Résumé
In seasonal environments, organisms use biotic and abiotic cues to time
various biological processes that are crucial for growth, survival and
reproductive success. Photoperiod is the best-known cue used to regulate
gonadal development, migration and moult of many animal species. In birds,
the relationship between photoperiod and gonadal development is clearly
established, but we have little understanding on whether photoperiod also
regulates actual timing of egg laying under natural conditions.
Elucidating the link between photoperiod and timing of breeding is however
key to understand whether an evolutionary change in sensitivity to
photoperiod is a possible mechanism through which organisms could adjust
their seasonal timing in response to climate warming. Here, we
investigated the causal relationship between photoperiod, gonadal growth
and laying date in wild female great tits. We experimentally increased the
photoperiod perceived by the birds in spring by clipping head feathers,
and we subsequently monitored gonadal development in the lab and egg
laying dates in the wild. We show that our manipulation increased the
photoperiod perceived by the birds to a level that approximately
corresponds to an advancement of ten calendar days. This increase in
perceived photoperiod led to an acceleration of gonadal development, but
not to an advancement of egg laying dates. Our results indicate that
photoperiod sensitivity is not constraining the advancement of laying date
under current environmental conditions and suggest that evolution of
sensitivity to other supplementary cues is necessary to advance
reproduction under global warming.