Intake Rates and the Timing of Crossbill Reproduction

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Abstract
I measured the foraging rates of breeding and nonbreeding populations of White-winged Crossbills (Loxia leucoptera) and Red Crossbills (L. curvirostra) throughout the year in the northeastern United States and adjacent Canada. I compared actual food intake rates (80 samples from 26 populations) to those predicted for five hypotheses that pertain to the timing of reproduction. To form eggs, crossbills required greater intake rates to meet energy demands than to meet protein requirements. Intake rates predicted to be necessary for the nestling stage (first five days after eggs hatch) were greater than those required for egg formation. Nearly all breeding and nonbreeding populations had intake rates adequate for egg formation. Thus, the ability to form eggs may only infrequently limit nesting. Crossbills nested when intake was sufficient for the nestling stage, but ceased nesting if intake rates were declining so that they would be inadequate for the nestling stage. When intake rates were less than those required for the nestling stage, crossbills nested if intake rates were increasing and would be sufficient for the nestling stage. The timing of crossbill reproduction was most consistent with the hypothesis that crossbills bred whenever intake was sufficient for egg formation and would be sufficient three weeks later during the nestling stage. That is, crossbills bred as if they were sensitive to both net intake and rates of change in intake. Intake rates may provide a proximate cue for crossbills to nest, but other factors such as maturing cone crops and changing day length probably affect breeding readiness.