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.
Amazing photos, Bruce – and a fascinating post. It’s a shame about the “cute” prey, of course, but the hawks have to survive, too.
I meant to comment on the photo of the “carnage” at Mount Hawk – atrocious, and kudos to Rosalie Edge (and to Rachel Carson for her role in the DDT ban).
I just looked at the Wikipedia Rosalie Edge article and see that she and Carson’s “conservation hero” efforts are intertwined: “In 1960, Hawk Mountain Sanctuary provided the scientist and author Rachel Carson with significant migration data that enabled her to link the decline in the juvenile raptor population to DDT, in her best-selling book, Silent Spring“. https://en.m.wikipedia.org/wiki/Rosalie_Edge
Very interesting! And those are some seriously dramatic dive bombing pictures.
Great pics.
You say it as a bit of a joke, but I think animals can and do react when they see a potential predator (which you are, to her) staring at them. Most animals “show you the butt” if they see you staring at them. I.e. they turn away so they can escape faster if you decide to come after them. But more aggressive animals may decide to warn off the human predator with some sort of display.
Wow, great photos and narrative. Thanks!
When I was growing up (Minneapolis/St. Paul area) in the 1970s, I never saw:
Cardinals
Bluebirds
Trumpeter Swans
Orioles
Sandhill Cranes
Cooper’s Hawks
Great Horned Owls
Turkeys
After about 1973, we hardly saw any eagles or ospreys.
With the cessation of DDT and hunting, these are all now abundant.
Same with mammals such as mink, otters, beaver, red foxes, opossums.
(I keep telling my son how lucky he is with the birds. Maybe I would have been a big bird enthusiast, as hes is, had there been more birds around when I was growing up.)
Trumpeter Swans were nearly made extinct in the “lower 48”. Now we have more than 20,000 nesting pairs in Minnesota every year.
On the other hand, I remember hearing Eastern Meadowlarks every (early) summer morning growing up. 40-50 years of tree growth has moved them away from here.
And, no one seems to put up Martin houses anymore, so they are rare and used to be ubiquitous (and most suburban homes had a Martin house out back).
Martin houses are quite popular on the Texas Gulf Coast. Maybe their range is changing?
I have spent much time outdoors in the Midwest US for the past 56 years. I agree with the anecdotal data; many more critters now than in the 70s. Far more common now are hawks, herons, vultures, cranes, turkeys, bald eagles, skunks, foxes, coyotes.
That’s great to see the comeback.
I recently had a back and forth online with someone (I am sure of a strongly right wing, anti-science bent) who claimed that all the reported declines in species was all a hoax, run by scientists and conservationists with “an agenda”. Showing him studies, data, charts and graphs had no effect.
I wonder what this alleged ‘agenda’ could be?
A plot to take muh guns and mah freedom.
(I can’t spell good)
In south central PA many farmers and others have martin houses. They may not all be occupied by martins; other species of swallows seem to favor them as well. At any rate, their value as insectivores is esteemed in this area.
Sub
… it’d be interesting if there were quiz questions after.
Thank you, a fascinating post! And me, I don’t think I would want to stand my ground with a camera watching that raptor coming at me.
Great report. The sexual dimorphism info is just fascinating. Looks like the best explanation is the different pray for each sex.
Rosalie Edge is a true hero.
Cracking stuff!
Great photos and very informative text! I have been fascinated by this species ever since I witnessed one nailing a pigeon at my backyard feeder quite a few years back.
Great post! Those shots of the female coming right at you are incredible.
These are amazing shots, Bruce! Your narrative is chocked full of good information too. I gather the female was strafing you to try to scare you off her territory, or what?
I get hawks hunting the backyard birds. They like to sit on the fence and pluck their kill. One year I was stooped really low doing some weeding and mostly hidden beneath a tree. One swooped down, and stopped short right above me when I suddenly lifted my head. Needless to say, we were both surprised. Maybe it mistook my full head of hair for a cat or some other critter.
Great photos, Bruce. We had a pair nest in front of our home for a couple of years. I miss them although I don’t really miss the grizzly body parts of prey birds on the sidewalk.
Very encouraging to know this research is ongoing.
Years ago, about 40 I think I was invited to the Northport point in Michigan on lake Michigan to observe the raptors, primarily hawks, resting before their flight over the lake going north. Those cedar trees literally vibrated with all the activity.
These photos are fantastic and the photographer fairly brave.
I’m puzzled by the diagram on sexual size dimorphism: The data range for mammals is about 1.01 to 1.10 on a y-axis labeled “female/male”. That would imply mammalian females to be 1% to 10% larger than males, which cannot be correct for, say, great apes (including humans), lions, wild pigs, or cattle.
Are there only species plotted where females are larger, or is the axis label to be interpreted not as actual quotient?
