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Evaluating nest box addition as a population augmentation strategy for tree swallows, Hirondelle bicolore,.. Pokorny, Stanley 2011

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DEAN'S OFFICE, FACULTY OF FORESTRY Undergraduate Thesis Evaluating nest box addition as a population augmentation strategy for tree swallows, Hirondelle bicolore, in interior British Columbia, Canada   Stanley Pokorny            Stanley Pokorny  2  TABLE OF CONTENTS Abstract ....................................................................................................................................................................................... 3 Introduction ............................................................................................................................................................................... 4 Study Area .................................................................................................................................................................................. 6 Methods ....................................................................................................................................................................................... 7 Results .......................................................................................................................................................................................... 8 Discussion ................................................................................................................................................................................... 9 Tables ......................................................................................................................................................................................... 13 Figures........................................................................................................................................................................................ 15 References ................................................................................................................................................................................ 18 Appendix ................................................................................................................................................................................... 21                Stanley Pokorny  3   ABSTRACT     Tree swallow populations across North America are exhibiting negative annual trends. An exception to the broader trend is interior BC, Canada where tree swallow populations are growing.  Augmentation of successful populations by the addition of nest boxes to cavity poor landscapes may be an appropriate strategy to mitigate or reverse the national decline.  This was evaluated through an analytical comparison of the reproductive performance (clutch size and hatchlings) of tree swallow breeding pairs nesting in natural cavities and nest boxes in the William’s Lake region of BC, Canada.  For the 2001-2003 breeding seasons mean clutch size (means ± SE, nest boxes: 5.94 ± 0.199 eggs (n=83), cavities: 4.2 ± 0.204 eggs (n=74)) and number of living hatchlings (means ± SE, nest boxes: 4.51 ± 0.265 chicks(n=83), cavities: 2.51 ± 0.27 chicks(n=74)) detected were significantly higher (clutch size: p-value=0.0001, hatchlings: p-value 0.0001, alpha=0.05) for pairs using nest boxes.  Nest boxes provide tree swallow breeding pairs with additional nesting territories and result in higher reproductive rates. Nest box addition has the potential to be a long term population augmentation mechanism but further studies on long term impacts at the community level are necessary before it can be implemented as a management plan.   Stanley Pokorny  4  INTRODUCTION   Species of the aerial insectivore guild utilize a broad range of habitats for nest site selection and foraging areas, which has allowed them to successfully colonize the majority of North America (McCracken 2008).  