Seasonal timing of reproduction and the number of clutches produced per season are two key avian life-history traits with
major fitness consequences. Female condition may play an important role in these decisions. In mammals, body condition
and leptin levels are correlated. In birds, the role of leptin remains unclear. We did two experiments where we implanted
female great tits with a pellet releasing leptin evenly for 14 days, to manipulate their perceived body condition, or a placebo
pellet. In the first experiment where females were implanted when feeding their first brood offspring we found, surprisingly,
that placebo treated females were more likely to initiate a second brood compared to leptin treated females. Only one
second brood fledged two chicks while five were deserted late in the incubation stage or when the first egg hatched. No
difference was found in female or male return rate or in recruitment rate of fledglings of the first brood, possibly due to the
desertion of the second broods. In our study population, where there is selection for early egg laying, earlier timing of
reproduction might be hampered by food availability and thus nutritional state of the female before egg laying. We
therefore implanted similar leptin pellets three weeks before the expected start of egg laying in an attempt to manipulate
the laying dates of first clutches. However, leptin treated females did not initiate egg laying earlier compared to placebo
treated females, suggesting that other variables than the perceived body condition play a major role in the timing of
reproduction. Also, leptin treatment did not affect body mass, basal metabolic rate or feeding rates in captive females.
Manipulating life history decisions using experimental protocols which do not alter individuals’ energy balance are crucial in
understanding the trade-off between costs and benefits of life history decisions.