Correlates and possible causes of fluctuations in a local wood thrush population
Date
2005
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Publisher
University of Delaware
Abstract
Declining abundance of many songbird species has prompted numerous and varied sampling procedures to determine present abundances and possible explanations for the decline. In this two-part study, I first tested if Wood Thrush (Hylocichla mustelina) abundance in an intensively studied 15-ha plot in Delaware (UDW) tracked long-term abundance trends and year-to-year changes from two widespread monitoring programs: the Breeding Bird Census (BBC) and the Breeding Bird Survey (BBS). Wood Thrush abundance was analyzed at 8 BBC sites over two periods. Four of the 12 tested abundance trends significantly increased or decreased at their particular site, but only three of those trends were similar to abundance trends at UDW. Due to the geographic scale of the data and the analytical method used, any annual fluctuation in Wood Thrush abundance at the BBS scale occurred was damped. However, a period of increasing abundance at UDW was correlated with a leveling off of the BBS trend. Abundance at UDW did not track changes at other local sites during the study period. Changes in Wood Thrush abundance at UDW, however, did correlate with a change in abundance in the BBS data at the regional level. While few connections were found between abundance at the local site (UDW) and in the broader data sets, both types of data have value. Extensive monitoring programs are important for identifying large scale trends in bird abundances and for providing perspective about the generality of those trends. At the same time, multiple local studies may indicate average population trends on a broader scale, but more importantly, they provide a basis for understanding causes of observed trends through collection of detailed demographic data. ☐ In part two, I investigated possible causes of fluctuations in abundance at UDW. Tests of relationships of breeding variables (measures of nesting success, parasitism and predation rate) and weather variables with Wood Thrush abundance at UDW revealed the possible causes to be multiple and complex. All of the breeding variables considered were significantly related to abundance in the following year (t) with number of successful nests in t-1 being the best single variable. However, multiple regressions revealed the importance of weather factors, such as the El Niño/Southern Oscillation index, when forecasting future abundance. The number of successful nests in year t was significantly correlated with number of breeders in year t+1 and young returning the next year. The results support the sometimes contentious argument that factors on the breeding ground are important to the future abundance of migratory songbirds but do not lead to conclusions regarding the relative importance of breeding, stopover, and wintering ground factors.