ENERGETIC LIMITATION IN THE EGG-LAYING PERIOD OF GREAT TITS

Individual birds of many species laying earlier than the population average produce more offspring which survive to enter the subsequent breeding population. A shortage of resources early in the season is commonly supposed to force some females to delay their egg production beyond the time which would be best for offspring production. We sought to test this hypothesis by experimentally manipulating the energy expenses of female great tits, Parus major, during the pre-laying and egg-laying period. By heating and cooling their roosting cavities (nestboxes) we decreased and increased, respectively, their energetic requirements for maintenance. The 'energy limitation hypothesis' predicts that our temperature manipulation would result in an earlier start of egg laying in heated boxes and a delay in cooled boxes. The temperature reduction in cooled nestboxes compared with heated nestboxes of about 1.5-degrees-C throughout the day seemed to affect the birds as they changed their nestbuilding behaviour and invested less in egg production (reflected in a reduction of egg volume of about 14 %). However, we could find no evidence for a later laying date of females breeding in cooled nestboxes as compared with females in heated nestboxes.