Despite this variation in acceptable habitat, long term and recent guild-wide declines in aerial insectivores have been detected by Regional Breeding Bird Surveys across the continent (McCracken 2008).  Typical limiting factors such as suitable nest site availability, competition from exotic species or large scale land use changes can be shown to be at least partly responsible for individual species’ declines (Aitken & Martin 2007, Carrascal & Alonso 2006, Cornelius et al. 2008, Gains et al. 2010, Kath & Dunn 2009, Koch et al. 2011, Miller 2010, McCracken 2008, Shustack & Rodewald 2010, Zhu et al. 2010) but are less explanatory at the guild level.  The life history trait that unites this otherwise diverse guild is a reliance on insect populations as a primary food source (McCracken 2008).  Accordingly, changes in insect populations arising from light sensitivity, deleterious aquatic conditions and climatic variation are being investigated as an alternate cause of this decline (McCracken 2008).    Although interesting and likely relevant, very little is known about broad scale insect population trends or demographics and even less is known about management (McCracken 2008).  Until the mechanism behind the decline is better understood, mitigation or reversal of this trend must rely on management strategies that have been proven to be effective in the past, namely habitat conservation and addition of critical limited resources such as suitable nest sites (Aitken & Martin 2011, Brandies et al. 2002, Holt & Martin 1997, Wesolowski 2007).    The overall population trend for Canada and the US is negative but there is notable temporal and geographical variation (McCracken, 2008).  In the William’s Lake area of British Columbia several members of the guild experiencing continental declines, tree swallows (Hirondelle bicolor), fly catchers (Empidonax spp.) and wood pewees (Contopus sordiulus) , were shown to have Stanley Pokorny  5  linear increases in abundance and detection over the last 13 years (Adamson et al. 2010).  Despite deforestation from forestry practices, massive habitat and food supply fluctuation from the MPB outbreak and competition from exotics such as the European Starling, Sturnus vulgaris, interior BC still acts as an aerial insectivore refuge (Adamson et al. 2010).  Augmentation of healthy growing populations may be an appropriate strategy to counteract the large scale decline, at least locally.     Secondary cavity nesters (SCNs) such as the tree swallow are reliant on excavated or naturally occurring cavities for nesting sites (Aitken and Martin 2007, Aitken and Martin 2008, Cornelius et al. 2008, Gains et al. 2010, Holt and Martin 1997, Kath & Dunn 2009, Koch et al. 2011, Martin et al. 2004, Miller 2010, Robertson and Rendell 1990, Saab et al 2009).  Density, diversity and abundance of SCN species are correlated with the abundance, nature (excavated or naturally formed), diversity (height on tree, cavity dimensions) and spatial distribution of cavities (Wiebe et al. 2006, Zhu et al. 2010).  Stable populations of woodpeckers, in the capacity of cavity excavators, play an important role in maintaining SCN populations and allowing for growth (Aitken and Martin 2007).  Nest box addition studies have conclusively shown that distributing appropriate nesting sites throughout cavity-poor areas can significantly increase abundance and diversity of mammalian and avian SCN species (Holt and Martin 1997).  Significant increases in abundance and breeding capacity have also been reported in SCN bird populations housed in grid arrangements of nest boxes when compared to adjacent populations in natural cavities (Robertson and Rendell 1990).  It should be noted that these results are limited in their representation of natural population performance and behavior as the nest grids limit the variation of cavity dimensions and alter the impacts of predation and parasitism while forcing breeding pairs to coexist at unnatural densities (Robertson and Rendell 1990).      