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Cooper’s Hawk abundance gives NMSU researcher insight on other raptor biology

Now fairly common in urban areas, the Cooper’s hawk has swooped in on Albuquerque and is capitalizing on the abundant dove population. Researchers from the U.S. Fish and Wildlife Service and New Mexico Department of Game and Fish close in on an active five-year study to gain insight on the biology of this raptor.

Man holding hawk
Brian Millsap, National Raptor Coordinator for the U.S. Fish and Wildlife Service is conducting a six-year study on Cooper’s Hawks. Millsap is using the study the research topic for his doctorate at New Mexico State University. (photo by James Walker)

“What we’re studying is the population biology of the Cooper’s hawks,” said Brian Millsap, national raptor coordinator for the U.S. Fish and Wildlife Service. “Their basic biology is similar to a lot of other raptors and their population biology is not that different from golden eagles, a species of great concern for us, but which are scarcer and therefore a lot harder to work on.”

What began as a Fish and Wildlife Service study, Millsap has carried forward for almost six years, making the study his Ph.D. topic at New Mexico State University. He and his team of researchers have banded almost 500 hawks, and tagged close to 100, which are monitored with radio tracking. Each year the tram tracks the fate of 50-80 nests.

The big question is the process which individuals use in selecting or settling on their first breeding territory. In raptors, it is an issue because nest sites are usually what limit populations, and there is a great deal of competition for those nesting territories.

In order to get a broad picture of the overall biology of the Cooper’s hawk, researchers are looking at survival rates, reproduction rates, diet, and population density. Information on territory acquisition and settling behavior is collected by following individual hawks from the time they leave their nest, through dispersal, until they eventually settle on a nesting territory. The behavior and biology of these floaters, which are the individuals old enough to breed but haven’t yet settled on a territory, is an important yet poorly understood area of raptor biology.

“In the study area where we’re working on Cooper’s hawks, which is an urban area, they’re increasing. Because of that, we’re able to collect a lot of information you can’t really get on an endangered species,” Millsap said. “They’re not rare, they’re abundant. We’re taking advantage of that abundance, kind of using them as a surrogate.”

Many aspects of Cooper’s hawk’s biology are unique. Common with all raptors, the males are smaller than females.

“There are over 20 hypotheses as to why males are the smaller sex in hawks, and there is still no agreement as to what evolutionary factors are responsible. Because this pattern, termed reversed sexual size dimorphism, is common among all predatory birds, it clearly has something to do with the predatory lifestyle.”

Cooper’s hawks are among the most size-dimorphic of all hawks, and there is a clear role partitioning between the males and females when it comes to breeding. Males do most if not all the hunting, and females provide all the care for the brood. Millsap believes this extreme role partitioning may help explain selection for reversed size dimorphism. Smaller males are better hunters, and larger females can lay larger clutches of large eggs.

The researches have been surprised to find that in this specific population every one-year-old female that has been tracked has been able to settle on a territory and breed. They expected to find only a small percentage of females breeding in their first year.

“The interesting thing with males, is that’s not the case,” Millsap said. “Because the male does all the hunting, provides all the food for the female and the brood, it turns out the males need a lot of experience to be successful and very few can do that at one year of age.”

In this population every female who reaches one year is ready to breed, but only a very few males are. This leads to a bias in the sex ratio of potential breeders, with the availability of breeding-age males limiting the rate of growth in the population. If males are a limiting resource, and females are competing for the males, researchers want to know which females are successful in getting the best males.

In Albuquerque, the primary food source for Cooper’s hawks is white-winged doves, which are abundant. In drought years, doves seem to concentrate in the urban areas where food and water are plentiful. However, in wetter years like this year, food and water are plentiful everywhere, and doves are more dispersed. This leads to less food for the urban hawks, and researchers are seeing a potential issue with food, for the first time this year.

“The doves are everywhere this year, not just packed into people’s yards,” Millsap said. “They’re not packed into the city as much as they usually are.”

Cooper’s hawks are being found less than football field length apart in urban areas where they previously were found about a mile apart.

“We are working in neighborhoods and parks,” Millsap said, “ and almost every time we are in the field, people are gathering in a crowd to watch us trap the birds, which makes it a great opportunity for them to learn.”

Because raptors are typically hard to study, are long lived, dispersed, many of these questions about basic biology population are not widely known.

“We’re going to answer some of those scientific questions and the neat thing about this project is, we get to do it in an urban area where we also get to teach neighborhood families about the hawks and how we study them.”