Does loss of mass during breeding correlate with reproductive success? A study on Blue Tits Parus caeruleus

Substantial loss of mass of female Blue Tits Parus cueruleus during breeding is commonly explained by three not mutually exclusive explanations: shrinking of gonadal tissues, cost of reproduction and adaptation to save energy during flight. This study showed that loss of body mass was inversely correlated with reproductive success of individual females. Female tarsus length and timing of breeding correlated with loss of body mass, whereas ectoparasite loading had no significant effect on body mass. Loss of body mass during the rearing of young could be a useful measure of cost of reproduction by accounting for individual variation in female quality.