Seasonal trends in the temporal plasticity of breeding in blue tits and great tits in the Loch Lomond area

Birds commonly increase their fitness by synchronising the emergence of their chicks with the peak in resource abundance in the environment. Climate change is driving earlier laying in many bird species, but within one season individuals may subsequently show additional plasticity. Presumably, birds benefit from being able to adjust the timing of their breeding activities according to environmental conditions. This plasticity in the timing of breeding is illustrated in blue tits (Cyanistes caeruleus) and great tits (Parus major) in the Loch Lomond area, Scotland. Birds use several mechanisms to fine-tune timing of breeding once laying has commenced. These include delays in clutch completion (laying gaps), modulating clutch size, and extending incubation periods beyond population average. We found a seasonal trend in these three mechanisms, with early breeding birds displaying larger laying gaps and extended incubation, and laying larger clutches, compared to later birds. Overall, we found increasing synchronicity with each breeding event across the population. No significant effects of delays in clutch completion and extended incubation period were found on hatching success or nestling weight. Whether the plasticity in the breeding process of tits arises due to a constraint (e.g. energy or nutrients), or a cue received from the environment is uncertain, but we need to improve our understanding of this plasticity in order to better predict the potential effects of climate change on breeding birds.