling survival was unaffected by other factors that were considered.  Results of this study The duckling survival rate observed in this study (48%) is similar to those reported for other duck species (Sargeant and Raveling 1992: Table 12-5). Duckling survival was higher in small broods, although large broods still produced more young than small broods. differences was statistically significant.  However, neither of these  Only two other studies have  examined duckling survival from experimentally manipulated  broods  (Rohwer 1985 on Blue-winged Teal, (Anas discors). Milonoff and Paananen 1993 on Common Goldeneyes (Bucephala clangula)). Neither study detected a significant relationship between brood size and duckling survival, however, in both studies, small broods appeared to have higher survival rates.  Previous research Descriptive studies relating brood size and duckling survival have generally found that survival is unaffected by brood size (Clawson et al. 1979, Dow and Fredga 1984, Eadie and Lumsden 1985, Bustnes and Erikstad 1991). The only exception has been Andersson and Eriksson (1982) who observed that survival of Common Goldeneye ducklings to 20 days of age was inversely related to brood size. However, subsequent observational studies of Common Goldeneyes have found no effect of brood size on survival (Dow and Fredga 1984, Eadie and Lumsden 1985). In the only other study that has manipulated brood size of waterfowl experimentally, Lessells (1986) found that fledging success of Canada Geese (Branta canadensis) was negatively related to brood size in one of two years.  12  LeBlanc (1987) also found that fledging success of Canada Goose goslings was negatively related to observed brood size in one of two years. Paradoxically, Lessells (1986) concluded that brood size dependent offspring survival does not limit clutch size in Canada Geese, whereas LeBlanc (1987) concluded that it may.  Interestingly, Lessells (1986) only observed reduced gosling survival  in broods that were larger than the observed range of brood sizes. Rockwell et al. (1987) found that survival of Lesser Snow Goose (Chen caerulescens caerulescens) goslings was also higher in small broods.  However, this  relationship was based largely on broods that were enlarged as a result of intraspecific nest parasitism. The lower survival of young from larger broods was considered to reflect developmental lags in parasitically laid eggs, rather than an inability of parents to rear more young (Rockwell et al. 1987).  Power of experimental studies Most studies relating brood size to offspring survival in waterfowl have failed to show a significant inverse relationship between brood size and offspring survival. Large broods in these studies fledged more young than small broods did, and thus variation in offspring survival alone is probably insufficient to limit clutch size in waterfowl. However, these data show a consistent trend toward  reduced survival in large broods.  Only  4  experimental studies have been conducted relating offspring number to survival (Rohwer 1985, Lessells 1986, Milonoff and Paananen 1993, this study) most of which have very small samples. In this study, 58 km  2  were searched for Canvasback nests in two years  and 197 active nests were found. However, 84% of these were lost due to predation or nest abandonment, yielding only 32 experimental broods, 26 of which I was able to follow until fledging. With my sample of 26 Canvasback  13  broods, I had an 80% chance of detecting a significant difference (P = 0.10, twotailed) of 37% in duckling survival between the two brood sizes (Steel and Torrie, 1980: 118-119). However, the observed difference in duckling survival was 17%, and was not detected as significant (t = 1.20, d.f. = 24, P = 0.25). To detect a difference of this magnitude would have required 120 broods (Steel and Torrie, 1980: 118-119).  Multi-factor hypotheses Arnold et al. (1987) proposed a model to describe clutch size of temperate-nesting ducks that incorporated nest predation and declines in egg viability.  temporal  Their model predicted a clutch size of 14,  approximately 4 eggs larger than average clutch sizes of temperate-nesting ducks (Bellrose 1980). They concluded that this discrepancy was a result of "fine tuning by other selection costs" (Arnold et al. 1987: p. 649).  Low  duckling survival in large broods may be such a cost. I was not able to demonstrate a statistically significant effect of brood size on duckling survival, however slight differences in offspring survival could act alone or in conjunction with other processes to determine clutch size. If the results of my study are interpreted as real, an increase in brood size of one duckling results in a decline in survival of 0.035. I incorporated this reduction in duckling survival into the model developed by Arnold et al. (1987) and determined the average number of fledged ducklings for each clutch size. This modification yielded a predicted clutch size of 8 (Fig. 6), equal to mean Canvasback clutch size (Sorenson 1993). The plateau of the function occurs over the range of most commonly observed Canvasback clutch sizes (i.e. 6-10: Sorenson 1993).  