Parenting frays some feathered friends.
This news may comfort, though not surprise, human parents. However, it does counter ornithologists' observations that sickly birds plagued by parasites tend to have fewer offspring.
There's a surprise for nonornithologists too. Only the male parents of large broods have a higher rate of illness than other bird parents, because the males take on more of the chick-care responsibilities, report Heinz Richner of the University of Berne in Switzerland and his colleagues.
Previous studies of mammals and birds have shown that producing a large brood or taking on more of the feeding burden can affect future fecundity, perhaps even survival.
The Swiss researchers suggest for the first time that heavy parenting demands may make birds more susceptible to protozoa, resulting in future fertility problems, says David R. Winkler of Cornell University. "I hope it serves to get more of us ornithologists more interested in parasites."
In addition, he says, researchers rarely do this kind of experimental -- as opposed to observational -- study of birds.
In their experiments, Richner and his colleagues monitored the health of 118 great tits (Parus major) nesting near Lausanne after manipulating the birds' brood sizes, they report in the Feb. 14 Proceedings of the National Academy of Sciences. The researchers moved enough pairs of nestlings to create similar numbers of diminished, enlarged, and untouched families.
When the chicks reached 13 days old, the age at which they require the most food from their parents, the investigators photographed the nests to find out how often the parents brought food. They then captured and tested the parents for malaria, a common infection in birds.
The males with enlarged broods made about 30 food deliveries per hour -- 50 percent more than any of the other birds, including their female partners.
In a recent, unpublished experiment, the team also found that chicks with the worst infestations of blood-sucking fleas beg for food more often than their less heavily infested peers. In response, the males feed them more. The females, however, don't bother.
Males may work harder than females to ensure that their offspring survive, because males outnumber females and may not have as many opportunities to reproduce, Richner speculates. Females may fare better in the long run by not putting all of their energy into one nest.
The busy males pay a price for their efforts, the researchers assert. More than 75 percent of the fathers of the bigger broods became infected with the protozoan that causes malaria -- that's twice the infection rate of the other males. Most of the females got off malariafree.
The scientists suggest that the added feedings tax the birds' energy and weaken their immune systems. Alternatively, the birds' schedules may somehow expose them to more of the protozoa.
In an earlier study, Richner and his colleagues found that about one-third of the males in 108 unmanipulated broods had malaria, twice the infection rate the researchers found among the females. A much smaller proportion of infected than uninfected males returned to breed, they report.
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|Title Annotation:||brood size effects health of male parents|
|Date:||Feb 18, 1995|
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