Tree swallows are experiencing a -2.7% annual trend across Canada(McCracken, J. 2008) but have been shown to have increasing populations in the Williams Lake region of BC (Adamson et al. 2010).  Adding nest boxes as surrogates in cavity-poor areas has the potential to allow for Stanley Pokorny  6  further growth in these populations (Aitken and Martin 2008, 2011).  However, the demographics associated with nest box populations in otherwise suitable habitat are poorly understood in comparison to populations in natural cavities.  Fecundity and associated cavity data were collected for tree swallows in the same area of the William’s Lake Region of BC for breeding pairs housed in cavities and in nest boxes.  By comparing fecundity rates, specifically clutch size and max number of observed hatchlings, between the two nest site types I hope to evaluate whether nest box addition is an appropriate management strategy for augmentation of growing tree swallow populations, where cavity limitation may be an issue.  If average clutch size and hatchlings from nest boxes are greater than or equal to values obtained from natural cavities, then it can be concluded that application of this strategy is appropriate, at least in the area of study. STUDY AREA   All data used for analysis was collected between 2001-2003 from adjacent study areas in the Cariboo-Chilcotin Region of central interior British Columbia (51 52’N, 122°21’W) southwest of the city of William’s Lake.  A map of the area is shown in Figure 1.  Located in the warm dry Interior Douglas-fir biogeoclimatic zone, the landscape is composed of mixed coniferous and deciduous forest surrounded by a grassland matrix.  Prevalent tree species include Douglas-fir, lodge pole pine, white and hybrid spruce and quaking aspen.  Cavity data specifically came from the Riske Creek study area where long term cavity monitoring has lead to multiple published articles.  Nest box data came from study sites on Becher’s Prairie where nest site limitation theory was being examined by the experimental addition of nest boxes to cavity poor areas.  For additional details describing the sites, experimental design and data collection techniques of the original studies see Martin et al. (2004) and Aitken and Martin (2011).   Stanley Pokorny  7  METHODS  The first step in comparing the data from the two studies was extracting the specific data I needed from the two Microsoft Excel spreadsheets supplied by K. Aitken (Boxes) and K. Martin (Natural cavity nests).  The cavity data were already in a usable format with one entry per cavity per year that included occupant fecundity measurements as well as cavity dimensions, height on tree, tree decay class and aspect.  Data covered all cavities shown to contain active tree swallow nests for years 2001-2003.   The nest box data were recorded in a different format but they contained observations for tree swallow nests in boxes for 2001-2003. The main difference between the spreadsheets was that, in the nest-box dataset, each visit to a box was recorded as its own entry resulting in 3-4 rows of data per box per breeding season.  As the original study was focused on changes in abundance and diversity of nest box occupants, general fecundity data was recorded in a comments section in the final column of each entry.  The parameters noted with consistency were clutch size and number of living or dead hatchlings seen on a given visit.  These two parameters were also recorded in the cavity data and so became the factors examined in the comparison.  The nest box data were made compatible for statistical comparison with the natural cavities data by combining observations from multiple visits into a single entry per box per year with an associated clutch size and number of hatchlings.   