The Desert Leaf July/August 2014 : Page 36
Y urban r apTor s by Claire Rogers Tucson’s es, Cooper’s hawks are on the “rise” as a matter of fact. For the last 20 years, their numbers have increased in and around Tucson. “Living in an urban environment is sometimes of mixed benefit, but overall a positive move, if and when raptors can overcome their sensitivity to human disturbance,” said William Mannan, professor of wildlife, fisheries and conservation biology with the University of Arizona’s School of Natural Resources and the Environment. “Some species are better at adapting than others—Cooper’s hawks clearly are one species that has overcome that sensitivity completely.” 36 DesertLeaf l July/August 2014
Tucson’s Urban Raptors
Yes, Cooper’s hawks are on the “rise” as a matter of fact.For the last 20 years, their numbers have increased in and around Tucson.
“Living in an urban environment is sometimes of mixed benefit, but overall a positive move, if and when raptors can overcome their sensitivity to human disturbance,” said William Mannan, professor of wildlife, fisheries and conservation biology with the University of Arizona’s School of Natural Resources and the Environment. “Some species are better at adapting than others—Cooper’s hawks clearly are one species that has overcome that sensitivity completely.”
When Mannan began studying Cooper’s hawks 20 years ago, their population consisted of 40 or 50 nesting pairs. He estimates more than 200 pairs live in and around Tucson today.
He added that Harris’s hawks, redtailed hawks and great horned owls are other examples of healthy populations of Tucson’s urban raptors.
“If they can get over that sensitivity to human disturbance, then they frequently have more abundant sources of food and water, particularly in the desert Southwest, than in the natural desert,” Mannan noted, pointing out that Tucson’s urban environment supports many prey species, such as doves, rabbits and rodents.
“Primary productivity is high in urban areas,” he explained. “For example, there is more biomass produced in Tucson than in a natural environment, and there is more water available, too.As long as there are some big trees, which these raptors use as nesting sites, then they have all the resources to do very, very well.”
According to the Arizona-Sonora Desert Museum, Harris’s hawks live in small groups of three or four, hunting cooperatively and even caring, as a group, for a single nest. This makes them much more noticeable around Tucson neighborhoods.
Mannan noted that there are some drawbacks to city life for raptors.“They have some mortality agents that the birds are not naturally adapted to dealing with,” he said. “Being hit by cars and flying into windows are some of the primary agents of mortality for urban Cooper’s hawks. The larger bodied birds, such as great horned owls, Harris’s hawks and red-tailed hawks suffer from electrocution. Because our [energy] history has this overhead distribution system, the large-bodied birds are big enough to make the connection between a phase wire and a ground wire.” That electrical connection often is lethal.
According to Mannan, Tucson Electric Power (TEP) has been proactive in retrofitting and insulating poles around nest sites to protect fledgling raptors looking for an easy first landing.TEP has even begun installing nesting platforms under established nests. Mannan has been helping TEP with the process, but he notes that tens of thousands of poles still need to be retrofitted.
“Periodically, some people are upset with Cooper’s hawks because the females can be particularly aggressive when they have nests; they can actually hit people,” said Mannan. He added that about 10 percent of the females will get aggressive: “Their general notion is to swoop down to drive predators away from the nest; that’s the female’s job.” Occasionally, they’ll go the extra step and rake their talons on a person’s head if they feel their nest is in danger. But, he noted, that behavior usually lasts only for a few weeks.
One potential cause for the boom of Cooper’s hawks is a recent decline of the nestlings’ susceptibility to trichomoniasis, Mannan explained. The disease is prevalent in doves, a main food source for Cooper’s hawks. Mannan found that adult hawks don’t usually contract the disease, yet it spreads to their offspring during feeding.
His research revealed that acidity in the mouths of newly hatched nestlings is near neutral, but after about 50 days, the acidity has increased sevenfold, enough to prevent infection. Fortunately, the spread of the disease appears to have declined in recent years.
Another interesting finding of this professor is that pair bonding seems to endure beyond nesting season, among urban pairs of Cooper’s hawks. He speculates that the abundance of food causes the male to continue feeding the female throughout the year in order to maintain the bond come nesting season.In conditions with less food abundance, pairs separate during the winter.
Mannan said most people are positive about having urban raptors in Tucson, though a few bristle at the mess that can come from a bird’s feeding perch or nest.
“If there’s a nest over your pool, you may find a lot of body parts falling into your pool,” he said. But again, he noted that it lasts only for a short season.
To support urban raptors, he recommends that Tucsonans report suspected electrocutions to TEP; otherwise it is best to leave hawks alone.
“If you see baby birds on or near the ground, please leave them alone.We get dozens and dozens of Cooper’s hawks picked up and brought to rehab centers and they don’t do real well once they leave the rehab center,” Mannan lamented.
He pointed out that flying is a developmental phase for raptors and, as a fact of life, not all fledglings will make it: “They’re not abandoned and they’re not orphaned unless they are really, really young; it’s best to let them alone, when you see them.”
Additionally, waiting to trim trees until after nesting season is over is always a good idea. (Raptors are protected by the National Migratory Bird Treaty Act.) Nests are usually active from February to August, so fall and early winter are better times to trim branches. Raptors seek denser trees for nesting, so pruning trees for an open arrangement will discourage nesting.
“From the standpoint of humans being able to see a predator in their backyard, that’s pretty uncommon,” Mannan said. “[But] a lot of people have that opportunity in Tucson. Cooper’s hawks are common and people have them in their backyards, along with bobcats and coyotes.
“We have a unique situation for where we live—in Tucson. It’s pretty special.”
Claire Rogers is a local freelance writer. Comments for publication should be addressed to letters@desertleaf.Com.
Read the full article at http://trendmag2.trendoffset.com/article/Tucson%E2%80%99s+Urban+Raptors/1738640/214061/article.html.