Dataset: The preference and costs of sleeping under light at night in forest and urban great tits

We studied 35 (17 forest and 18 urban) male great tits. The birds were caught in the wild (see electronic supplementary material, figure S1, for a map of catching locations) and transported to the Netherlands Institute of Ecology (NIOO-KNAW) Wageningen, the Netherlands. Birds were housed in individual cages (90 × 50 × 40 m), initially spread over three adjacent rooms. Each cage had two light sources, one for day and one for night. The front of each cage was covered with a wooden board to exclude any external light from the outside and neighbouring cages. Birds were provided with food and water ad libitum. Over the course of the first experiment (experiment 1), which was from 9 October until 28 October (21 days), birds had a constant photoperiod of 10.15 h light : 13.45 h dark, and for the second experiment (experiment 2), which was from 6 November until 17 December (42 days) birds had a constant photoperiod of 8.15 h light : 15.45 h dark. This was the average of natural daytime and night-time hours throughout the dates that the experiments were carried out.