The extracted data for clutch size and max observed living hatchlings, included as an appendix, were analyzed with GLM procedures using the SAS 9.2 statistics package, 2002-2008, SAS Institute Inc, Cary, NC, USA, to test the significance of the nest site origin as a treatment and to assure the residuals met the assumptions of normality and equal variance.  The GLM procedures used 2 treatment classes (box and nat) over 3 breeding seasons (2001, 02 and 03) with maximum of 43 replicates per treatment per year and an alpha level of 0.05.  The number of replicates varied Stanley Pokorny  8  by nest type and year.  For natural cavities n=22 in 2001, n=23 in 2002 and n=29 in 2003.  For nest boxes n=19 in 2001, n=43 in 2002 and n=21 in 2003. Blank entries indicate the lack of an accurate observation and were excluded in the analysis.   Concurrence with assumptions was judged using normality test p-values (p-value>0.05= residuals normally distributed), visual examination of normality plots and histograms as well as the Bartlett’s test.  Assistance in writing the SAS programming was supplied by Dr. Tony Kozack.   RESULTS   Results of the GLM procedures for clutch size and hatchlings (Table 3) yielded significant findings.  However, using untransformed data, neither output met the normality assumption (all test p-values<0.01).   The clutch size output also failed to meet the equal variance assumption (Bartlett’s test p-value=0.002) while the hatchling output met the assumption (Bartlett’s test p- value= 0.0346).  By applying a power transformation of X^1.8 both outputs were able to meet the equal variance assumption (clutch size^1.8 p-value=0.0124, hatchlings^1.8=0.0172) but had no effect on the normality test values.   Tree swallow pairs breeding in natural cavities laid significantly smaller clutches size (p- value<0.0001) than those using nest boxes (Table 2).  Mean clutch sizes for cavities were smaller in each individual year (Figure 2) and when annual data was pooled cavities on average contained 1.742 fewer eggs than nest boxes (2001, 2002 and 2003 combined, means ± SE, nest boxes: 5.94 ± 0.199 eggs (n=83), cavities: 4.2 ± 0.204 eggs (n=74)).  Figure 2 illustrates that clutch sizes increased similarly in 2003 for both nest types (nest boxes: +0.62, cavities: +0.68) in comparison to the 2001 and 2002 seasons.  This difference was significant between 2001 and 2003(p-value 0.011, alpha 0.017) and nearly so between 2002 and 2003(p-value 0.032, alpha 0.017).    Stanley Pokorny  9  Similar results were obtained for number of maximum observed living hatchlings with significantly fewer chicks detected in cavities (p-value<0.0001) (Table 3).  Mean number of hatchlings observed was lower in each individual year for nests in cavities (Figure 3) and when annual data was pooled cavities on average contained 1.9994 fewer hatchlings than nest boxes (2001, 2002 and 2003 combined,  means ± SE, nest boxes: 4.51 ± 0.265 chicks(n=83), cavities: 2.51 ± 0.27 chicks(n=74)).  Figure 3 shows separate trends for each nest type.  Cavities hit a low point in 2002 (1.9 hatchlings) and rebounded in 2003(3.4 hatchlings), an 81% increase.  Nest boxes exhibited a slight negative trend -7.1% between 2001(4.68 hatchlings) and 2003 (4.35 hatchlings).   No other significant interactions were detected.      I calculated the number of hatched chicks/egg laid for the three year period and for each year by combining the mean clutch size and hatchling values (Table 4).  For nest boxes an overall average of 5.94 eggs led to 4.506 hatchlings, a 75.85% hatching rate or 0.76 chicks per egg.  For cavities an average of 4.2 eggs led to an average of 2.5 hatchlings, a 59.52% hatching rate or 0.60 chicks per egg (Figure 4). The cavities data set also contained information on the number of hatchlings that reached the fledgling stage.  Of the cavities used for the GLM procedure 29 had reliable fledgling counts.  The mean for these 29 cavities was 1.433 fledglings and was incorporated into Figure 4.  With this new data point, Figure 4 now shows the survival trend , estimated by a best fit line( y=-1.385x+5.48), for tree swallow offspring in cavities through three life stages.  