The Influence of Spatio-Temporal Variation in Food Availability and Nest-Predation Risk on Clutch-Size Decisions

Abstract

The role of both temporal and spatial variability in nest predation and food availability in influencing birds' decisions about clutch size has not been studied. I examine patterns in nest-predation risk and food availability across a range of severity of wildfire to investigate their relationship to clutch size in the Dark-eyed Junco (Junco hyemalis), which breeds commonly in burned forests. Spatial variation in burn severity led to lower risk of nest predation in patches of intermediate severity in the first two years after the fire, while food availability was inversely related to burn severity during only the first year post-fire. Spatial variation in risk of nest predation explained variation in clutch size only during the first year post-fire when food was limited, consistent with parents investing less in risky nest locations during periods of nutritional stress. Nest-predation risk increased seasonally during both year 1 and year 2 post-fire. Clutch size rose dramatically over the first breeding season post-fire, paralleling a unique seasonal increase in food availability in year 1, consistent with juncos tracking temporal variation in food availability by investing more in eggs. Results are consistent with parents balancing spatio-temporal variation in resource limitation with predation risk in their investment in eggs. Disagreement between existing studies as to the relative importance of food and nest predation further highlights the need for carefully designed experimental approaches that integrate explanations for both temporal and spatial trends in sources of selection likely to shape the evolution of clutch-size decisions.