To anticipate future breeding conditions in spatiotemporally dynamic environments, any proximate aspect of breeding is expected to be causally associated with environmental cues or constraints. Photoperiod, ambient temperature, and food are considered to be among the most important environmental determinants of breeding performance. However, the importance of these determinants in the expression of nest size has not previously been investigated experimentally in laboratory conditions. We report that captive Blue Tits (Cyanistes caeruleus) and Great Tits (Parus major) did not adjust the size of their cavity nest to experimental changes in photoperiod or ambient-temperature regimes. Results did not change when we controlled for first-egg date and clutch size. In addition, captive birds fed ad libitum did not build larger nests than free-ranging breeders in standardized nest boxes. Thus, we report no evidence that nest size per se is proximately influenced by commonly studied environmental factors like photoperiod, temperature, food abundance, or the level of investment in egg production—factors that can all pose energetic challenges to breeding birds. However, we do not exclude the possibility that, in natural conditions, energy- or resource-associated constraints might influence aspects of nest design other than nest size (e.g., rare nest components) or that constraints on nest building might only be revealed when birds need to build substantially larger nests (i.e. in larger cavities). Therefore, additional experimental work will be required to determine which are the proximate determinants of nest design in cavity breeders.
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Vol. 135 • No. 2