| | 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, |
| | Principal | D. C Hahn, USGS Patuxent Wildlife Research Center: caldwell_hahn@usgs.gov; |
| | Investigators |
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