The costs of egg production and incubation in great tits (Parus major)

The costs of egg production and incubation may have a crucial effect on avi an reproductive decisions, such as clutch size and the timing of reproducti on. We carried out a brood-size enlargement experiment on the great tit (Pa rus major), in which the birds had to lay and incubate extra eggs (full cos ts), only incubate extra eggs (free eggs) or did not pay an) extra cost (fr ee chicks) in obtaining a larger brood. We used female fitness (half the re cruits produced plus female survival) as a fitness measure because it is th e female which pays the costs of egg production and incubation, and because clutch size is under female control. Female fitness decreased with increas ing costs (fitness of free chicks females is higher than that of free eggs females which is higher than that of full costs females). These fitness dif ferences were due to differences in female survival rather than in the numb er of recruits produced. This is the first time that the costs of egg produ ction and incubation have been estimated using such a complete fitness meas ure, including, as our measure does, the local survival to the following ye ar of both the female and her offspring. Our results emphasize that reprodu ctive decisions cannot be understood without taking egg production and incu bation costs into account.