The environmental predictors of spatio-temporal variation in the breeding phenology of a passerine bird

Author(s): Shutt, J.D., Cabello, I.B., Keogan, K., Leech, D.I., Samplonius, J.M., Whittle, L., Burgess, M.D. & Phillimore, A.B.

Published: August 2019   Pages: 9pp

Journal: Proceedings of the Royal Society B Volume: 286

Article No.: 20190952

Digital Identifier No. (DOI): 10.1098/rspb.2019.0952

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Warmer springs have seen many of our birds breeding earlier, but scientists still do not know how they actually decide when to start building nests and laying eggs. Do they respond directly to temperature or do they respond to other aspects of their environment that change in spring like days getting longer, trees coming into leaf or more food becoming available? A team of biologists from the University of Edinburgh in collaboration with the RSPB, BTO and the Woodland Trust, set out to answer this question for Blue Tits, collecting information from 40 sites in Scotland over five years.

The team found that temperature does play a role, but that night-time temperature is particularly important. This could show that cold weather constrains when Blue Tits are able to start the very energetically costly processes of building nests and laying eggs. On top of this they found that birds lay earlier when birch trees come into leaf earlier and this is some of the first evidence that birds are using trees as a cue for timing their breeding. That birch trees are used more than other tree species in the landscape may be due to their comparably early leafing.

As a follow up step, the authors wanted to check whether their findings could be transferred to a much larger geographic scale and longer time-period. To do this they used observations from two citizen science projects that focus on phenology (the timing of biological events) in the UK, the Woodland Trust’s Nature’s Calendar and the BTO’s Nest Record Scheme. Encouragingly they found very similar effects of temperature and birch leafing in these larger data sets, giving confidence in the findings. Working out what information a bird uses to time breeding is key to us accurately predicting how their timing may change under future conditions and what effect this will have on them.


Establishing the cues or constraints that influence avian timing of breeding isthe key to accurate prediction of future phenology. This study aims to identify the aspects of the environment that predict the timing of two measures ofbreeding phenology (nest initiation and egg laying date) in an insectivorous woodland passerine, the blue tit (Cyanistes caeruleus). We analyse data collected from a 220 km, 40-site transect over 3 years and consider spring temperatures, tree leafing phenology, invertebrate availability and photoperiod as predictors of breeding phenology. We find that mean nighttime temperature in early spring is the strongest predictor of both nest initiationand lay date and suggest this finding is most consistent with temperature acting as a constraint on breeding activity. Birch budburst phenology significantly predicts lay date additionally to temperature, either as a direct cue o rindirectly via a correlated variable. We use cross-validation to show that our model accurately predicts lay date in two further years and find that similar variables predict lay date well across the UK national Nest Record Scheme. This work refines our understanding of the principal factors influencing the timing of tit reproductive phenology and suggests that temperature may have both a direct and indirect effect.
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