Date of Award:

5-1-1980

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Life Sciences:Biology

Committee Chair(s)

Keith L. Dixon

Committee

Keith L. Dixon

Committee

James T. Bowman

Committee

Barrie K. Gilbert

Committee

Ivan G. Palmblad

Committee

Allen W. Stokes

Abstract

Free-living American Avocets and Black-necked Stilts were studied during 1977 and 1978 in northern Utah. The broad objectives of the study were to: (1) describe the breeding biology of the birds and identify aspects related to predation pressure, and (2) examine the antipredator behavior of parent birds and their young. Nest success was approximately 30% at 75 avocet and 18 stilt nests studied. Fifty-seven percent of nest failures were due to predation on eggs, and another 11% were due to predation on incubating adults. Juvenal-plumaged broods contained fewer chicks than downy broods, indicating partial brood loss, which was probably due to predation. Nest dispersion varied from relatively solitary to loose colonies of 3 to 50 nests. Possible functions of coloniality in recurvirostrids are discussed. Five categories of adult antipredator displays were recognized: Dive-bombing, diversionary aerial displays, Crouch-run, terrestrial wing displays, and incubation-like displays. Individual avocets and stilts exhibited mobbing and distraction behavior only when they had eggs or chicks. Group displays of up to about 50 birds may result from social attraction, but there is no cooperation between individuals. The occurrence of mobbing behavior is examined in North American shorebirds. Mobbing is exhibited only by the larger species, with the size threshold being about 100-120 grams body weight. Possible determinants of this pattern are discussed. Evidence is presented that avocets and stilts discriminate a large number of species. Responses to different species seem to be adjusted to the degree of threat posed to eggs and young, and to the adults themselves. Avocets and stilts approached more closely and displayed more rapidly to a human, and mobbed a crow model more vigorously, as incubation progressed. Response strength peaked near hatching. This increase in display intensity and risk is discussed from an evolutionary perspective. Males, especially in stilts, took the leading role in defending eggs and chicks. Analysis of sexual role specialization suggests that the reproductive strategies of the sexes tend to equalize male and female parental investment over a breeding season. Relatively greater investment in a clutch of eggs by female stilts may have resulted in stronger sexual selection in stilts than in avocets; this could have favored relatively larger males with a stronger defense role. Young avocets and stilts crouch or hide when danger threatens, and then rely on the antipredator behavior of their parents. After their second week, avocets had a greater tendency than stilts to run when approached. This may be related to the greater tendency of avocet chicks to forage in open areas, and to their less cryptic juvenal plumage, both of which render hiding less effective. Chicks dive when pursued in the water; propulsion is with the wings, so that underwater swimming ability was correlated with wing size and age. Avocets were more likely to dive than stilts. Avocet chicks often feed and rest in open areas where they are vulnerable to diurnal visual predators. Two mimetic systems may reduce predation on them. Resemblances of avocets during their first 2 weeks to adult Wilson's Phalaropes may cause some predators to perceive chicks as unsuitable prey so that they fail to attack them. This benefit would result from predators' experience with the difficult-to-capture phalaropes. During their third and fourth weeks avocets acquire a juvenal plumage that is adult-like in pattern; together with adult-like behaviors, this probably makes it difficult for distant predators to distinguish flightless chicks from volant adults. This form of mimicry, characterized by accelerated (and deceptive) acquisition of adult or adult-like features, is designated adult automimicry.

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