A trend line fitted to the nest box data had a similar slope (y=-1.43x+7.37), indicating a similar rates of loss in offspring across live stages for the two nest types.  Extending the nest box best fit line to the fledgling interval results in an estimated average of 3.08 fledglings per nest box for 2001-2003.       DISCUSSION  Stanley Pokorny  10  The purpose of the project is to evaluate whether the addition of nest boxes to cavity poor landscapes can be a population augmentation mechanism for tree swallows in interior BC, Canada as part of a proactive effort to mitigate nationwide declines.  It is established that adding nest boxes to cavity poor areas can increase the local abundance of tree swallow breeding pairs and other SCN species (Aitken & Martin 2011, Holt & Martin 1997).  Less examined but equally important to evaluation of nest box addition as a management strategy is the reproductive performance of breeding pairs in nest boxes and the survivability of their offspring.  The studies that have analyzed these properties used data collected from nest box grids and rarely compared results to data from adjacent natural populations (Robertson & Rendell 1990).  I used data from a population where nest boxes were distributed throughout otherwise unaltered cavity poor habitat and compared them to another component of the population breeding in natural cavities.   Analysis of this comparison reported that mean clutch size and number of hatchlings found in nest boxes significantly exceeded those found in natural cavities for 2001-2003. More eggs were laid in nest boxes and a higher percentage hatched.   Further, based on trends observed in the data, nest boxes may result in two times the number of hatchlings surviving to the fledgling stage.  Based on these results, long term next box addition is an appropriate strategy for tree swallow population augmentation.   While the results are significant and provide evidence in favor of nest box addition, this analysis is notably limited by the number of breeding seasons with data available for comparison and in the number of species considered.  After the 2003 breeding season, the nest boxes were removed and the abundance of occupant species returned to pre-addition levels (Aitken & Martin 2011).   This highlighted nest site availability as the limiting factor on population density but precluded a longer term dataset.   Additional years of data would allow for better interpretation of the trends detected in Figures 3 and 4.  Clutch sizes increased for nest boxes in 2003 while number of hatchlings decreased slightly.  Timing of breeding initiation, influenced by annual stochastic Stanley Pokorny  11  variation in temperature and cavity availability can alter the date of first egg which has been shown to impact subsequent clutch sizes and chick survival (Koch et al 2011).  Alternately the reduced number of hatchlings may have been due to increased targeting of nest boxes by parasites and predators, which has been documented in other nest box studies (Robertson & Rendell 1990).   Identification of the causal factors of trends based on data from three breeding seasons is purely speculation.   Only a single species was examined in this analysis.  While nest boxes function as high quality nesting sites tree swallows they may have deleterious or negligible effects on other species.   The datasets used in this project contain nest box and cavity data for multiple species.  It is possible to compare clutch size and hatchlings across nest types for these species as well.  The mountain bluebird, Sialia currucoides, for example is a conspecific of the tree swallow with similar data available.  Mountain bluebirds are dominated by European starlings when competing for nest sites and may also benefit from the addition of nesting territories (Aitken & Martin 2008, Koch et al 2011).  The use of nest boxes as a bluebird population augmentation strategy could be evaluated using similar methods.  