The mammal graphs I have seen usually have the ratio flipped: male to female. The interpretation of the raptor graph I included is that a value of 1.2 on the y axis (which would be a Cooper’s hawk incidentally) means the female is 1.2 times longer than the male. Made up example- if male is 10 inches long, female is 12 inches. To get volume and mass we cube the ratio which gives close to 1.7 which is the actual mass ratio for Cooper’s hawks.
I have reservations about interpreting this graph as showing the female/male linear size quotient for birds but the reciprocal for mammals. The question seems to be which species are shown.
Size dimorphism in birds can evidently go either way, i.e., either males or females are larger, depending on family. A reference I found starts with Darwin’s assertion that usually males are larger, and its Fig. 3.3 shows extremes for certain families, plotted as Log_10 (male/female), so equality is at 0:
DOI: 10.1093/acprof:oso/9780199208784.003.0004 ·
In book: Sex, size and gender roles, Chapter: 3, Publisher: Oxford University Press.
https://www.researchgate.net/publication/236174109_Sexual_size_dimorphism_in_birds
OK I think I understand the confusion. The only data plotted on this graph are relative size of female over males. The height of the bar on the Y axis indicates the value of the ratio. The X axis indicates the diet of raptor species. So for the dots with mammal on the X axis, these are for raptors that eat mainly mammals. Does this clarify things?
Yay! Thank you, that makes sense, and I am glad I learned about that! Sorry for the confusion!
Oh, and let me add to the congratulations on the pictures! Raptor eyes are ever fascinating when so well in focus, and mildly alarming when being focused by them.
Thanks for the great pics!
I was trying to think of reasons the hawks leave Jerry’s ducks alone. Do Cooper’s Hawks eat trash in town and, perhaps, prefer it? You mentioned they particularly like squirrel. Perhaps they find the squirrel population in the Botany Pond area satisfies them.
I wonder if they are a bit more generalist or opportunist then, in feeding. Have noticed where peregrines nest on church towers etc & I still see pigeons on those towers. That has always seemed strange! I wonder if the layout of the buildings makes it more awkward? No, more likely swooping on ducks on a pond risks you getting in the water & drowning.
Do they take birds flying or on the ground/on trees? Or both?! Anyone?
The ones we had relied on birds the size of sparrows and robins. They would capture them by swooping through trees and grabbing the prey especially around bird feeders.
I was just thinking that most creatures that survive and thrive in cities are ones that have adapted to a different diet, one that includes leftover human food. After all, there are presumably more ducks and squirrels outside of cities. On the other hand, they need territory so will spread to whatever areas work.
Stunning photos! I found the article to be informative. I found it very interesting that the female hawk recognized Bruce and seemed to want to play. I am glad she wasn’t really interested in attacking. We have a couple of Red Tail Hawks that soar overhead and I sometimes see them perched on light poles and trees.
Great photos. Plenty of Coopers here in Tennessee. I’ve seen them dive bomb doves in my back yard, causing explosions of feathers.
Fine biology post! Essentially they look very similar to the other sparrow hawks & goshawks that are also Accipiters. You do not mention they were named after the conchologist William Cooper by his friend & Napoleon’s nephew, Charles Lucien Bonaparte https://en.m.wikipedia.org/wiki/Charles_Lucien_Bonaparte
Thanks – that made me learn biology & then teach myself some biology history! Only surprised it has not been a candidate for renaming – there’s a move in the US particularly to rename animals, birds, plants etc removing ‘colonial’ names & names of people who are now w considered politically unacceptable.
https://undark.org/2020/07/17/mccowns-longspur-confederate-name-calls-for-change/
Yes I have been noting this and and curious where it goes. There has been a move specifically for birds and there is a petition trying to get support for the move to change names of ALL species named after people, irrespective of whether the person was a scumbag.
Great photos especially on the wing.
Very good story and photos, as always.
In Wisconsin the urban Cooper’s Hawks have learned to flush birds into windows. After the birds hit the windows they are “sitting ducks” (pardon the expression, Jerry) for the hawks.
The sharp-shinned hawks in Princeton New Jersey also learned this and used to chase mourning doves from the feeder outside the Biology Building into windows. One left a perfectly clear imprint from the feathers on the window that we referred to it an avian version of the Shroud of Turin. Princeton Professor Henry Horn called the feeders that lured the birds to their demise by raptors as a two-trophic level bird feeders.
Terrific photos and commentary. I always learn something from the science posts!
Nice commentary, we have observed very different behavior! We have had cooper’s hawks nest in trees a few feet from suburban streets close to our house and successfully fledge between 3 and 5 chicks. Ours are very tolerant of people, often sitting on a branch 5′ above our heads, near their nest, as we walk down a sidewalk and never showing aggression. Last year, one calmly sat about 10′ above men using a huge, noisy jackhammer to break up a concrete street under their nest.
Beautiful pictures showing beautiful creatures doing beautiful things, amazing, thanks.
Magnificat. I wish we had them in Manhattan – we do get some kind of hawk/falcon sometimes.
If you know the sad story of the passenger pigeon you might just loose your faith in humanity. 🙁
D.