14  Although the modified model fits the observed data well, this result may  not apply generally. The brood size related decline in duckling survival  reported for Blue-winged Teal (0.012; Rohwer 1985) is less than half that observed in this study. When these data are used, the modified model yields a predicted clutch size of 12, 2 eggs larger than the average Blue-winged Teal clutch size (Rohwer 1985). Despite the discrepancy, this result is closer to observed clutch size than that originally predicted by Arnold et al. (1987). Thus, although brood size per se does not determine clutch size, even a slight brood size dependent reduction in duckling survival may have a significant influence on clutch size.  Support for multi-factor hypotheses Erikstad et al. (1993) observed that clutch size in Common Eiders may also be determined by a combination of pre-hatch and post-hatch factors. Clutch size and female body mass at hatching in their study were positively correlated to the probability that a female would tend a brood, suggesting that in Common Eiders there is a trade off between resources allocated for egglaying and those allocated for incubation and brood-rearing. To date, single factor hypotheses regarding determinates of clutch size have generally failed to predict avian clutch size (Winkler and Walters 1983, Ydenberg and Bertram 1989, Arnold and Rohwer 1991). Hypotheses incorporating the interaction of several processes acting throughout the breeding period may be more appropriate for predicting clutch size. Arnold et al. (1987) demonstrated that predation risk and temporal declines in egg viability can significantly affect clutch size. Brood size dependent duckling survival (this study) and energetic costs to females of brood-rearing (Erikstad et al. 1993) also appear to be important.  15  Duckling survival in creches In some circumstances duckling number may be positively correlated with duckling survival. Newly hatched broods of many waterfowl species combine to form multi-brood creches (Eadie et al. 1988). Creche formation predominates in situations where nest density is high, hatching is synchronized and ducklings are relatively cold tolerant (Afton and Paulus 1992), and under some conditions, duckling survival is positively related to creche size (Munro and Bedard 1977, Brown and Brown 1981, Kehoe 1989). Survival benefits associated with large creches seem to occur primarily in breeding areas where gull predation is intense and frequent (Munro and Bedard 1977, Brown and Brown 1981, Kehoe 1989).  However, duckling  survival is not always higher in large creches (Savard 1987, Bustnes and Erikstad 1991, Afton 1993), and the relationship may even vary within species. Bustnes and Erikstad (1991) found no relationship between survival of Common Eider (Somateria mollissima) ducklings and creche size in Norway, yet in Quebec, where gull predation was particularly intense, duckling survival of Common Eiders was positively associated with creche size (Munro and Bedard 1977). Relationships between creche size and survival are often confounded because several females often tend creches, and active defense of ducklings contributes to higher survival in large creches (Munro and Bedard 1977). Thus, the higher survival rates observed in large creches may  be determined by the number of tending females, rather than the  absolute group size.  16  Seasonal effects Seasonal decline in clutch size and offspring survival is a common and widespread phenomenon in birds (Jarvis 1974, Cooke et al. 1984, Toft et al. 1984, Martin and Hannon 1987, Hochachka 1990, Spear and Nur 1994). Among waterfowl, seasonal declines in duckling survival have been observed in Mallards (Anas platyrhynchos) (Orthmeyer and Ball 1990, Rotella and Ratti 1992), Black Ducks (Anas rubripesHRingelman and Longcore 1982) and Common Goldeneyes (Dow and Fredga 1984). Moreover, Cooke et al. (1984) observed a seasonal decline in recruitment rates of Lesser Snow Goose goslings into the breeding population. However, Savard et al. (1991) observed no consistent effect of timing of breeding on duckling survival in Barrow's Goldeneye  (Bucephala islandica) or Buffleheads (Bucephala albeola).  Canvasback clutch size declines seasonally (Serie et al. 1992), however, duckling survival appears to be unaffected by timing of breeding (this study, Leonard 1990).  Affects of parental condition Female body mass may affect duckling survival if females in poor condition are less attentive than females in good condition.  Talent et al.  (1983) observed that some Mallard hens left young ducklings unattended while they went to feed in other areas and that many of these females subsequently lost their entire brood. Talent et al. (1983) suggested that these females were renesting birds in poor condition, although they did not measure condition. I observed a weak, but significant, effect of female body mass on duckling survival. However, this relationship was not born out by a larger sample. A 10 year data set from the same study area (which included  17  the two years of data from this study) showed no effect of female body mass on duckling survival (Arnold et al. Unpubl. MS).  Environmental factors Bad weather is often cited as an important contributor to mortality of waterfowl young (Seymour 1982, Talent et al. 1983, Sargeant and  Raveling  1992). Lessells (1986) found a significant effect of weather during the first 5 days after hatch on fledging success of Canada Goose goslings. However, the variation in temperature and precipitation that I measured did not affect duckling survival. Duckling survival is probably determined primarily by a combination of stochastic events that includes predation and severe weather, but the susceptibility of individual ducklings to inclement weather appeared to be independent of brood size in this study.  Conclusions Brood-size dependent survival of young is probably not the sole factor limiting clutch size of waterfowl, yet it could be an important one.  Previous  studies examining the relationship between brood size and offspring survival in waterfowl have generally found a trend toward reduced survival in large broods, yet many of these studies have lacked the statistical power to detect slight differences in survival. The idea that clutch size may  be determined by  offspring survival has generally been rejected because large broods tend to produce more young. The possibility that brood-size dependent declines in offspring survival may  act in conjunction with other mechanisms to affect  clutch size has not been considered before and deserves further exploration.  18  Other factors acting after hatch may also affect clutch size. In the following chapter I employ the experiment used here to examine if brood size affects parental care in a manner that could limit clutch size of Canvasbacks.  19  Table 1. May field estimates of nest success in Canvasbacks.  1992  1993  Number of nests found  56  141  Depredated  36  73  Abandoned  3  9  Hatched  17  59  1022.5  1408.0  0.9619 ± 0.0060  0.9418 ± 0.0062  0.27+ 0.006 *  0.13 ± 0.006 *  Exposure days Daily nest survival ± 1 SE Estimated nest success ± 1 SE  * Z - 2.33, P = 0.02  20  Table 2. Fates of manipulated Canvasback clutches.  Fate  Brood Size  Total  Small  Large  Depredated  9  2  11  Abandoned  8  5  13  Brood not located  4  6  10  Brood observed  16  16  32  37  29  66  Not Hatched  Hatched  Total  21  1.0 n  Brood size  0.9  10 8  0.8  > co  _c  0.7 -  Small  •  Large  •  10 16 *  0.6 0.5 -  3E o  0.4 -  Q  0.3 -  13 16 13 13  0.2 0.1 0.0 0-14 days  15-28 days  29-42 days  0-42 days  Duckling Age  Figure 1. Canvasback duckling survival (± 1 SE) in relation to brood size during each of three 14-day periods, and during the entire 42-day interval that broods were under observation. Small broods had 3 ducklings at hatching; large broods had 8 ducklings at hatching. Numbers below points indicate sample sizes for each period. The asterisk indicates a significant difference in survival between the two brood sizes.  22  1.0 0.9 0.8 -  5  z  0.7 0.6 -  D  CO  O) 0.5  c  3E  0.4 H  Q  0.3 -  o 13  3 10 11  0.2 0.1 0.0  Large  Small  Brood Size  Figure 2. Duckling survival to 42 days in relation to brood size and year. Numbers below points indicate sample sizes.  23  1.0 0.9 0.8 0.7 0.6 0.5  -  0.4 -  Brood Size  0.3 0.2 -  Small  •  Large  A  0.1 0.0  *120  • A  1  125  —I 130  1  1  1  135  1— 140  i  145  150  Nest Initiation Date  Figure 3. Duckling survival to 42 days plotted as a function of nest initiation date.  24  800  850  900  950  1,000  1,050  1,100  1,150  1,200  Female Body Mass (g)  Figure 4. Duckling survival to 42 days as a function of late-incubation body mass of brood-rearing females.  25  Figure 5. a. Duckling survival to 14 days as a function of the average minimum daily temperature during the 7 days folowing hatching.  b. Duckling survival to 14 days as a function of the average daily precipitation during the 7 days following hatching.  26  1.0 -  •  •  A A  MA  0.9 0.8 0.7 -  A A  0.6 0.5 0.4 0.3 0.2 0.1  -  0.0  L.  Brood Size Small  •  Large  •  A  T 4  6  m T  A  Ait  8  -i  10  12  Minimum Daily Temperature (C)  1.0 -  •  0.9 -  A  A  M • A  •  A  A  A  0.8 0.7 0.6 0.5 0.4 -  Brood Size  0.3 -  Small  •  0.2 -  Large  A  0.1 0.0  -  t  T  4  A A  T  A  A  8  -i 12  16  Mean Daily Precipitation (mm) 27  20  1  1 24  o.o  1  0  i  1 2  1  1 4  1  1 6  .  