Experimental manipulation of Blue Tit nest height does not support the thermoregulation hypothesis

Abstract

Birds show immense variation in nest sizes within species. At least siz different hypotheses have been forwarded to explain intraspecific variation in nest size in cavity nesting species, but very few of those hypotheses have been tested experimentally. In our study, when nestlings were 2 days old, we manipulated the height of 182 Blue Tit (Cyanistes caeruleus) nests to either 5 cm or 11 cm while standardizing their ectoparasite load and genetic and maternal background. In line with the the hypothesis that larger nests to show increased growth of nestlings compared to shallow nests, or to imrove female somatic condition. We found that the nest height manipulation affected the tail length of 16-day old nestlings, but did not affect any other morphometric measure (tarsus length, body mass, head size and wing length). In addition, nest height manipulation had no impact on nestling survival and did not affect female body condition. Our results do not therefore provide strong support for the thermoregulatory hypothesis and suggest regional differences in the relationship between nest size and reroductive success.