[Photo] A male Kentish plover with newly hatched chicks. (Photo Credit: Larry Wan)

The battle over who cares for the kids has played a key evolutionary role in deciding whether different species of shorebird are monogamous or polygamous, according to new research in the journal BioScience.

A demanding youngster means that parents are more likely to stay together to help rear their young, yet those with more hardy offspring are likely to battle it out to see who gets to leave the nest.

Played out over evolutionary time, it is this childcare tug-of-war which has shaped the varied breeding systems found amongst the world’s shorebirds, say researchers.

Scientists from the universities of Bath, Bristol and Imperial College London (all UK) focused on the Kentish plover (Charadrius alexandrinus) to help investigate the influences on the shorebirds’ breeding systems.

The Kentish plover is particularly interesting because whilst some pairs are monogamous, in other pairs either the male or female can be sequentially polygamous, mating several different times throughout the breeding season and leaving the abandoned partner to raise the chicks.

The scientists also found that whilst both males and females are equally adept at raising the offspring, the female parent is more likely to leave childcare responsibilities to her partner if there is a high ratio of unpaired males around. Mathematical modelling showed that if the sex ratio was reversed, so that there were more females than males, it would be likely that males would fly the nest in search of a mate.

The scientists also discovered that unpaired females would find a new mate faster (on average less than two days) after deserting the nest than males (average 12 days).

“Having systems of independent self-feeding young, compared to those that require feeding by the parents, opened the possibility for the evolutionary divergence of breeding systems to those where either females or males had more than one mate,” said Dr Tamas Szekely from the Department of Biology & Biochemistry at the University of Bath.

“In those species which have demanding young, the parents are more likely to share responsibilities, suggesting that the burden of childcare has shaped breeding systems.

“Care is costly to parents because it takes time and energy, and incubating eggs and feeding young may put a parent at risk of predation.

“Unless they are likely to breed again in the future, each parent has only a short-term interest in its mate’s welfare.

“These short-term interests may be at odds with long-term interests in securing its own reproductive potential.

“An outcome of this is that a parent may gain by shunting parental care duties to its mate, so that it is free to mate with a new partner.

“We have much to discover about how and why population sex ratios are maintained and regulated by nature.

“We also need to grasp better how breeding systems function in nature, for example ??? if a youngster is reared in a father-only family, will this influence how it will behave with its own family.

“Although we assume the deserted parent loses out, we also need to find out if the deserted parent gets any benefit ??? for example by being able to demonstrate that it is a competent parent.

“We noted in our fieldwork that female Kentish plovers were unusually receptive to the courtship of males caring for nearly fledged and apparently healthy offspring.”

The research was funded by the Natural Environment Research Council, the Biotechnology & Biological Sciences Research Council and the Leverhulme Trust.

BioScience is the monthly journal of the American Institute of Biological Sciences.