Nest Success of Yellow Warblers in Willow Habitats: The Role of Surface Water and Snakes

Abstract

Yellow Warblers (Dendroica petechia) are migratory songbirds found in high abundance in the tall willow (Salix boothii) habitats of Grand Teton National Park (GTNP). Willows are found in wet soils with high water tables and varying densities of exposed surface water. Dense surface area of water leads to thick, well foliated, continuous patches of S. boothii, which is the favored nesting habitat of Yellow Warblers. This water, however, also supports the favored prey base of the wandering garter snake (Thamnophis elegans vagrans), which has been known to prey on songbird nest contents when the opportunity arises. We hypothesized that nest territories containing high densities of surface water would also attract garter snakes, and increase the probability of nest failures. We found and monitored 28 Yellow Warbler nests, recorded their locations and fates (success or failure) and measured the density of surface water within each nest territory. We analyzed nest success with the logistic-exposure method coupled with comparisons of models with and without water density as an explanatory variable. Information theoretic model comparison consistently supported models with water density over those without. A significant correlation was found between water density within a nest territory and that nest's daily survival probability. The estimate of this effect was -0.049 (a logistic model parameter) with a standard error of 0.019. Expressed alternatively, each 5-meter per territory increase in waterway density decreases the odds ratio of nest survival by 21%. While water density provides a trade off between nesting habitat and predation pressure, other predation causes and temporal water density variation likely contribute to overall warbler productivity in important ways as well.