1  •  8  ,  ,  10  , 12  ,  , 14  ,  , 16  ,  ,—j 18  1 20  Clutch Size  Figure 6. The average number of fledged ducklings per clutch in relation to predation risk and egg viability, adapted from Arnold et al. (1987). The dashed line represents the relationship predicted by Arnold et aL (1987). The function is maximized when clutch size is 14. The solid line represents the outcome of the modified model, incorporating a brood size dependent decline in duckling survival. The function is maximized when clutch size is 8.  28  Chapter II: The effect of brood size on parental behaviour  Introduction  Clutch size could be constrained by parental care if the costs of such care vary with brood size (Lazarus and Inglis 1986). Although many precocial birds do not feed their young, they do provide care to offspring by brooding them, leading them to foraging areas and by providing vigilance against predators (Kear 1970).  Lazarus and Inglis (1986) suggested that these  behaviours are not likely to vary with brood size, and consequently, that clutch size in precocial birds is probably not limited by parental care. However, other workers have observed that certain forms of parental care do vary with brood size in precocial birds (Walters 1982, Schindler and Lamprecht 1987, Sedinger and Raveling 1990, Forslund 1993). This has led some to conclude that clutch size in precocial birds may be determined by the costs of parental care (Walters 1982, Schindler and Lamprecht 1987, Forslund 1993). Parental care in precocial species is not generally considered to be energetically costly (Winkler and Walters 1983, Lessells 1986), yet for waterfowl, egg-laying and incubation are demanding (Alisauskas and Ankney 1992, Afton and Paulus 1992). Females typically reach annual mimima in body condition at the end of incubation (Ankney 1980, Noyes and Jarvis 1985, Hohman 1986), and must forage extensively during brood-rearing to meet maintenance costs and to regain lost body condition (Noyes and Jarvis 1985, Afton and Paulus 1992).  Parental care in precocial species could be  energetically costly if it interferes with the ability of parents to regain condition lost during laying and incubation (Afton 1993). Such a cost could be  29  exacerbated by brood size if larger broods require more time and effort to monitor (Sedinger and  Raveling 1990, Schindler and  Lamprecht  1987,  Forslund 1993). Costs incurred during incubation are not always ameliorated during the brood-rearing period. Hepp et al. (1990) found that return rates of female Wood Ducks (Aix sponsa) were positively related to post-incubation body mass in some years. Body condition during fall and early winter is positively correlated with overwinter survival of Canvasbacks and Mallards (Haramis et al. 1986, Hepp et al. 1986). If brood rearing interferes with a female's ability to regain condition, birds with low body mass at the end of the brood-rearing period may not be able to improve their condition before migration, and  may  consequently suffer higher overwinter mortality. Previous work regarding the relationship between brood size and parental behaviour in waterfowl has been inconclusive. Some studies have shown an increase in parental care with brood size (Sedinger and Raveling 1990, Schindler and Lamprecht 1987, Forslund 1993), whereas others have shown no change (Lazarus and Inglis 1978, Rushforth-Guinn  and Batt 1985,  Afton 1993, Seddon and Nudds 1994). None of these studies manipulated brood size. Unless brood size is manipulated experimentally, variation in the amount of care provided with respect to brood size cannot be distinguished from variation in parental ability. In this portion of my  study I used the  experiment described in Chapter 1 to examine the relationship between brood size and parental behaviour of Canvasbacks.  30  Methods  Techniques used to manipulate clutches and mark adult females are presented in Chapter 1. Brood sizes are as in Chapter 1; small broods had 3 ducklings at hatching, and large broods had 8 ducklings at hatching.  Behavioural observations Individual brood hens were observed once per week for one hour from a vehicle or portable blind using a 20-60x scope. I recorded the activities of females and their proximity to ducklings every 30 seconds at the prompting of a metronome (Wiens et al. 1970). Successive observation sessions alternated between morning (0600-1100) and evening periods (1800-2000). To ensure that focal birds were not affected by my presence I waited 15 minutes after arriving at a wetland before beginning a sampling session. If a female and brood were not seen within 20 minutes, another female was observed.  