June 20, 2006 — Bird parents, like human couples, may fight over parental duties and some mothers and fathers even favor certain offspring over others, according to a new study on territorial songbirds.
The finds suggest humans are not the only ones that prefer certain young and struggle with the difficulties of parenthood. Like divorced parents, some bird pairs even split up and divide offspring between the mother and father. Such splits may even benefit the birds in the long run.
"Brood division could possibly result from a conflict between the two sexes over the parental investment: each parent should try and do less work and push the other parent to work harder, in order both to maximize current reproductive success and to save energy for future reproductive events," said Tudor Draganoiu, lead author of the study.
Draganoiu, a post doctoral researcher at Leiden University in the Netherlands, explained to Discovery News that he and his colleagues focused on bird communication, and specifically how avian parents relate to their offspring.
The researchers spent five summers studying black redstarts, Phoenicurus ochruros, in the small mountain village of La Valla sur Rochefort, France. Draganoiu and his team first noticed that couples among these birds would sometimes "divorce," due to conflicts or when one of the birds would abandon the brood.
In such cases, the birds would often move with favored offspring to a different location. Each parent, therefore, wound up preferentially feeding and caring for certain fledglings.
The researchers recorded begging calls from all of the fledglings and determined that parents responded more to the calls of individuals they doted on. The study, recently published in the journal Animal Behavior, provides the first direct evidence that birds acoustically discriminate between two categories of their own offspring: those that they preferentially feed and those that are cared for by the other parent.
It remains a mystery as to why some bird parents gravitate towards certain fledglings. Size does not seem to be a factor, nor does the sex of the chick. Male parents, however, tended to care for fewer chicks than mothers did.
In broods of three to five fledglings, fathers usually took care of one chick by feeding it insects while mothers fed the rest.
Such brood division occurs in a number of other songbirds, including blackbirds, robins, bluethroats, dunnocks, prairie warblers, song sparrows, white-throated sparrows and even some aquatic species, such as the great crested grebe and the coot.
"I suspect that brood division could be a common characteristic across bird species and not only for insectivorous birds (there are examples also among nocturnal raptors and waders), but parent-offspring interactions after the fledgling (stage) are difficult to observe for many species," said Draganoiu.
In another paper accepted for publication in the same journal, Iain Woxvold and colleagues from the University of Melbourne, Australia, studied apostlebirds, Struthidea cinerea. For these birds, parenting commune-style, where individuals other than the parents help to take care of offspring, seems to benefit old and young alike.
"Mothers and female helpers, and to some extent fathers, provisioned less when in larger groups," the researchers said. Feeding rates per nestling actually increased under such an arrangement because of all of the available extra help.
The researchers hope additional studies will be conducted in future to determine more about bird family arrangements and the interactions between avian parents and their broods.
