Quantification of the effects of hatchling number on nest weight as a proxy for nest size when measured post-fledging

Abstract The intraspecific variability of nest construction behavior in birds underlies environmental—as well as builder-dependent drivers. To draw correct conclusions about the bird’s nest construction investment, an accurate assessment of the nest size or weight is crucial. The weight of a nest depends on the material collected by the builder but may still increase over time with the presence of chicks, for instance if fecal material accumulates. Here, we provide evidence from a nesting box population of European Pied Flycatchers ( Ficedula hypoleuca ) that nest weight was positively correlated to hatchling number when weighed post-fledging. We use a novel approach of sorting, washing, and sieving the nesting material to reduce the effects of the chick-rearing phase on nest weight. Finally, we compare nest height and nest bottom thickness as indirect measurements of nest size to nest weight. Both nest height and nest bottom thickness became significantly smaller throughout the course of breeding and chick-rearing, which is why these parameters should be measured right after nest completion. While neither pre-incubation nest height nor nest weight (post-sieving) was related to number of eggs or number of hatchlings, we found a significant positive correlation between nest weight before sieving and number of hatchlings, showing the impact of chick presence on nest weight. Based on our data, we want to highlight the importance of an accurate and well-timed assessment of nest size and weight to facilitate an accurate characterization of the investment of birds into nest construction.