As temperatures begin to fall past zero in Iowa, it’s hard to believe that for some birds, especially birds of prey, Iowa is a southerly destination when it comes to migration.
“There are some that are coming from the arctic, there are some that are coming from the boreal forests,” Says wildlife biologist Jim Pease. “But in general, if we think of raptors, owls tend to stay, hawks tend to move, and eagles do both.”
For snowy owls, Iowa can provide food sources that their usual arctic homes cannot.
“There aren’t a lot of prey and there aren’t a lot of predators, so when one or the other goes up or down dramatically, the others have to respond in some way,” says Pease. “In this case, lemmings and voles, which the owls depend on for food. If there is a shortage of them, the owls have nothing to eat, so the owls will come south to find food.”
Since these visiting owls tend to stay through the winter, do they cause any competition with native Iowan owls?
“It certainly does, to some extent,” Says Pease, “Since snowy owls are a prairie species, the only owls we have a lot of out on the prairies are short eared owls. And short eared’s tend to migrate a little bit further south. So whether it would be competitive with them? I tend to doubt it. Most of our other owls are woodland or woodland edge species, so there is certainly some competition.”
As the snowy owls move in, the Iowan hawk populations move out.
“Hawks tend to go south. We will see a few harriers, a few red-tails, and some kestrels stay, but hawks tend to head south,” says Pease. “The two that tend to hang around are the sharp shin hawk, and the cooper’s hawk, because they eat other birds.”
For the hawk populations in Iowa, the data shows a slow increase, but for only one species of hawk.
“The only hawk population that has been on the rise nationwide has been red-tailed hawks,” says Pease.
“If you look at the Breeding Bird survey data for the last fifty years, it’s one of the only birds of prey that has shown increase.”
Finally, eagles. Or more specifically, bald eagles.
“They come here because it’s warmer, and because we have open water,” says Pease. “Because we have a state that is rich in rivers, and we’ve had a lot of dams on those rivers where the water below the dam remains open year round, we have always had a lot of northern eagles spend their winter here. We’ve often had three to four thousands eagles every winter.”
“Now, we do not know for sure if our eagles go south for the winter and come back, because some of them will actually begin nesting in January, February, and March,” says Pease. “Or if they stay in Iowa, and compete for the same fish resources that the northern eagles eat.”
During this edition of Wildlife Day on Talk of Iowa, host Charity Nebbe speaks with wildlife biologist Jim Pease about the migration of predatory birds in Iowa. Pease also answers listener questions.