Analysis of reproductive performance at the community level in this type of nest box arrangement is necessary to accurately model the effects of long term nest box addition.     In conclusion, the regional tree swallow population of interior BC breeding in natural cavities is growing and will potentially be limited by the number of available nest sites (Adamson et al 2010).   Long term nest box addition to otherwise suitable sites with limited natural or excavated cavities is an inexpensive and direct way to increase the number of nesting territories available to breeding pairs (Aitken & Martin 2011). Other methods of increasing nest site availability, such as inoculating trees with fungi to hasten decay and promote excavation by woodpeckers, are indirect, operate on long time scales, may be economically unfeasible at the landscape scale and have had only limited success (Brandies et al 2002, Filip et al 2004).   Additional territories would also be Stanley Pokorny  12  available to other SCN species, possibly reducing competition for quality nest sites with conspecifics such as the mountain bluebird and invasive species such as the European starling.  The results this analysis indicate that pairs breeding in these boxes will have increased reproductive output compared to neighbors in cavities.  The scope and applicability of the results are limited. Future studies will need to expand the time frame of data collection and number of species considered to better understand the impacts of nest box addition before long term nest box addition can be implemented as a management strategy at broad scales.                    Stanley Pokorny  13   TABLES Table 1: Summary of statistical parameters, mean and standard error, calculated for tree swallow clutch size data (number of eggs).   Year Natural Nest Box  Mean SE Mean  SE 2001 3.89 0.360 5.63 0.337 2002 4.01 0.360 5.79 0.177 2003 4.69 0.273 6.41 0.312 Total  4.20 0.204 5.94 0.199  Table 2: Summary of statistical parameters, mean and standard error (SE), calculated for tree swallow maximum observed living hatchling data (number of chicks).   Year Natural Nest Box  Mean SE Mean  SE 2001 2.26 0.593 4.68 0.589 2002 1.86 0.575 4.48 0.296 2003 3.40 0.516 4.35 0.514 Total  2.51 0.270 4.51 0.265  Table 3: GLM procedure results for the clutch size and living hatchlings of tree swallow breeding pairs housed in nest boxes or natural cavities in interior BC, Canada for years 2001-2003. Clutch Size Source of Variation Degrees Of Freedom  Sum of Squares  Mean Square  F-  value  P-  Value  Model 74 214.59 2.8998 2.14 0.0014  Error 59 79.81 1.3526   Corrected Total  133 294.40     Stanley Pokorny  14   Hatchlings  Source of Variation Degrees Of Freedom  Sum of Squares  Mean Square  F-  value  P-  Value  Model 72 307.23 4.2671 1.18 0.2643  Error 55 199.24 3.6224   Corrected Total  127 56.47     Table 4: Annual hatched chicks/egg laid values for of tree swallow breeding pairs housed in nest boxes or natural cavities in interior BC, Canada for years 2001-2003. Year  Natural Cavities Nest boxes 2001 0.58 0.83 2002 0.46 0.77 2003 0.72 0.68 Total  0.60 0.76   Stanley Pokorny  15  FIGURES    Figure 1: Map of interior British Columbia, Canada with nest box addition sites indicated (Aitken and Martin 2011).  Stanley Pokorny  16   Figure 2: Histogram of average annual clutch size for tree swallow breeding pairs in cavities and nest boxes in years 2001 (boxes:n=19, cavities:n=22), 2002(boxes:n=43, cavities:n=23), and 2003(boxes:n=21, cavities:n=29) in interior British Columbia Canada.    Figure 3: Histogram of average annual number of hatchlings for tree swallow breeding pairs in cavities and nest boxes for years 2001(boxes:n=19, cavities:n=22), 2002(boxes:n=43, cavities:n=23) and 2003(boxes:n=21, cavities:n=29) in interior British Columbia, Canada.   