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Thursday, March 13, 2014

Raptor on Raptor Violence

This is a follow up to my tweet from Floyd Bennett Field this past Saturday about finding a raptor kill site.

During the winter months one of the places that I like to explore around New York City are owl habitats. In other words, coniferous forests. Unfortunately, there are very few pine groves left around the five boroughs and in the coastal areas those pine groves are primarily composed of Japanese Black Pines. In the 20+ years since I've been birding I've watched nearly all those black pines slowly dying off. According to this study by Cornell it is due to the effects of black turpentine beetles, pinewood nematodes, pine engraver beetles, pine voles and cenangium canker. The pine grove at Floyd Bennett Field used to be a reliable location to find overwintering Long-eared Owls, as well as, the occasional Northern Saw-whet Owl and Barn Owl. Back in 1996 Steve Nanz and I spotted a "parliment" of Long-eared Owls here stacked in one pine tree like books on a bookshelve. I also spotted my life Barn Owl here in the early 1990's. Despite the National Park Service's sporatic plantings of native conifers around Floyd Bennett, I estimate that the pine grove at the park's "Ecology Village" will be completely decimated in 3 to 5 years. For now, though, I optimistically check this area throughout the winter, hoping to find a sleeping owl.

Last Saturday was probably my 12th visit to the pine grove since January 1st. There were signs that an owl had been roosting in the pines in the form of whitewash spatter and several fresh pellets. I also had sparks of optimism every time I heard the sound of crows cawing nearby. Still, nothing. My birding buddy Heydi and I frequently joked that the definition of insanity was, "Doing the same thing over and over, hoping for different results".

On Saturday, almost reluctantly, we decided to check the pines one more time. As we approached a spot where we'd recently found a couple of pellets, Heydi gasped. It was almost as if we'd stumbled on a violent crime scene. A large section of the pine needle covered ground was littered with feathers. I've become somewhat accustomed to finding raptor kills, but they are usually piles of tiny passerine feathers contained within a relatively small, confined area. I'd picture in my mind a Sharp-shinned Hawk, Cooper's Hawk or Red-tailed Hawk with its freshly killed prey, plucking the feathers off. It usually doesn't really bother me, but something about this discovery did. Perhaps it was the size of the feathers, the largest of which was about 9" long. It also disturbed me because the trail of feathers seemed to indicate that there was a prolonged struggle. There was one area below a tree stump with a fairly concentrated collection of flight feathers. It appeared that the victim had been carried up to the top of the stump where it was then consumed:

We spent a few minutes searching for a carcass, but there didn't appear to be one. Was it eaten whole? Who was this victim and who was the predator? I decided to collect a selection of wing and tail feathers. That night I would photograph them and email my friend Paul, who is the Collections Manager for the Division of Vertebrate Zoology - Ornithology at the American Museum of Natural History. If anyone would be able to shed some light on this mystery it would be him.

In the meantime, Heydi and I ran down the list of raptors that we either knew or suspected were around Floyd Bennett Field. Of the diurnal raptors there was Northern Harrier, Sharp-shinned Hawk, Cooper's Hawk, Red-shouldered Hawk, Red-tailed Hawk and Rough-legged Hawk. The owl list consisted of Barn Owl, Great Horned Owl, Snowy Owl, Long-eared Owl and Short-eared Owl. Based on the color, pattern and feather size we eliminated from the list Northern Harrier, Sharp-shinned Hawk, Cooper's Hawk, Red-tailed Hawk, Barn Owl and Snowy Owl. I've never looked closely at Red-shouldered Hawk feather patterns, but given that one had been hanging around the area, we thought it was a possibility. I kept going back to Long-eared Owl, though, because the color palette seemed right, and also because this pine grove was, historically, a winter roosting area for them. Neither Heydi nor I wanted to believe it was a long-eared because we'd been trying so hard to find one. It would be like one of those "Good News/Bad News" jokes, only the punch line would really suck. Also, if it was a Long-eared Owl, which predator was large enough and powerful enough to prey on it. A Great Horned Owl, perhaps?

About an hour after I tweeted the photo of a section of the kill site scattered with feathers Paul responded with, "Long or Short-eared Owl". He included a link to the US Fish and Wildlife Service online Feather Atlas. On first look, it seemed likely that the feathers from the consumed bird was a Long-eared Owl, but I wanted to wait until I got home and examined them more closely.

At my apartment I placed a piece of black cloth on my kitchen table and arranged the feathers from large to small with a vertical ruler at the edge, to show scale. The resulting image is similar to the format that the US Fish and Wildlife Service uses for their online Feather Atlas. It also made it very obvious that the bird that was killed and eaten was, indeed, a Long-eared Owl. You can check out the USFWS feathers image here.

During this exercise in wildlife "crime scene" forensics I learned a key feature of owl flight feathers that would have quickly answered the question about whether the prey bird was a hawk or owl. The following is from the website "The Owl Pages" (with a close up of one of the feathers I found):

The most unique adaptation of Owl feathers is the comb-like or fimbriate (fringe-like) leading edge of the primary wing feathers referred to as "flutings" or "fimbriae". With a normal bird in flight, air rushes over the surface of the wing, creating turbulence, which makes a gushing noise. With an Owl's wing, the comb-like feather edge breaks down the turbulence into little groups called micro-turbulences. This effectively muffles the sound of the air rushing over the wing surface and allows the Owl to fly silently. There is also an alternate theory that the flutings actually shift the sound energy created by the wingbeats to a higher frequency spectrum, where most creatures (including prey and humans) cannot hear. Silent flight gives Owls the ability to capture prey by stealth, and also allows the Owl to use its hearing to locate potential prey. This adaptation is not present on some Owl species that hunt in the daytime.

Here is a close-up of the fimbriate at the leading edge of the feather in the above image.

So, who killed and ate the Long-eared Owl? I've ruled out any of the hawks because, as Paul pointed out in his message to me, "I don't think a raptor would pull wing feathers like this? Wings are usually left untouched - no meat". Which leaves another, larger owl. At first I thought that a Great Horned Owl would have to be the culprit because these two birds would be competing for the same woodland prey. Except for two points. To my knowledge, there is only one report of a Great Horned Owl here and in the grove's current declining state, it would stand out like a sore thumb and get mobbed by crows daily. Also, Long-eared Owls only roost in the pine grove. This is also from "The Owl Pages":

Long-eared Owls hunt mainly by ranging over open rangeland, clearings, and fallow fields. They rarely hunt in woodlands where they roost and nest.

There is one, very obvious choice for the "villain" here. The edge of the grassland of Floyd Bennett Field is only about 30 yards away from the kill site. If the Long-eared Owl was hunting over the grassland, it is very likely that it encountered one of several Snowy Owls that have spent the winter on those fields. A hungry snowy will eat pretty much anything from rodents to waterfowl to cats. They will also eat other owls. My guess is that it spotted the long-eared flying over the field and pursued it into the pine grove. It may seem sad, but it's merely part of the survival game that every animal is programmed to follow. Maybe next winter I'll find one in the pine groves.

Here's a photo of one of the members of that parliment of owls that Steve Nanz and I spotted back in 1996:

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