Brood Parasites "Diversify Portfolios" In Times of Environmental Flux

During periods of environmental flux, brood parasites "diversify their portfolios" by laying their eggs in different nests to be raised by different bird species, a new study reveals.

Brood parasites are birds that lay eggs in the nests of other birds, employing different imitation strategies (mimicry) to let the host raise a young that is not theirs. The research team was comprised of Nicholas D. Antonson and Mark E. Hauber from the Department of Evolution, Ecology, and Behavior at the University of Illinois Urbana-Champaign; Dustin R. Rubenstein from Columbia University's Department of Ecology, Evolution, and Environmental Biology; and Carlos A. Botero from Washington University in St. Louis' Department of Biology.

Their study was published in the latest journal Nature Communications on Friday, August 21.

Tracking Brood Parasites' Host Diversification

"We found that, in unstable environments, brood parasite birds choose to not put all their eggs in one basket," Antonson said. He explained that the results from their study support the theory that brood parasites diversify their reproductive risks in locations with uncertainty in terms of ecology, behavior, or environment.

To observe the brood parasites' host diversification, the researchers collected data from host species of birds connected to brood parasites spanning 19 genera and five different bird families. The list they used for reference reportedly covers 86 percent of all known species of brood parasites. Data from all prospective host species were provided by the Field Museum of Natural History in Chicago, Illinois as well as the findings compiled by ornithologist Paul Johnsgard in his book "The Avian Brood Parasites: Deception at the Nest."

In terms of environmental variables, researchers calculated monthly mean values of precipitation, in mL per month, and temperature, in degrees Celsius, covering 1850 to 2005 based on Ecoclimate.org's data through the Community Climate System Model (CCSM4).

"This research begins to answer a longstanding question about how species first interact and then coevolve in environments that are also changing," Hauber explained. He added that theoretically, both predators and parasites should include a variety of prey and hosts.

Avian Brood Parasites' Imitation Strategies

The reproductive success of brood parasites is largely dictated by whether the prospective host accepts the outsider's eggs, as well as the host's capability to raise the parasite young as its own.

To do this, avian parasites employ a variety of imitation strategies, or mimicry, to make their young blend in with the rest of the host egg. Some species have eggs that resemble those of the host (egg mimicry), some birds learn to imitate the call and cry of their host species.

However, some host birds learn to distinguish between those eggs that are theirs and those that are not - they can either eject the imitation egg or build new nests elsewhere. Meanwhile, for hosts that fail to identify, they go on and incubate, hatch, feed, and care for the brood parasite as one of their own.

The study noted that there are factors that affect the selection of host species and the success of the parasite species. Another point brood parasites consider is whether the host cares for its eggs alone or together with its mate. Having both parents around makes it more difficult for the brood parasite to slip their own egg into the nest.