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Long-term demographic changes in a Bicknell’s Thrush population

Principal Investigator(s):

John D. Lloyd, PhD and Jason M. Hill, PhD

Institution:

Vermont Center for Ecostudies

Project Term:

2016 - 2017

Bicknell’s Thrush is a species of the highest conservation concern and is the target of ongoing, multi-national conservation planning under the auspices of the International Bicknell’s Thrush Conservation Group. Although many aspects of the ecology of this species are well-understood, numerous information gaps persist that hinder the design and implementation of effective conservation practices and policies. In particular, we lack a clear understanding of the factors that limit population growth of Bicknell’s Thrush, which in turn limits our ability to target conservation actions efficiently. In this study, we directly addressed this problem by testing a series of hypotheses about causes of variation in Bicknell’s Thrush apparent survival. We used banding data from atop Mt. Mansfield, Vermont (2001-2015), and compared Cormack-Jolly-Seber models using an information theoretical approach and a four-step model-selection procedure. Our 15 year dataset included unique capture histories for 178 Bicknell’s Thrush, most of whom (59%) were captured in only one year of our study. Bicknell’s Thrush survival and recapture probability did not vary between sexes or age groups. Apparent survival averaged 0.61 between 2001 and 2015, from a low of 0.51 (95% CI: 0.35, 0.68) to a high of 0.71 (95% CI: 0.48, 0.86). Contrary to our hypotheses, apparent survival was higher after a mast year (0.71, 95% CI: 0.58, 0.81) then after a non-mast year (0.50, 95% CI: 0.41, 0.60). Balsam fir cone production in Eastern Northern America is partially driven by warm growing seasons in the year prior to masting. These weather conditions may also be favorable to Bicknell’s Thrush (e.g., by increasing insect abundance), with the result being higher apparent survival. Apparent survival was negatively affected by drought conditions on the wintering grounds (SOI β=-0.32, 95% CI: -0.72, 0.08) which are known to influence insect biomass. Our results suggest that a suite of weather conditions influence Bicknell’s Thrush apparent survival, given that an index of weather (SOI) was selected over rain data. We hypothesized that tropical storms create favorable habitat disturbance on the wintering grounds, and the number of tropical storms and hurricanes passing near Hispaniola (July through December) was positively related to apparent survival (β=0.30, 95% CI: -0.11, 0.71). Covariates presenting deforestation levels and rainfall on the wintering grounds and Swainson’s Thrush (a potential competitor) relative abundance on the breeding grounds were not selected as informative. The results of this research have provided insight into the relative importance of factors operating during the winter, including rainfall-driven variation in winter habitat quality and deforestation, and during the breeding season, including competition with an invading competitor and annual cycles of nest predation.

Project Report: Long-term demographic changes in a Bicknell’s Thrush population

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