Activities of  brood hens were classified into three broad categories: 1) Parental Carebrooding or vigilant (female in a head-up alert posture) 2) Feeding- diving or dabbling at the water surface 3) Comfort Movements- preening, sleeping, or resting, and 4) Out of Sight - female was not visible. The Out of Sight category includes observations in which  females were concealed in emergent  vegetation, as well as those when I could not see a female because the entire wetland was not visible to me. Sessions in which a female was out of sight continually for 50% of the period were excluded from the analyses. Time spent concealed in vegetation probably represents periods when females were brooding ducklings and is thus an important part of the time budget.  31  Sessions were generally excluded because my position prevented me from seeing birds, and not because they were in vegetation. The distance between females and the nearest duckling in the brood was used as another index of parental care. Distances were classified into three categories: 0-5 m, 5-10 m, and 10-plus-m (female greater than 10 m away from the brood). Rare events are not well represented by instantaneous sampling techniques (Altmann 1974), so I counted all aggressive interactions that I observed during a sampling session.  Analyses Most females were observed more than once, so I used the mean of all observation periods for each female as an independent observation. To assess the effect of timing of breeding on female behaviour, I divided nest initiation dates into "early" and "late" periods using the midpoint of the 47 day nesting period as a division. I transformed time budget data by arcsine square-root (Steel and Torrie 1980). Time budget and proximity data were compared with respect to brood size and nest initiation date using a multivariate analysis of variance (SYSTAT 1991).  If the overall M A N O V A model was significant,  individual behaviours were tested using analysis of variance to determine which behaviours varied with which independent factors. To examine the effect of duckling age on female behaviour, I used a matched-pairs analysis to compare the behaviour of individual brood hens when their ducklings were "young" (< 24 days old) and when their ducklings were "old" (> 24 days old). A n arcsine square-root transformation did not normalize these data, so the effects of age were analyzed with a Wilcoxen Matched-Pairs Signed-Ranks test (SYSTAT 1991).  Data on social interactions and brood size were analyzed  32  using a t-test, and with respect to age by a Wilcoxon Matched-Pairs SignedRanks Test. Estimates of duckling  survival  were taken from  Chapter 1.  Relationships between parental behaviour, female body mass, and duckling survival were analyzed using least-squares regression (SYSTAT 1991).  Results  Behavioural observations I collected behavioural data on 9 females with large broods and 9 females with small broods. On average, each female and brood was observed 3 times (range: 1-6). In total, 51 observation sessions were used for these analyses. Females devoted about half of their time to Parental Care activities (50.7% ± 3.5). Comfort Movements composed 24.7% (± 2.6) of the time budget and Feeding accounted for 17.3% (± 2.2) of the time budget. Females were Out of Sight for the remaining 7.3% (± 1.4) of the time. Brood size did not affect the amount of time that females spent in any activity (Fig. 7; Wilks' Lambda = 0.75, d.f. = 4, 11, P = 0.50). Parental behaviour was also unaffected by nest initiation date (Fig. 8; Wilks' Lambda = 0.72, d.f. = 4,11, P= 0.41). Duckling age had a slight effect on parental behaviour; females with old broods fed less than females with young broods (Fig. 9; Wilcoxon Matched-Pairs SignedRanks Test, Z = 1.75, P = 0.08), and females with older broods were also Out of Sight more often (Fig. 9; Wilcoxon Matched-Pairs Signed Ranks Test, Z = 2.02, P = 0.04).  33  Aggressive interactions Brood hens engaged in 0-9 (mean = 1.5 ± 0.35) aggressive interactions per behavioural session.  These interactions generally involved other  waterfowl, but on a few occasions, displays were directed at beavers (Castor canadensis). Brood size did not affect the number of aggressive interactions that females engaged in (small; 1.33 ± 0.33 vs. large; 1.67 ± 0.62; t = -0.47, d.f. = 16, P = 0.65), nor did the number of aggressive interactions change as ducklings grew older (young; 1.11 ± 0.80 vs. old; 1.30 ± 0.71; Z = 0.13, P = 0.89 ).  Distance to brood Females spent most (74.9% ± 2.9) of their time within 5 meters of their brood, and only a small proportion of time between 5 and 10 meters (11.7% ± 1.6) or more than 10 meters (7.1% ± 1.8) from their brood. Neither brood size (Fig. 10a; Wilk's Lambda = 0.83, d.f. = 4,12, P = 0.66), nor duckling age (Fig. 10b; P > 0.34) affected the distance between a female and her brood.  Parental behaviour in relation to other factors Duckling survival to 42 days was unrelated to the amount of time that females spent in Parental Care or maintenance activities (Fig. 11a; r = 0.08, P = 2  0.41, n = 11 and Fig. l i b ; r = 0.02, P = 0.72, n = 11). If parental behaviour was 2  costly to females in terms of reduced foraging time, then females in poor condition might have allocated more time to feeding and less time to brood care.  