Sunday, October 31, 2010

The Northern Goshawk!

It's been a great and busy fall as I continue to get settled into my Master's Program in Raptor Biology at Boise State University. The great news is that my top candidate thesis project received funding from the forest service! I will be studying breeding ecology of Northern Goshawks. Focal areas include the prey base in a uniquely structured forest, prey's effects on nest productivity, and some work on sex ratio of the offspring. I am very excited about that. While the project is funded, it is not fully funded so my past few weeks have been filled with applying for additional grants. Oh yeah, in case I wasn't busy enough, I also have my classes and teach my labs.

Another activity filling my time this fall was being trained on trapping and banding raptors. My thesis does not require migration trapping, but I wanted to gain the handling experience which I will need during the breading season. The Idaho Bird Observatory, where I have worked for 2 years, operates two raptor trapping stations, one at Lucky Peak (location of songbird banding, owl banding, and Hawkwatch) and one on Boise Peak. These locations are chosen to align with the flight paths of migrating raptors as they move down the Boise ridge. Both stations reside near the tops of mountains between 6000 and 6500 feet. I spent most of my time at Boise Peak. It is more difficult to get to in a vehicle, but my enduro motorcycle makes easy work of the rough roads. While Boise Peak isn't quite as successful in total numbers of birds trapped as the Lucky Peak station, it does offer its rewards in great views, close up action, and fewer people. The bird I most wanted to catch, the Northern Goshawk, did grace my presence on a number of occasions. On the first occasions I was still being coached on the process. Dave instructed as the goshawk dialed in on the prey from a mile away!

My first goshawk! A juvenile male.

One thing you quickly learn about goshawks is that they are much higher strung than any other bird we catch. A non-stop "I want to rip your eyes out" sort of intensity. They are also very powerful! They are forest raptors making them very maneuverable. They can catch birds in flight, pick squirrels out of trees, and even capture large items such as Snowshoe Hares or jackrabbits. This one really wanted my pigeon!

The trapping station consists of a blind with a series of nets and lures. The nets and lures are all operated by strings. The lure birds, all non-native species, wear harnesses which allow us to pull them up into the air so that they flap their wings. The harnesses also protect them from attacks from the raptors. We use pigeons to attract birds from far away and to capture the larger species, Eurasian-collared Doves for middle size prey, and House Sparrows for the smaller raptors. Some nets are spring loaded bow nets on a string trigger and some are vertical nets positioned in front of the lure birds.

Boise Peak Blind and Sparrow Traps.

Pigeon trap (far right), Dove trap (near left).

View from inside the blind.

Raptors in the hand provide a unique way to learn the subtleties of the species. Most of the species that I caught belong to the family of Accipiters. These are all highly maneuverable forest species. The male Sharp-shinned Hawk is the smallest bird we catch, even smaller than an American Kestrel.

Juvenile male Sharp-shinned Hawk (~100grams).

All of the Accipiters, and most all raptors in general, exhibit reverse sexual dimorphism. This means that the female is larger than the male. There is a correlation of the amount of dimorphism and the speed of their prey. Since Accipiters capture fast prey, including birds in flight, the females are much larger than the males, up to 80%!

Adult male Sharp-shinned Hawk.

Another thing to notice from the two pictures above is that adults have very different plumage than the juveniles and their eyes change from yellow to a rich dark red. While eye color cannot be used to provide a definitive age, other observations tell me the adult pictured above was at least 2 years old (we call it "after second year" on a scale of: hatch year, second year, after second year). Sharp-shinned Hawks and Coopers Hawks look much alike. A large female Sharp-shinned is almost the same size as a small male coopers. They both have different juvenile and adult plumage and the eyes change from yellow to red. One very distinguishing feature is the shape of the head. Sharp-shinned have very small heads, where coopers are much more blocky.

Adult male Cooper's Hawk (~250 grams).

Adult female Cooper's Hawk (~470grams).

Juvenile female Cooper's Hawk.

Adult male Cooper's Hawk (probably 2nd year-note eye color).

Which brings us back to the largest North American Accipiter, and the baddest of the bad, the Northern Goshawk! After my first "coached" trapping of a goshawk, I would have a number of good solo experiences as well. On my final day in the blind this year, I would successfully trap three goshawks! I missed the first and another flew by un-interested, for a total of five sightings. A biologist friend of mine, Julie, and her husband would join me in the blind for this remarkable day. Within 15 minutes of their arrival I demonstrated how its done by catching the first bird. The video linked below was taken by Julie's husband John.

Julie with juvenile male goshawk (~850grams/2 pounds).

Juvenile male Goshawk.

But that wouldn't be all. A short while later we would catch another male, this one had much more white than the first.

Male juvenile goshawk.

Male juvenile goshawk.

Male juvenile Goshawk.

But the day wasn't over yet. A third goshawk, this one a big female, went for my pigeon.

Female juvenile Northern Goshawk (~980grams).

Female juvenile goshawk.

Weapons of mass destruction.

Amazing birds!

What a way to finish out the season. I really enjoyed my days on the mountain. I now spend my time writing grant proposals so I can spend more time with the birds!