S05.2: Energetic or nutritional constraints on the timing of reproduction

Division of Evolutionary and Environmental Biology, University of Glasgow, Glasgow G12 8QQ, UK, e-mail rn6f@udcf.gla.ac.uk

Timing of reproduction is believed to be affected by food availability. Supplementary feeding usually results in an advancement of laying. Also within individual changes in timing between years can be linked to changes in food availability. However, the proximate mechanism is not yet clear. Food may affect the timing of reproduction through the availability of energy or nutrients, or through information about the future environment. Here I report on a series of experiments to disentangle energetic and nutritional limitations on the timing of reproduction in a population of Great Tits Parus major. First, energetic demands independent of nutritional effects can be affected through manipulating the thermal environment that roosting females experience at the beginning of the breeding season. Although this manipulation affected egg production it did not changed the timing of egg laying. Second, the supply of some key nutrients, such as proteins, can be manipulated through supplementary feeding experiments. There was no effect of protein availability on timing of reproduction. Thus these results for Great Tits do not support the physiological constraint hypothesis that suggests a limitation of either energy or protein quality acts as the key proximate factor of timing of reproduction. This leaves the possibility open that the food early in the season acts as a cue for future feeding conditions.

Note: Full paper not submitted