I therefore correlated late-incubation body mass of brood-rearing  females to the amount of time that they spent Feeding and the amount of time that they spent in Parental Care behaviours (Fig. 12). Female behaviour was unaffected by body mass (Fig. 12a; r = 0.00, P = 0.94, n = 17, Fig. 12b; r = 2  0.13, P = 0.16, n = 17).  34  2  Discussion  Brood size did not affect parental behaviour in this study, and additional evidence provides little support for the idea that brood-rearing entails a cost of reproduction for Canvasbacks.  Brood size and parental behaviour Brood  size had no effect on the behaviour  of brood-rearing  Canvasbacks. These results are similar to those observed in Northern Pintails (Anas acuta). Lesser Scaup (Aythya affinis) and Common Eiders, (Table 3). Results regarding the relationship between parental care and brood size in other waterfowl are less clear cut. Several studies have found no difference in parental behaviour with respect to brood size, while others have observed significant correlations between brood size and the amount of time parents spent in feeding or vigilant behaviours (Table 3). Some of these latter studies have even concluded that clutch size in precocial birds is therefore limited by parental care (Schindler and Lamprecht 1987, Forslund 1993). One study even concluded that parental care limits clutch size even though they did not measure time budgets in relation to brood size (Pellis and Pellis 1982) Parental behaviour could limit clutch size if the costs associated parental care increase with brood size. Although many studies have demonstrated that the level of care provided increases with brood size, they have not demonstrated that there are costs to such an increase.  The affect of brood size on parental condition There is little evidence to suggest brood  size related costs of  reproduction in waterfowl. A few studies have examined whether brood size  35  affects parental condition at the end of the brood-rearing period (Table 3). Lessells (1986) found that female Canada Geese rearing  experimentally  enlarged broods had a delayed molt and a lower body mass at molting than did females rearing broods of normal or reduced size, although the subsequent survival of these birds was unaffected.  Williams et al. (1994)  measured both body mass and time budgets of parental Lesser Snow Geese; although behaviour varied with brood size, this variation did not affect parental condition. The body mass of female Blue-winged Teal at the end of the brood-rearing period was also unrelated to brood size (Rohwer 1985).  Brood size and parental survival The relationship between brood size and parental survival has been examined in only 5 species of waterfowl (Table 3) and brood size was unrelated to parental survival in all studies (Table 3).  When measured,  future fecundity was also unaffected by brood size (Lessells 1986, Milonoff and Paananen 1993, Williams et al. 1994). I was unable to measure adult return rates (i.e. survival) in this study; however, in a non-manipulative study, Serie et al. (1992) found that return rates of female Canvasbacks were positively correlated with reproductive success during the previous breeding season, suggesting that current and future fecundity are not negatively related in Canvasbacks.  Evidence for costs of parental care I found additional evidence to suggest that costs of brood-rearing are minor in Canvasbacks. If brood-rearing interferes with the ability of parental females to regain condition, late-nesting females or females in poor condition might have fed more. Neither timing of breeding nor late-incubation body  36  mass affected how Canvasbacks allocated their time in this study. Contrary to expectation, females with young ducklings tended to feed more than those with older ducklings. Thus, although females may have needed to forage extensively soon after hatching to regain mass lost during incubation, broodrearing does not appear to interfere with this process. Variation in female behaviour had no effect on duckling survival in this study, suggesting that brood-rearing females are not constrained by the amount of time that they have available for foraging. Arnold et al. (Unpubl. MS) found that lateincubation body mass of Canvasbacks is unrelated to return rates, supporting the idea that brood-rearing does not interfere with the ability of females to regain condition lost during egg-laying and incubation, and thus, probably does not have a big effect on subsequent survival.  