Paul Cuchot, Timothée Bonnet, Olivier Dehorter, Pierre-Yves Henry, Céline Teplitsky

Phenological adjustment is the first line of adaptive response of vertebrates when ancestral seasonality is disrupted by climate change. The prevailing response is to reproduce earlier in warmer springs, but habitat changes, such as conversion of ancestral (pre-human) habitats into cities and agricultural lands, are expected to affect phenological plasticity, for example due to loss of reliability of environmental cues used by organisms to time reproduction. Relying on two-decade, country level capture-based monitoring of common songbirds’ reproduction, we investigated how the conversion of forest habitat into agricultural lands, and into artificialized surfaces affected the deviation of average phenology and plasticity to local temperature from its presumed ancestral state, i.e., in non-urbanized forests. For this, we built a hierarchical model that simultaneously estimated fledging phenology and its response to spring temperatures based on the changes in the proportion of juveniles captured over the breeding season. Both species fledge earlier in warmer sites (blue tit: 2.94 days/°C, great tit: 3.83 days/°C), in warmer springs (blue tit: 2.49 days/°C, great tit: 2.75 days/°C) and in most urbanized habitats (4 days for blue tit and 2 days for great tit). The reaction norm of fledging phenology to spring temperature varied across sites in both species, but this variation was explained by ancestral habitat only in the deciduous forest specialist, the blue tit. In this species, the responses to spring temperature were shallower in agricultural landscapes. The ancestral reaction norm is preserved in the habitat-generalist species (great tit), as well as along the urbanization gradient and was correlated to mean fledgling phenology (i.e., steeper response in later sites). The effects of habitat change on ancestral adaptive reaction norms provide another way through which combined environmental degradations may threaten populations persistence, to an extent depending on species and changes in their prey phenology and abundance.