DO GREAT TITS ADJUST HATCHING SPREAD, EGG SIZE AND OFFSPRING SEX-RATIO TO CHANGES IN CLUTCH SIZE

1. Many hole-nesting birds lay smaller clutches in small than in large nestboxes. Breeding success may be limited by cavity space in small boxes, but by food availability in large boxes. If so, birds nesting in small boxes may adopt a 'brood survival strategy' (synchronous hatching and large final eggs), whereas those nesting in large boxes may adopt a 'brood reduction strategy' (asynchronous hatching and small final eggs). 2. We studied initial (n = 203) and repeat nestings (n = 91) of great tits (Parus major L.) in nestboxes with small (70 cm2) and large (180 cm2) basal areas, in two study plots in Norway during 1984-89. Renesting was induced by clutch removal. In 1986, birds laying their first clutches in small boxes were experimentally made to renest in large boxes, and vice versa. 3. A clutch size adjustment to nestbox size was verified, with a difference between small and large boxes of one to two eggs. However, contrary to expectation, no significant differences were found in either hatching spread, or in the relative size of the final egg laid. 4. Alternative explanations for variation in hatching spread and egg size are discussed. Limitation, by energetic constraints, on the start of incubation and on egg formation may be of great importance. 5. From assumptions made about the costs of rearing male and female offspring, and about the fitness distribution of the young in relation to their body weight, we expected that the sex ratio found for hatchlings would be higher in favour of males in small than in large nestboxes. An insignificant tendency in this direction was found.