Resource competition among nestling red-winged blackbirds (agelaius phoeniceus)

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Glassey, Barb C.
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Red-winged blackbird ('Agelaius phoeniceus') females hatch their offspring asynchronously, creating mixed-aged broods comprising first-hatched "core" nestlings and later-hatched "marginal" offspring. Nestlings communicate their requirements to the parents using a combination of vocal and visual behaviours. Other studies of begging have focused solely on the need for food; here I incorporate thermal care as a resource. I begin by outlining the ontogeny of begging by red-winged blackbird nestlings using data collected from video-taped nests. My results indicate that parents use the collective begging efforts of the brood to assess both the thermal and nutritional requirements of the brood--the female parent responds to a weak collective effort by increasing nest attentiveness whereas a strong effect stimulates foraging. I compare the begging behaviour of core vs. marginal offspring at three phases of brood d6elopment, to determine whether developmental disparities influence the outcome of begging competitions. My results indicate that the outcome of begging competitions is primarily determined by size. Consequently, larger nestlings consistently receive more food. I develop a novel, non-surgical technique to mute nestlings temporarily in order to separate vocal from visual begging displays. My results indicate that food allocation is determined principally on the basis of visual displays, but that the foraging is regulated by the cumulative vocalizations of the brood. A secondary effect of the muting treatment is to reduce the length of time that nestlings beg, to which parents respond by increasing nest attentiveness. Finally, I compare parent-offspring interactions in unparasitised broods of red-winged blackbirds, to broods parasitised by the brown-headed cowbird ('Molothrus ater ') across the nestling period. Cowbird nestlings differ from host nestlings by maintaining a consist nt begging effort, and by begging for a lengthy period of time, particularly following the allocation of food. Host nestlings increase their begging efforts in response to the presence of the cowbird, but as they are unable to sustain the effort, parents do not increase foraging.