Do hormones dictate breeding success in birds?

WASHINGTON - Scientists at Max Planck Institute for Ornithology, Radolfzell, and Princeton and Edinburg Universities have found that hormones like prolactin and corticosterone can exercise a crucial influence on the behaviour of birds in the breeding season and subsequently on their reproductive success.

They have demonstrated that these levels also dictate, long in advance, how many eggs a breeding pair will lay, when they will lay them and how often.

An animal’s hormonal constitution is thus of major significance for its reproductive success, and is possibly an important driving force of evolution.

Jenny Ouyang, Michaela Hau and colleagues examined how hormone concentrations in individual wild house sparrows (Passer domesticus) before and after the breeding season correlate with reproductive success.

These birds often differ significantly in terms of the number of clutches they produce, the number of eggs they lay over the course of a breeding season and how many of their young become fully fledged.

The scientists counted the number of eggs, clutches and fledglings for each breeding pair.

They also took regular blood samples three weeks before the beginning of breeding and during the rearing of the first clutch, both in natural situations and under artificially generated stress, to determine the corresponding concentrations of the hormones corticosterone and prolactin in the birds’ blood.

“We were surprised that we could predict how many offspring a breeding pair would have based on hormone levels three weeks before the breeding season,” said Ouyang.

“Sparrows that had low corticosterone values before the breeding season raised the most offspring. Especially birds with low values before, but high values during the season had the highest reproductive success. They apparently invested a lot of energy in the brood,” she said.

In contrast, the animals that had a very strong hormonal reaction to stress fed their young less and produced fewer offspring.

According to the scientists, prolactin plays a crucial role in the timing of laying the first egg: females with higher prolactin levels started laying earlier and had more offspring as a result.

“The fact that the two parents have very similar hormonal values is also fascinating for us,” said Ouyang.

“It is not yet clear whether the couples influence each other’s hormonal status or whether they select partners with similar hormone levels,” she added.

The results enhance knowledge about the physiological mechanisms that can dictate when a bird breeds, how many eggs it produces per clutch and how often it breeds.

It also showed that prolactin and corticosterone played a more important role in the regulation of individual investments before reproduction begins than was previously assumed.

The findings are published in the Proceedings of the Royal Society B. (ANI)