Individual variation in avian reproductive physiology does not reliably predict variation in laying date

Most animals reproduce seasonally. They time their reproduction in response to environmental cues, like increasing photoperiod and temperature, which are predictive for the time of high food availability. Although individuals of a population use the same cues, they vary in their onset of reproduction, with some animals reproducing consistently early or late. In avian research, timing of reproduction often refers to the laying date of the first egg, which is a key determinant of fitness. Experiments measuring temporal patterns of reproductive hormone concentrations or gonadal size under controlled conditions in response to a cue commonly assume that these proxies are indicative of the timing of egg laying. This assumption often remains untested, with few studies reporting both reproductive development and the onset of laying. We kept in total 144 pairs of great tits (Parus major) in separate climate-controlled aviaries over 4 years to correlate pre-breeding plasma luteinizing hormone (LH), prolactin (PRL) and gonadal growth with the timing of laying. Individuals varied consistently in hormone concentrations over spring, but this was not directly related to the timing of gonadal growth, nor with the laying date of the first egg. The timing of gonadal development in both sexes was similarly not correlated with the timing of laying. This demonstrates the female's ability to adjust the onset of laying to environmental conditions irrespective of substantial differences in pre-laying development. We conclude that stages of reproductive development are regulated by different cues, and therefore egg laying dates need to be studied to measure the influences of environmental cues on timing of seasonal reproduction.