Artificial light at night, in interaction with spring temperature, modulates timing of reproduction in a passerine bird

The ecological impact of artificial light at night (ALAN) on phenological
events such as reproductive timing is increasingly recognized. In birds,
previous experiments under controlled conditions showed that ALAN strongly
advances gonadal growth, but effects on egg-laying date are less clear. In
particular, effects of ALAN on timing of egg-laying are found to be
year-dependent, suggesting an interaction with climatic conditions such as
spring temperature, which is known have strong effects on the phenology of
avian breeding. Thus, we hypothesized that ALAN and temperature interact
to regulate timing of reproduction in wild birds. Field studies have
suggested that sources of ALAN rich in short wavelengths can lead to
stronger advances in egg-laying date. We therefore tested this hypothesis
in the great tit (Parus major), using a replicated experimental setup
where eight previously unlit forest transects were illuminated with either
white, green, or red LED light, or left dark as controls. We measured
timing of egg-laying for 619 breeding events spread over six consecutive
years and obtained temperature data for all sites and years. We detected
overall significantly earlier egg-laying dates in the white and green
light versus the dark treatment, and similar trends for red light.
However, there was a strong inter-annual variability in mean egg-laying
dates in all treatments, which was explained by spring temperature. We did
not detect any fitness consequence of the changed timing of egg-laying due
to ALAN, which suggests that advancing reproduction in response to ALAN
might be adaptive.