0 1 2 3 4 5 6 7 2001 2002 2003 A ve ra ge  A n n u al  C lu tc h  S iz e Year Nat Box 0 1 2 3 4 5 6 2001 2002 2003 A ve ra ge  A n n u al  # o f H at ch lin gs  Year Nat BoxStanley Pokorny  17   Figure 4: Plot with trend line of average number of offspring for tree swallow breeding pairs housed in nest boxes (n=83) and natural cavities (n=74) (n=29 for hatchlings) for years 2001-2003 in interior British Columbia, Canada as offspring pass through progressive life stages.  Fledgling detection not recorded in nest box dataset.       0 1 2 3 4 5 6 7 A ve ra ge  # o f L iv in g O ff sp ri n g Life Stage nat boxStanley Pokorny  18  REFERENCES  Adamson, C., Drever, M., Martin, K. & Koch, A. (2010). Species richness and population trends of forest wildlife species in interior BC in response to an outbreak of mountain pine beetle and other habitat change. Unpublished report, UBC.   Aitken, K. E. H., & Martin, K. (2007). The importance of excavators in hole-nesting communities: Availability and use of natural tree holes in old mixed forests of western Canada. Journal of Ornithology, 148, S425-S434. doi:10.1007/s10336-007-0166-9  Aitken, K. E. H., & Martin, K. (2008). Resource selection plasticity and community responses to experimental reduction of a critical resource. Ecology, 89(4), 971-980.  Aitken, K.E.H., & Martin, K. (2011). Experimental test of nest-site limitation in mature mixed forests of central British Columbia, Canada. Journal of Wildlife Management, not yet published.   Brandeis, T. J., Newton, M., Filip, G. M., & Cole, E. C. (2002). Cavity-nester habitat development in artificially made Douglas-fir snags. Journal of Wildlife Management, 66(3), 625-633.  Carrascal, L. M., & Alonso, C. L. (2006). Habitat use under latent predation risk. A case study with wintering forest birds. Oikos, 112(1), 51-62.  Cornelius, C., Cockle, K., Politi, N., Berkunsky, I., Sandoval, L., Ojeda, V., Rivera, L., Hunter, M. Jr., Martin, K. (2008). Cavity-nesting birds in neotropical forests: Cavities as a potentially limiting resource. Ornitologia Neotropical, 19, 253-268.  Filip, G. M., Parks, C. G., Baker, F. A., & Daniels, S. E. (2004). Artificial inoculation of decay fungi into Douglas-fir with rifle or shotgun to produce wildlife trees in western Oregon. Western Journal of Applied Forestry, 19(3), 211-215.  Gaines, W., Haggard, M., Begley, J., Lehmkuhl, J., & Lyons, A. (2010). Short-term effects of thinning and burning restoration treatments on avian community composition, density, and nest survival in the eastern cascades dry forests, Washington. Forest Science, 56(1), 88-99.  Stanley Pokorny  19  Holt, R. F., & Martin, K. (1997). Landscape modification and patch selection: The demography of two secondary cavity nesters colonizing clear cuts. Auk, 114(3), 443-455.  Kath, J., Maron, M., & Dunn, P. K. (2009). Interspecific competition and small bird diversity in an urbanizing landscape. Landscape and Urban Planning, 92(2), 72-79.  Koch, A.J., Martin, K., & Aitken, K.E.H. (2011). The relationship between introduced European starlings and the reproductive activity of mountain bluebirds and tree swallows in central British Columbia.  Not yet published Lohmus, A., & Remm, J. (2005). Nest quality limits the number of hole-nesting passerines in their natural cavity-rich habitat. Acta Oecologica-International Journal of Ecology, 27(2), 125-128. doi:10.1016/j.actao.2004.11.001  Martin, K., Aitken, K.E.H. & Wiebe, K.L. (2004). Nest sites and nest webs for cavity-nesting communities in interior British Columbia, Canada: nest characteristics and niche partitioning. The Condor, 106(1), 5-19.  McCracken, J. (2008). Are aerial insectivores being bugged out? BirdWatch Canada, 42, 1-7. Miller, K. E. (2010). Nest-site limitation of secondary cavity-nesting birds in even-age southern pine forests. Wilson Journal of Ornithology, 122(1), 126-134.  Moreno, J., Lobato, E., Gonzalez-Braojos, S., & Ruiz-de Castaneda, R. (2010). Nest construction costs affect nestling growth: A field experiment in a cavity-nesting passerine. Acta Ornithologica, 45(2), 139-145.  