Does nighttime light change when great tits lay their eggs?

Details

The ecological impact of artificial light at night (ALAN) on seasonal events such as reproductive timing is increasingly recognized. Previous controlled experiments in birds showed that ALAN strongly advances gonadal growth, but its effects on egg-laying dates are less clear. Effects on laying dates have been found to depend on the year, suggesting an interaction with climatic conditions such as spring temperature, which strongly affects the timing of bird breeding. The researchers hypothesized that ALAN and temperature interact to regulate reproductive timing in wild birds. Field studies have also suggested that ALAN sources rich in short wavelengths can produce stronger advances in egg-laying dates.

The researchers tested their hypothesis in great tits (Parus major) using a replicated field experiment. Eight previously unlit forest transects were illuminated with white, green or red LED light, or left dark as controls. They measured egg-laying timing for 619 breeding events over six consecutive years and obtained temperature data for every site and year.

Overall, egg-laying dates were significantly earlier under white and green light than in the dark treatment, with similar trends under red light. Mean egg-laying dates varied strongly from year to year in all treatments, and spring temperature explained that variation. The researchers detected no fitness consequence of the change in egg-laying timing due to ALAN, suggesting that earlier reproduction in response to ALAN might be adaptive.