Conclusions Results of this, and other studies, suggest that costs of parental care generally do not vary with brood size in waterfowl, and consequently, are not likely to affect clutch size.  37  Table 3. The effect of brood size on parental behaviour, body mass and survival in waterfowl. Parental behaviour Species  Manip  a  Effect  b  Source  Feed Vigilance no  none  none  brachyrhynchus Anser  Lazurus and Inglis 1978  indicus  0  no  +  Schindler and Lamprecht 1987  Chen caerulescens  no  none  none  Lessells 1987  caerulescens no Branta canadensis maxima  +  Williams et al. 1994  no  none  none  Astrom 1993  no  none  none  Seddon and Nudds 1993  minima  no  +  Sedinger and Raveling 1990  Branta leucopsis  no  Anas acuta  no  + none  none  Forslund 1993 Rushforth-Guinn and Batt 1985  Ay thy a affinis  no  none  none  Afton, 1993  Somateria  no  none  none  Bustnes and  mollisima  Erikstad 1991  38  Table 3. The effect of brood size on parental behaviour, body mass and survival in waterfowl (continued).  Parental body mass Species  e  Manip  Effect^  a  Source  Chen caerulescens caerulescens  no  none  Branta canadensis  yes  -  11  Anas discors  yes  none  Williams et al. 1994  Lessells 1986 Rohwer 1985  Parental survival Species  Manip  Effect^  a  Chen caerulescens caerulescens no  no  Branta canadensis  none 0  yes  39  Source  none  Rockwell et al. 1987  Williams et al. 1994 none  Lessells 1986  Table 3. The effect of brood size on parental behaviour, body mass and survival in waterfowl (continued).  Parental survival (continued) Species  Aix sponsa  Manip  no  a  Effect  none  b  Source  Rohwer and Heusmann 1991  Ay thy a affinis no  none  Afton 1993  Bucephala clangula  no  none  Dow and Fredga 1984  yes  none  Milonoff and Paananen 1993  T3rood size experimentally manipulated.  a  ^The direction of statistically significant effects are indicated by +'/ -. °Semi-captive flock. ^Vigilance measured as proportion of foraging time spent alert. e  Body mass of parents at, or close to, the time of fledging.  40  1.0 -  Brood Size  0.9 0.8 -  Small (n=9)  •  Large (n=9)  •  0.7 " 0.6 0.5 "  0.4 -  t +  0.3 0.2 0.1 0.0  L, Parental Care  Comfort Movements  Feeding  Out of Sight  Female Behaviour  Figure 7. Time-activity budgets of brood-rearing Canvasbacks in relation to experimentally manipulated brood size. Small broods had 3 ducklings at hatching; large broods had 8 ducklings at hatching. Means are presented + 1 SE.  41  1.0 -  Nest Initiation Date  0.9 0.8 -  Early (n=9)  •  Late (n=9)  •  0.7 0.6 0.5 -  0.4 -  t I  0.3 0.2 0.1 0.0  L. Parental Care  Feeding  Comfort Movements  Out of Sight  Female Behaviour  Figure 8. Time-activity budgets of brood-rearing Canvasbacks in relation to nest initiation date.  Early nests were initiated prior to the midpoint of the nesting  period; late nests were initiated after the midpoint of the nesting period. Means are presented + 1 SE.  42  1.0 -  Duckling Age  0.9 -  Young (n=6) Old (n=6)  0.8 -  • A  0.7 0.6  H  0.5  I  0.4 0.3  H  0.2  -I  t  0.1 0.0  Parental Care  Feeding  Comfort Movements  ~  '—t  Out of Sight  Female Behaviour  Figure 9. Time-activity budgets of brood-rearing Canvasbacks in relation to duckling age. Young ducklings were < 24 days old; old ducklings were > 24 days old. Means are presented ± 1 SE. Asterisks indicate a significant difference.  43  Figure 10. a. The distribution of distances of brood-rearing females from their brood, in relation to brood size. Means are presented ± 1 SE. b. The distribution of distances of brood-rearing females from their brood, in relation to duckling age. Means are presented + 1 SE.  44  1.0 Brood Size  0.9 0.8 -  Small (n=9)  •  Large (n=9)  A  0.7 -  o c a>  cr a> i—  LL  0.6 0.5 0.4 H  0.3 0.2 -J oi -1  0.0 0-5m  5-10m  > 10m  Out of Sight  Distance  1.0 Duckling Age  0.9 -  Young (n=6)  0.8 0.7 O  c  CD  D  cr  CD  £  0.6 -  Old  •  (n=6) A  t  0.5 0.4  -I  0.3 0.2 H  • 4  0.1 A 0.0  0 -5m  5-10m  > 10 m  Distance 45  Out of Sight  Figure 11. a. The relationship between duckling survival to 42 days and the mean proportion of time that females spent in Parental Care activities.  b. The relationship between duckling survival to 42 days and the mean proportion of time that females spent in maintenance behaviours (Feeding and Comfort Movements).  46  a.  1.0  Brood Size  0.9 0.8 -  1 D  CO Ui  c  Small  •  Large  •  0.7 0.6 0.5 -  o  0.4 -  Q  0.3 0.2 -  -  0.1 0.0  0.0  0.1  T  T  0.2  0.3  T  1—p  0.4  0.5  T  0.6  0.7  ••—r  0.8  0.9  1.0  Frequency of Parental Care Behaviours  b.  • •  1.0 0.9  Brood Size  -  0.8 0.7  Small  •  Large  A  -  A  0.6 -  A  0.5 0.4 0.3  -  0.2 0.1 0.0  *• 0.0  -i— —r1  0.1  0.2  0.3  1  0.4  0.5  0.6  0.7  "I  r  0.8  Frequency of Maintenance Behaviours 47  -  0.9  1.0  Figure 12. a. The relationship betwen the mean proportion of time that females spent Feeding and their late-incubation body mass.  b. The relationship between the mean proportion of time that females spent Parental Care activities and their late-incubation body mass.  48  a.  1.0 0.9  Brood Size  -  Small • Large A  0.8 0.7  O) "O CD  0)  U.  -  0.6 - | 0.5  -  0.4 0.3  -  0.2 0.1  H  0.0 900  •  I 950  A —I—  1,000  1,050  1,100  1,150  1,200  Female Body Mass (g)  1.0  Brood Size  0.9 - \  52 3 O >  0.8  H  0.7  H  CD  0.6  H  gJ 8> CO  0.5  CD  O  (0 0) (0  0.4  H  0.3  H  0.2  H  Small • Large  •  0.1 H 0.0 900  I 950  —I  —I  1,000  1,050  1,100  Female Body Mass (g) 49  1,150  1,200  General Conclusions  Results from this study suggest that clutch size of Canvasbacks may  be  limited by post-hatching factors. Ducklings in large broods appeared to have lower survival, yet, this difference was probably not sufficient to limit clutch size per se, because large broods appeared to produce more young. However, incorporating the observed decline in duckling survival into an existing model of clutch size determination (Arnold et al. 1987) lowered the predicted optimal clutch size to a value equal to observed Canvasback clutch size. I propose that clutch size of ducks is not determined by any single factor, but by a combination of processes. Other factors that may  influence clutch size of waterfowl include  predation risk, egg viability and the availability of nutrient reserves for incubation. Arnold et al. (1987) have demonstrated that predation risk and temporal declines in egg viability during laying are likely to influence clutch size in temperate nesting waterfowl. Clutch size does not appear to have a direct affect on other aspects of incubation. Hatching success and the duration of the incubation period are both independent of clutch size (Rohwer 1985, Erikstad et al. 1993, Milonoff and Paananan 1993) and mass loss by incubating females is also unrelated to clutch size (Gatti 1983, Rohwer 1985). However, clutch size may have an indirect effect on costs associated with incubation. The role of nutrient reserves in the determination of clutch size of waterfowl has been a contentious issue (Ankney et al. 1991, Arnold  and  Rohwer 1991, Drobney 1991). Stored nutrients do not appear to limit eggproduction in temperate nesting waterfowl (Rohwer 1984, Milonoff Arnold and Rohwer 1991), although their use during incubation may  1989, effect  clutch size. Female waterfowl use nutrient reserves for egg production, and  50  females that lay smaller clutches will begin incubation with more nutrient reserves (Alisauskas and Ankney 1992). Many researchers have suggested that females beginning incubation with few reserves take more feeding recesses and thus have lower incubation constancy.  A decrease in nest  attentiveness could result in reduced hatchability or longer incubation periods, and thus, higher predation risk (Aldrich and Raveling 1983, Gloutney and Clark 1991, Erikstad et al. 1993, Arnold et al. MS). However, the idea that condition at the onset of incubation is important to reproductive success has not been tested directly. Erikstad et al. (1993) suggested that female Common Eiders balance the allocation of nutrient reserves to egg-production and incubation, however, it is unclear whether this occurs in temperate nesting waterfowl. Gloutney and Clark (1991) showed that the body mass of Mallards and Northern Shovelers at the end of incubation is positively correlated to nest success (Gloutney and Clark 1991), however, this is not the case for Blue-winged Teal, (Gloutney and Clark 1991) Canvasbacks, or Redheads (Arnold et al. MS).  These studies used  body mass at the end of incubation to examine the relationship between nest success and body condition. This comparison may not be valid if individuals lose mass at different rates, as Wood Ducks do (Harvey et al. 1989). To determine if nutrient reserves are important during incubation, we need to determine how condition at the onset of incubation is related to both clutch size and subsequent nest success. Female Canvasbacks did not adjust their behaviour with respect to brood size, and I found little evidence to suggest a cost of reproduction associated with brood size.  I did not measure adult survival or future  fecundity, but my results suggest that there are not likely to be large costs  51  associated with parental care in Canvasbacks. It is thus unlikely that brood size dependent costs of parental care limit clutch size. If there are costs associated with brood-rearing they might be more evident in species that nest in areas with a shorter breeding season, such as Common Eiders. Work by Erikstad et al. (1993) suggests that this may be the case. They found that female eiders with low late incubation body mass were more likely to abandon their broods to be reared in creches, and that females in good condition were more likely to tend creches. These differences were unrelated to brood size, indeed females with smaller clutches tended to be those in poorer condition. Thus, even if there is a cost of brood-rearing, it appears to be unrelated to number of young reared.  52  Literature Cited  Afton, A.D. 1993.  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