Experimental manipulation of nocturnal nest cavity temperature in wild blue tits

Advances in the timing of reproduction in temperate species are some of the most well documented biotic responses to increasing global temperatures. However, the magnitude and rate of these advances in timing are not equal across all taxonomic groups. These differences can lead to disruption of interspecific relationships if species respond differently to temperature changes. Understanding the relationship between temperature and phenology is a key step in predicting future population trends for species living in seasonal environments. However, experimentally manipulating temperature in the wild is logistically challenging and has consequently rarely been attempted. In this study we experimentally test whether in-nest temperatures in early spring act as a cue for breeding phenology in a population of wild blue tits ( Cyanistes caeruleus ). We split nests into three treatments; heated, cooled, and control. In-nest temperature in the heated and cooled boxes was manipulated by an average of ± 0.6 °C from control temperatures using heating devices and ice packs respectively. We assessed the impact of our experimental manipulation on box occupancy and reproductive timing. We found trends towards earlier phenology in heated nest boxes in addition to a higher occupancy rate in cooled boxes, however neither of these trends was found to be statistically significant. Our ability to distinguish statistical signals was hampered by unexpectedly low occupancy rates across all experimental treatments. Based on the results we cannot say if nocturnal in-nest temperature is an important cue for nest box choice or the timing of laying.