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Accession Number 5004034

Title Endocrine baseline values in songbirds and a cowbird-host comparison

Project Description Project Description EPA and DOI have made it a priority to investigate in wildlife models how

environmental contaminants may mimic vertebrate hormones and compromise endocrine functions

at several life states, including embryonic development, sexual differentiation, mating behavior,

and reproduction. One major component of endocrine disruptor research is the establishment of

baseline endocrine values at key life stages so that future contamination events can be

recognized. Songbirds are one group for which almost no baseline endocrine data exist, and the

broad geographical distribution of songbirds, their use of multiple habitats and food sources

makes them a useful group to monitor. We propose to measure levels of testosterone and

estradiol in eggs of 10 songbird species collected in the eastern, central, and western US.

Species have been selected to represent three geographical regions as well as both forest, edge,

and grassland habitats, and both insect and seed diets. One of the species we will study is the

brood parasite, the Brown-headed Cowbird, and we propose to compare relative hormone levels in

cowbirds and their hosts. Conservation biologists do not yet know all the factors that allow the

brown-headed cowbird to parasitize over 200 host species successfully across the U.S., but we

have identified a mechanism that may explain why they are so successful and may also identify

which species are most at risk. Ornithologists have long been aware anecdotally that cowbird

chicks are particularly aggressive and competitive at feeding time (Friedmann 1929, Nice 1937,

Payne 1977). However, until recently there was no quantitative behavioral study of competition

between cowbird and host nestlings similar to the many within-species studies that established

the importance of begging vigor and feeding hierarchy to chick survival (e.g. Hahn 1981, Smith and

Montgomerie 1991, Price and Ydenberg 1995). Now Lichtenstein and Sealy (1998) have used

video cameras at the nests of yellow warblers to document that brown-headed cowbird nestlings

were fed significantly more often than host chicks due to their greater begging vigor and more

frequent begging calls. Concurrently, Schwabl (1993, 1996, 1997) made a series of advances in

the laboratory that explain the neuroendocrine mechanism underlying begging vigor in nestlings.

He discovered that female canaries deposit increasing levels of testosterone in successive eggs in

a clutch, such that the fourth egg contained a dose level three times the level in the first egg.

Schwabl also manipulated testosterone levels and showed that level of testosterone is directly

related to three traits that fundamentally affect survival: the begging vigor, rate of growth, and

relative dominance as a juvenile (see OConnor 1984, Ricklefs 1994). We propose to examine

whether the mechanism Schwabl discovered is also the basis for the unusual competitiveness

observed in cowbird nestlings. Several other species of birds, both songbirds and others,

manipulate hormone levels systematically within a clutch (both increasing and decreasing) in the

same way that canaries do (e.g., red-winged blackbird,house sparrow, zebra finch, cattle

egrets,and roseate terns ; see Schwabl 1993, 1997, Schwabl et al. 1997, Lipar et al. 1999, French

et al., unpublished data). Similarly, a growing literature on mammals has illuminated the profound

physiological, morphological, and behavioral effects of exposure in utero to different hormonal

regimes (e.g. vom Saal 1989, Zielinski et al. 1991, 1992). Thus, it appears that the phenomenon of

variable embryonic exposure to hormones with resulting post-hatch (or post-natal) differences

occurs widely in vertebrates. Our proposed study is the first to examine whether such differences

in embryonic hormone levels are exploited between species. We hypothesize that the brood

parasitic cowbird uses this hormonal mechanism to give its offspring behavioral dominance and

thus a feeding advantage over the

Keywords brood parasite, contaminant, cowbird, development, endocrine disruptor, songbird,



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