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Study examines conflict between survival of parents versus their young

Study examines conflict between survival of parents versus their young

(April 19, 2001)

When faced with a predatory threat, should parents behave in a way that reduces their own risk of being attacked, or should they act in a way that best insures the survival of their offspring?

Scientists have long used evolutionary theory to predict how animals should behave when faced with such conflicting demands.

Now evolutionary ecologists at the University of California, Riverside and the University of Montana have gathered experimental evidence that confirms predictions from "life-history theory" on the factors that influence decisions different birds make when threatened by predators. Life-history theory makes predictions about how the forces of natural selection shape they way organisms allocate time and energy to the competing demands of growth, survival and reproduction.

Writing in the April 20 issue of the journal Science, the scientists first analyzed data on the adult survival rates and clutch sizes - numbers of eggs laid - for 182 species of birds occurring in North America, South America, Europe, Australia, New Zealand, and South Africa. The data showed a widespread trade-off between survival and reproduction, where southern hemisphere birds have higher survival rates and smaller clutches and northern hemisphere birds have lower adult survival and larger clutches.

Given the differences in survival and reproduction between the northern and southern hemispheres, they then conducted field studies on 10 species in Arizona and Argentina to test a fundamental prediction of life-history theory.

In species that are generally long-lived and produce small clutches of eggs, parents should behave in a way to maximize their own survival - even at a risk to their nestlings - because their prospects for breeding again are relatively high. On the other hand, birds with larger clutches and loweradult survival rates should behave in ways that maximize survival of the offspring - even at a cost to themselves - because the chances of surviving to breed again are low and chances of survival for at least some of the offspring is high.

"If there is truly an evolutionary trade-off between survival and reproduction...we can make predictions about how parents value themselves vs. their offspring," said Cameron Ghalambor, a postdoctoral researcher at UCR and lead author of the study.

Ghalambor and Thomas E. Martin, an adjunct professor in the Division of Biological Sciences at the University of Montana, artificially manipulated predatory risks in natural populations of five northern hemisphere birds and five closely related species in the southern hemisphere. The species chosen for this phase of the study show the same general pattern of lower adult survival rates and larger clutch sizes in the northern hemisphere compared to their southern hemisphere counterparts.

A stuffed hawk model near the nest was used to simulate a predatory risk to adults, while tape-recorded vocalizations of a jay - a common nest predator - served as the experimental offspring predator. Parent birds making frequent visits to the nest often risk attracting the attention of these two predators, thus the experiments forced parents to choose between feeding their young or reducing the risk of attracting the attention of a predator to themselves vs. their young.

The studies took place in El Rey National Park of Argentina for four months in the winter of 1998-1999 and near Flagstaff, Ariz. for nearly four months in the summer of 1999. Assisting in the fieldwork were Susana Peluc, currently a graduate student at UCR, and Mariel Bazzalo, a graduate of the University of Buenos Aries.

As expected, adult birds in both study locations reduced the number of feeding visits to the nest when faced with the mock predatory risk directed either at themselves or their offspring.

But, there were important differences in behavior between the northern and southern birds.

When the predatory threat was directed at the offspring, the shorter-lived North American birds with larger clutches visited the nest less frequently than the South American birds, a reaction that more strongly reduces risk to the nestlings compared to the parents. In contrast, when the predatory threat was directed at the adults, the response was reversed, South American birds reacted more strongly to reduce their own risk.

"The parents change their behavior because they know they are being watched," Ghalambor said. "Not only are these birds very aware of their environment and very sly about not giving away the location of their nests, they also are distinguishing between different types of threats."

For instance, when threaten by the hawk model, the saffron-billed sparrow of Argentina would not only reduce the frequency at which it fed its young, it also modified the manner in which it visited nest. Instead of flying directly to the nest like a typical feeding visit, when confronted with a predator parents would walk long distances under the brush to their well-concealed ground nests.

Similarly deceptive, in response to the calls of the jay, the orange-crowned warbler of Arizona would often drop the food intended for its young as if to give the impression its nest was not nearby.

The study confirms predicted differences in adult behavior between northern and southern hemisphere birds based on their life-history strategies - northern birds place greater emphasis on survival of their offspring while southern birds place greater value on their own survival, Ghalambor said. "Theoretical models predict there should be such trade-offs - you can't be both long-lived and raise many offspring each year, but empirically documenting how these sorts of trade-offs are manifested in nature has up to now proven to be difficult," Ghalambor said.

The birds studied in Arizona and their Argentinean counterparts in parentheses are as follows: Cordilleran flycatcher (Euler's flycatcher), American robin (rufus-bellied thrush), house wren (house wren), dark-eyed junco (saffron-billed sparrow), and orange-crowned warbler (two-banded warbler).

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