Today’s science-and-photography post comes courtesy of evolutionary ecologist Bruce Lyon. His words are indented. (Note: this counts as a science post, so please read it all!)
Jerry has had a family of Cooper’s hawks (Accipiter cooperi) around his duck pond. I too have a family of these hawks by my house. Jerry writes that the hawks have not been bothering his ducks, which surprises me since these are ferocious predators that can take surprisingly large prey. I would have though that ducklings would be ideal prey, especially when they were smaller. These hawks hunt birds and mammals and, sorry Jerry, but they really love squirrels! Warning: this post will include a couple of photos of the hawks with dead prey items, some of which are cute.
Below: A Cooper’s hawk with some unidentified bird prey. Cooper’s hawks are considered bird eaters—hence the old nickname ‘chicken hawk’—but some individuals eat a lot of mammals too. Note the hawk’s orange eye, which contrasts with the juvenile’s blue eyes in the next photo. Many birds have colorful eyes—I have often wondered if colorful eyes could be some sort of social signal in birds but I could find almost no papers on this. In fact, the only study I found was specifically about eye color in Cooper’s hawks. This study, following known-age banded birds, found that the eyes get increasingly redder and darker with age. The authors also tested the idea that male eye color might be a signal of a male’s quality, but their data did not support this hypothesis.
Below: A juvenile Cooper’s hawk with a squirrel it parent brought it. In the words of Monty Python, this squirrel is an ex-squirrel (i.e., it is dead even though it looks alive). This photo and the next are from nests in Santa Cruz parks in previous years.
Cooper’s hawks used to be rare in much of their geographic range—and accordingly a species of conservation concern—due to the combined effects of pesticides (DDT) and persecution (people love to hate predators and used to shoot these so-called ‘chicken hawks’). In the early 1900s there were organized annual hunts to kill as many migrating raptors as possible, and people would go to migration hotspots like Hawk Mountain in Pennsylvania to shoot the ‘varmints’ (and the state government apparently paid bounties on some species). The photo below, found online, shows the carnage of one of these hunts. The hunt at Hawk Mountain suddenly stopped when an early conservation hero, Rosalie Edge, bought the property at Hawk Mountain, and turned it into a sanctuary (see here for an account of the life of this remarkable woman written by her son).
Once people stopped shooting raptors and DDT use ended in North America, Cooper’s hawks bounced back and they are now thriving. They are the most common raptor around towns but other species of raptors also seem to colonizing suburban areas and even cities, including various species of hawks, falcons and owls. For Cooper’s hawks, suburban areas provide great nesting habitat, and the bird feeders around houses may increase the supply of small birds for prey. I often see Cooper’s hawks hunting at my neighbor’s bird feeder and wondered if these hawks might focus on feeders. However, one radio tracking study of the closely related sharp-shinned hawk (Accipiter striatus) showed that the hawks were not spending more time at feeders than expected by chance. Feeders may, however, increase the density of birds in an area and contribute to better hunting success overall.
Below: My Cooper’s hawks nested in a sycamore tree in a heavily used university parking lot, and at least 100 people walked right under their nest on most days. Unlike many hawks, Cooper’s hawks build dinky nests—so dinky that the tail of the incubating female often protrudes over the edge as you can see in this photo.
Cooper’s hawks, and other members of the genus Accipiter, are famous examples of ‘reverse’ sexual size dimorphism in birds. The term reverse was used because the opposite pattern—males much larger than females—is way more common in many taxonomic groups, In western populations of Cooper’s hawks, females average 470 grams, males 280 grams—a 1.7-fold difference. Over a dozen hypotheses have been proposed to explain why females are larger than males in raptors. Many of these focus on single factors that can affect body size, but some researchers suggest that the answer is likely to be a combination of several factors.
Body size affects almost everything an organism does, and the size difference between female and male raptors will be the result of natural or sexual selection operating in different ways on the body sizes of two sexes. Body size affects the amount of food a bird needs to sustain itself, the cost, speed and agility of flight, the bird’s strength and the size of prey that can be taken, among many other things. The figure below, from a review paper by Malte Andersson and Åke Norberg (1981, Biol J Linn Soc), nicely shows all of the factors that could potentially cause natural and sexual selection to differ between male and female raptors.
Below: One interesting factor that correlates with the sex differences in size in raptors is diet: the more vertebrates in the diet, the greater the relative size difference between the sexes. And the biggest sexual differences are seen in bird-eating raptors, as shown below. The graph shows the ratio of linear measurements, not body mass. I use this graph in teaching and have forgotten the source.
The bird-eating raptors are not only the most extreme in sexual size differences, but they also show the most extreme differences in sex role specialization when nesting, which suggests that size difference and division of labor could be linked. In bird-eaters, the female incubates the eggs and defends the nest from predators while the male does most of the hunting for the entire family, at least until the chicks are quite large. This means that the male is not just hunting for himself, but for his mate and for three to five chicks. Given this, small males would need less food for themselves and they may also be more agile and effective hunters. Large females might be favored because they can lay larger eggs or because they can better defend the nest.
Female Cooper’s hawks and other Accipiter hawks are indeed famous for their ferocious defense of their nests. Researchers who visit nests to band chicks have to wear protective gear or risk losing an eye or having their scalp opened up. Given this fearsome reputation I worried that there might be chaos around the parking lot if the female hawks started aggressively defending the nest. However, she mostly ignored people, even those who walked right under the nest to get to their car. For some reason, however, the female really loved divebombing me. She was polite about it and never came close to hitting me, but she regularly swooped a couple of times whenever I showed up in the parking lot, even when I was farther from nest tree than many other people that she ignored. Maybe she does not like paparazzi! I did not take it personally and it almost seemed like a game for her (she did not seem too upset because after swooping at me she would perch and preen and otherwise seem relaxed). The fact that she singled me out suggests that the female recognized me; recognition of individual humans and singling them out for mobbing has been shown in a few birds like crows and Australian magpies.
Below: The next few photos show the female swooping at me. She was moving too fast for the focus tracking on my camera to work so I just chose a fixed manual focus setting, blasted away with the motor drive and hoped that I occasionally got a sharp photograph. I got several.
Below: The female perched in a sycamore tree near the nest, in nice early morning light.
Below: The female with the remains of what I think is a California ground squirrel (Otospermophilus beecheyi). These ground squirrels are very abundant in a large meadow just across the road from the nest. While returning from shopping the other day, the male hawk flew right above my car and pounced on a ground squirrel in the field. Then, seconds later, the female flew in, supplanted him, took the squirrel and flew with it back to the nest.
Below: A nestling flutters along a branch, having been just kicked out of the nest by a sibling (nest just out of frame to the right). This chick had been the first to get a prey item the female brought to the nest, and it ate peacefully for ten minutes before the second chick jumped onto the nest, stole the prey and forced this chick to leave. Even after the chicks fledged and could fly well, the female continued to use the nest as the dining area and dropped off prey for the chicks at the nest. The pair raised three chicks and they are now terrorizing the neighborhood. I just returned from a walk close to my house where I watched two of the chicks repeatedly swooping at a gray squirrel. The squirrel did not seem too concerned so either it is not very bright or it is smart enough to know that these chicks are inept and pose little threat.