Pre‐breeding energy requirements: thyroid hormone, metabolism and the timing of reproduction in house sparrows Passer domesticus

Abstract

We measured thyroid hormone (T3) levels and energy expenditure of pre‐breeding house sparrows Passer domesticus in relation to the timing of breeding and reproductive success. The onset of reproduction was synchronised in two waves, separated by a three‐week interval. On an annual basis, early breeders (birds that bred for the first time during the first wave) made significantly more breeding attempts, laid significantly more egg and raised 2.3 times more chicks to fledging than late breeders (birds that bred for the first time during the second wave). By the end of March, about one month before the first egg was laid in the population, plasma titres of testosterone in males and estradiol in females were still low and did not differ between early and late breeders. However, birds that subsequently bred early had higher titres of plasma triiodothyronine (T3) than birds that started to breed late. We show for the first time in a free‐living bird population that Basal Metabolic Rate (BMR) is positively correlated with the plasma titre of T3. Differences in plasma T3 accounted for 48% of the inter‐individual variation in BMR. Elevated T3 levels indicate that energy requirements increase prior to breeding. Although early breeding appears to be advantageous in terms of the number of offspring raised on an annual basis, the increased energy requirements prior to breeding are thought to delay the onset of reproduction in those birds that cannot afford the additional energy expenditure early in the season.