Remm, J., Lohmus, A., & Rosenvald, R. (2008). Density and diversity of hole-nesting passerines: Dependence on the characteristics of cavities. Acta Ornithologica, 43(1), 83-91.  Robertson, R.J. & Rendell, W.B. (1990). A comparison of the breeding ecology of a secondary cavity nesting bird, the tree swallow ( Tachycineta bicolor), in nest boxes and natural cavities. Canadian Journal of Zoology, 68, 1046-1052.  Stanley Pokorny  20  Saab, V. A., Russell, R. E., & Dudley, J. G. (2009). Nest-site selection by cavity-nesting birds in relation to postfire salvage logging. Forest Ecology and Management, 257(1), 151-159.  Shustack, D. P., & Rodewald, A. D. (2010). A method for detecting undervalued resources with application to breeding birds. Ecological Applications, 20(7), 2047-2057.  Wesolowski, T. (2007). Lessons from long-term hole-nester studies in a primeval temperate forest. Journal of Ornithology, 148, S395-S405. doi:10.1007/s10336-007-0198-1  Wiebe, K. L., Koenig, W. D., & Martin, K. (2006). Evolution of clutch size in cavity-excavating birds: The nest site limitation hypothesis revisited. American Naturalist, 167(3), 343-353.  Zhu, X., Srivastava, D.S., Smith, J.N.M., & Martin, K. (2010) Habitat selection and reproductive success of Lewis’s woodpecker (Melanerpes lewis) at its northern limit. The Auk, Submitted.    Stanley Pokorny  21  APPENDIX  Raw extracted data of clutch size and max observed living hatchlings for Tree Swallows in nest- boxes (box)and natural cavities (nat) for years 2001, 2002 and 2003. Treatment Year Replicate Clutch size  Hatchlings   Box 2001 1 5  Box 2001 2 6 5 Box 2001 3 6 6 Box 2001 4   Box 2001 5 6 6 Box 2001 6 6 5 Box 2001 7 6 6 Box 2001 8 6 6 Box 2001 9 4 0 Box 2001 10 6 5 Box 2001 11 5  Box 2001 12 6 4 Box 2001 13 4 4 Box 2001 14 6 6 Box 2001 15 4 3 Box 2001 16 7 3 Box 2001 17 4 4 Box 2001 18 3 3 Box 2001 19 4 3 Box 2002 1 6 6 Box 2002 2 4 4 Box 2002 3 6 0 Box 2002 4 6 6 Box 2002 5 6 6 Box 2002 6 5 4 Box 2002 7 5 2 Box 2002 8 4 4 Box 2002 9 5 5 Box 2002 10 5 3 Box 2002 11 6 4 Box 2002 12 6 3 Box 2002 13 6 6 Box 2002 14 5 1 Box 2002 15 4 4 Box 2002 16 8 8 Box 2002 17 5 5 Box 2002 18 6 5 Box 2002 19 5  Box 2002 20 5 3 Box 2002 21 5 4 Box 2002 22 7 5 Box 2002 23 7 6 Box 2002 24 6 6 Box 2002 25 6 6 Box 2002 26 6 5 Box 2002 27 6 6 Box 2002 28 6 4 Box 2002 29 6 6 Stanley Pokorny  22  Box 2002 30 7 7 Box 2002 31 5 5 Box 2002 32 7 5 Box 2002 33 7 4 Box 2002 34 6 1 Box 2002 35 6 4 Box 2002 36 6 0 Box 2002 37 5 4 Box 2002 38 7 6 Box 2002 39 6 5 Box 2002 40 6 6 Box 2002 41 6 4 Box 2002 42 7 7 Box 2002 43 5 4 Box 2003 1 6 3 Box 2003 2 4 4 Box 2003 3 5 4 Box 2003 4 5 3 Box 2003 5 6 5 Box 2003 6 5 3 Box 2003 7 6 4 Box 2003 8 6 4 Box 2003 9 6 5 Box 2003 10 6 3 Box 2003 11 5 3 Box 2003 12 6 4 Box 2003 13 6 4 Box 2003 14 8 7 Box 2003 15 6 2 Box 2003 16 7 4 Box 2003 17 7 4 Box 2003 18 7 5 Box 2003 19 6 4 Box 2003 20 7 5 Box 2003 21 6 4 Nat 2001 1 3 3 Nat 2001 2 4 2 Nat 2001 3   Nat 2001 4 2  Nat 2001 5 4 3 Nat 2001 6 2 0 Nat 2001 7 2 0 Nat 2001 8  0 Nat 2001 9   Nat 2001 10   Nat 2001 11 7 7 Nat 2001 12 3 3 Nat 2001 13 2 2 Nat 2001 14 1 0 Nat 2001 15 6 5 Nat 2001 16 6 5 Nat 2001 17   Nat 2001 18   Nat 2001 19 5 4 Nat 2001 20 5 0 Nat 2001 21   Nat 2001 22 2 0 Stanley Pokorny  23  Nat 2002 1   Nat 2002 2  4 Nat 2002 3 3 3 Nat 2002 4 5  Nat 2002 5 3 0 Nat 2002 6 4 0 Nat 2002 7 1  Nat 2002 8 5 5 Nat 2002 9   Nat 2002 10 5  Nat 2002 11  0 Nat 2002 12  0 Nat 2002 13 3 3 Nat 2002 14 4 3 Nat 2002 15 4 0 Nat 2002 16  3 Nat 2002 17   Nat 2002 18  0 Nat 2002 19 4 4 Nat 2002 20 4 3 Nat 2002 21 3  Nat 2002 22 3 0 Nat 2002 23 5 0 Nat 2003 1   Nat 2003 2 4  Nat 2003 3 5  Nat 2003 4 2 0 Nat 2003 5 7 4 Nat 2003 6 3 0 Nat 2003 7 5 4 Nat 2003 8 7 5 Nat 2003 9 6 4 Nat 2003 10   Nat 2003 11   Nat 2003 12 6  Nat 2003 13 2 2 Nat 2003 14   Nat 2003 15 3 3 Nat 2003 16 4 4 Nat 2003 17 6 6 Nat 2003 18 3 3 Nat 2003 19   Nat 2003 20 4 0 Nat 2003 21 5 4 Nat 2003 22 4 4 Nat 2003 23   Nat 2003 24   Nat 2003 25 3 3 Nat 2003 26 7  Nat 2003 27 3 3 Nat 2003 28 6 6 Nat 2003 29 5 3  

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