UBC Theses and Dissertations
The control and organization of parental feeding and its relationships to the food supply for the glaucous winged gull, Larus glaucescens Henderson, Bryan Alexander
From work previously done by John Ward, it was shown that the productivity on Cleland Island was greater than that on Mandarte Island. The mean weight and fledging rate for all brood sizes from one to six, was higher on Cleland Island. It was thought that an analysis of the parental feeding system and a comparison of the feeding biology would provide a partial explanation for the difference in productivity between Mandarte and Cleland Islands. The main objectives of this comparative study were to distinguish between the ration requested by the chicks, i.e., parental feeding system, and the ration received by the chicks, i.e., how the feeding biology modifies the ration. Both the quality and quantity of foods given to the chicks differ between the two colonies. Mandarte adults eat a greater variety of foods than the Cleland adults. The observations of courtship feeding and the collections of stomach contents, show that Mandarte adults eat garbage, intertidal organisms, and fish, whereas Cleland gulls eat fish and intertidal organisms. If courtship feedings do significantly contribute to the requirements for egg production, the superior quality of food given to the Cleland females may be correlated with the higher hatching success on Cleland. In addition to the effect on the hatching success, the quality of food may also influence the fledging success. On both Mandarte and Cleland, the adult's food differs from the chick's food. Initially, the Mandarte chick predominantly receives fish but as the chick stage progresses, less fish and more garbage is given to the chicks. This deterioration in the quality of the foods may be correlated with the low fledging success of chicks raised late in the season. Cleland' s chicks receive only fish throughout the parental period so that chick mortality could not be associated with a change in the quality of the food. The lower fledging rate recorded for Mandarte chicks may also be associated with the smaller quantities of food given to the chicks. Although no significant difference exists, the weight of food given to the Cleland chicks exceeds that given to the Mandarte chicks. Some of the differences of quality and quantity of food may be explained by the temporal and spatial patterns of food availability. The parental feeding frequency is thought to be dependent on the needs of the chicks but is modified by prey availability. Parental feeding frequency reaches a peak in the early morning, afternoon, and evening on Mandarte but is relatively constant until just before dusk on Cleland Island. The sandlance, obtainable in the immediate vicinity of Cleland, is available throughout the day; the herring, rarely caught near Mandarte Island, has a periodic availability during the day. A Mandarte adult, seeking an alternate food during a period of low herring availability, may have to make a long trip to some garbage dump or intertidal zone. Not only the patterns of feeding activity but also the total feeding activity differ between the colonies. The frequency of feeding after a trip is similar for both colonies but the foraging frequency is significantly greater for Mandarte parents. This may mean that the Mandarte parent presents fewer feedings than Cleland adults from the catch of one foraging trip. Although parents from both colonies increased the foraging and feeding frequency when chicks were added to normal broods, neither Mandarte nor Cleland adults increased the feeding frequency in direct proportion to the brood size. The adjustments made in the foraging and feeding frequency are made possible through a series of signals constituting the parental feeding control system. The control of parental feeding is achieved through the use of two signals, calling and pecking. A functional relationship exists between the hunger level and the call frequency, peck frequency, and call intensity. For any hunger level the frequency and amplitude of the call change with age but the pecking response does not change with the chick's age. From the field observations of feeding, it is evident that a particular cumlative pecking value and call frequency elicits a feeding and the change in call amplitude associated with satiation is instrumental in terminating a feeding. The foraging frequency, proportional to the brood size, may be modulated by the cumulative calling of the chicks. A simple experiment did show that the activity of the parent is influenced by the hunger of the brood; this fact was demonstrated by an experiment in which I satiated the gull chicks thus increasing the time that the parents remained on the territory. In summary, the feeding frequency is adjusted by the pecking and calling frequency while the foraging frequency of the parents is adjusted by the cumulative calling of the chicks. A feeding is initiated by a specific calling frequency and cumulative pecking value and terminated when the food is consumed or when the call amplitude indicates satiation. The ability of the parents to adjust their feeding activity raises some interesting questions. The relationship of both the feeding biology and parental feeding system to the productivity of an island is discussed from a comparative viewpoint. The importance of the feeding system and other factors is discussed in relation to the evolution of clutch size.
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