Costs and Benefits of Hatching Asynchrony in Blue Tits Parus caeruleus

Abstract

We studied the significance of hatching asynchrony in blue tits Parus caeruleus during a 4-year period at Oslo, Norway, by comparing breeding success of broods manipulated to hatch over a shorter or longer period than average. Nestling mortality was high and mainly caused by starvation. Mean body mass of fledglings on day 14 was significantly higher for asynchronous than for synchronous broods. No significant difference was found in the mean number of young fledged between the treatment groups, nor in the number of offspring recruited into the local breeding population, even in a year with heavy nestling mortality. However, the number of recruits was few. The results are consistent with the view that asynchronous hatching ensures high quality of some offspring (the offspring quality assurance hypothesis). This benefit is achieved even when no brood reduction occurs. We suggest a potential cost to hatching asynchrony (the diminishing return hypothesis): in asynchronous broods, early hatched offspring may require so much food that their own subsequent survival is reduced or at least no further improved. Resources may therefore be wasted on first hatched offspring that might have improved the growth of younger siblings. Female parents had lower post-breeding survival with asynchronous than with synchronous hatching; the opposite result was found for male parents. We propose the exploitation of mate hypothesis to explain this result. The hypothesis presumes there is sexual conflict over the amount of parental investment in current versus future reproduction, and over the investment in particular offspring within the brood. We suggest that with asynchronous hatching, females have to invest more to keep less competitive, late-hatching nestlings alive.