Variation in Wood Thrush Nest Dimensions and Constructio

We collected 48 nests from radio-marked and nonradio- marked female Wood Thrushes (Hylocichla mustelina) at the Piedmont National Wildlife Refuge in central Georgia and conducted statistical analyses to determine the sources of variation in nest size. Each nest measurement had a large coefficient of variation, but we found trends in six of eight nest measurements related to date, nest number, female occupant, or nest success. Regression analyses showed that cup depth, nest wall width, and exterior nest height decreased during the season, while cup width increased (0.14 < R 2 <0.24). A repeated-measures linear model, comprised of study plot, nest number, and occupant effects, adequately described much of the variation in the multi-broodedr,a dio-markedf emales' (n = 24 nests) nest size (0.55 < R 2 < 0.84). Cup depth, cup width, exterior nest height, and nest weight showed differences among nest attempt number, while weight and exterior nest height showed differences related to individual females. We also found evidence that nestlings in successful nests apparently enlarged the nest cup. Very little is known about short-term variation in nest size and shape, but our results indicate that female Wood Thrushes may change the size of their nests for various reasons during the breeding season.