Dead Bird Quiz answers

26 09 2014

Another set of unanimous decisions on these three specimens. Edward and James both came back with answers of A) cormorant (probably double-crested); B) Juvenile herring gull; C) no idea/?

That’s where I came down on these as well, but I wanted to post them because they each illustrate one of the challenges in identifying dead birds, even when the carcass is lovely and intact, as in Bird B.

For Bird A, we mainly have the skull, and thus the bill, to go on. The bill is thin, but has a substantial curve, and a prominent hook at the tip. Cormorant does immediately come to mind given these features, but I always try to think “What else could this possibly be?” For Bird A, the only other species group that seemed even remotely possible was the shearwaters. Their bills can have a similar curve and hook. “But wait!” I hear you shriek, “Shearwaters are tubenoses, and I don’t see any tubes on top of that bill!” Well I don’t either, but we must always contend with the possibility that decomposition and general falling apart can alter features rather profoundly. Bird A has lost the keratin sheath that overlies the bones of the bill during life. That sheath sloughs off rather readily as the carcass weathers, and if the tubes on the noses of the tubenoses were only a feature of the bill sheath, and not the underlying bone, then we might indeed see a skull that looks like this.

This image of the skulls of both an extinct (top) and extant species of shearwater from a paper by Ramirez et al shows the general features of shearwater skulls when the bill sheath is no longer present.

extinct Lava shearwater skull (top) and Manx shearwater skull. (from Ramirez et al, 2010.)

extinct Lava shearwater skull (top) and Manx shearwater skull. (from Ramirez et al, 2010.)

In these images, you can see that the overall curve of the bill is not as great as in our Bird A, and you can see distinct nares (nostrils) atop the bill, though the distinctive tube structures are indeed, far less evident without the sheath in place. Cormorants, on the other hand, have a rather different looking skull:


Note the lack of evident nares in this cormorant skull. (Photo: Dominique Harre-Rogers, copyright Smithsonian Institution)

In this photo from the Smithsonian Institution, it’s clear that there is no rise in the bone at the level of the nares as there is in the shearwaters. In fact, it’s hard to really see the nares all that much at all here. This is a feature of pouchbills like cormorants and gannets; in fact,¬†they lack external nares entirely. We who handle the birds live must always pay attention to how we restrain the bill since, if we hold the bill closed entirely, the bird has no way to breathe.

All this explanation is really just to tell you what you all apparently already knew: Bird A is a cormorant. It’s difficult to say which species, since an accurate culmen length relies on the bill sheath being intact and seeing where it meets the feathers or facial skin. Not possible here, for obvious reasons. Based on their ubiquity, and the overall shape of the skull, Double-crested is the more likely.

Bird B was beautifully intact, so it may seem strange that I used it here as a challenge. But there were features on this carcass that I think could throw some people off. Everyone concurs that this is a juvenile Herring Gull. mlyons6759-15258-1Indeed, there are many features that would lead us there: the overall gray over the belly and underwings are characteristics of young Herring Gulls that differ from Great Black-backed, Ring-billed, and Laughing Gulls, the three most likely alternatives. So why not just call this a Herring Gull and move on? The color of the legs and the color of the mantle (the back between the wings) caught my eye in this bird. The legs look reddish to me, which makes me think of Laughing Gulls as they have reddish black legs as adults. Our Bird B also has quite a distinct brown pattern to the mantle, where most of the juvenile Herring Gulls we see have more of a grayish wash. The Larusology blog does a good job helping people identify gulls, and this post about Herring vs. Thayer’s gulls points out that this phase is a normal one in young gulls–the feathers over the mantle are large and have a scaled appearance, while the wings have a smaller pattern that looks more checkered, very much like our Bird B.

Overall, the weight of the evidence falls on the side of Herring Gull, especially the conspicuous features like the color of the rump at the tail base. In young Herring Gulls, this is dark, where in Laughing Gulls, Ring-billed and Black-backeds, it’s white. None of those species have this much gray and brown over the entire breast, belly, and under the tail. So we’re left with that strange reddish color to the legs. My only guess is that they usual pinkish color has been altered by some trick of the light in combination with the effects of desiccation. In any case, this bird didn’t fool anyone, apparently, and really only had me wondering what was up with it. I’m always hoping to find some weird gull hybrid too, so maybe I see oddities where they are not actually present.

Finally, for Bird C, I agree with our respondents: no way to tell from what we have, which is a rather shredded wing missing a bunch of its coverts. “Unknown bird” it is. You can’t win ’em all.


More on gillnets: an inadvertent experiment

29 05 2013

Coming close on the heels of last week’s post about estimated worldwide seabird bycatch in gillnets, an experiment that basically did itself. Paul Regular, William Montevecchi and several colleagues have a paper out in Biological Letters examining the effect of the closure of two major Canadian fisheries on murre populations. In 1992, cod and salmon gillnetting was effectively closed down in Atlantic Canada. This resulted in the removal of tens of thousands of gillnets from the foraging waters of seabirds like murres and razorbills, which are heavily impacted by bycatch in these nets. What followed was thus a kind of de facto experiment on the population impact of removing bycatch mortality for these birds. The study authors looked at impacts not only on birds likely to be entangled in gillnets, but also surface feeding seabirds like gulls which are comparatively unlikely to suffer that fate.

Populations of Herring Gulls and Common Murres before and after gillnet moratorium (Regular, et al.)

Populations of Herring Gulls and Common Murres before and after gillnet moratorium (Regular, et al.)

Using both bycatch data and breeding colony census information, the paper tracks the populations of the various species over the decades since the fisheries closures. Data was sufficiently robust to look closely at two species: Common Murres and Herring Gulls. The researchers found a dual and approximately opposite effect on the two species. Murre numbers climbed substantially once pressure from gillnets was removed, but Herring Gull numbers showed a decline. This is likely due to the withdrawal of offal and other fisheries waste when the gill nets were hauled up and stowed away in 1992. Opportunistic feeders, gulls feed heavily on discarded bait and other material tossed overboard by fishermen. As those anthropogenic food sources become less available, gull survival declines. The same phenomenon seems to have followed the closure of many open landfills after the 1970s. The declines in breeding gull populations over the past 30 years seem to reflect a “correction” of sorts–a return to baseline numbers in the absence of man-made buffets. Simultaneously, more specialized feeders like murres have benefited not only from closure of some fisheries, but also from stricter laws governing the discharge of petroleum products by ships off the coasts of the U.S. and Canada.

The closure of the cod and salmon gillnet fisheries in Canada afforded this opportunity to study the effects of bycatch pressure on seabirds, and it appears to be substantial. Fisheries don’t generally remain closed forever though, and if and when the salmon and cod fisheries off the Canadian Maritimes do reopen, the authors of the study point to potential changes in methods to continue to protect murres and other diving birds. One such possibility is cod pots, which operate similar to lobster traps, sitting on the ocean bottom with bait inside. Fish swim in and can’t swim out. This type of gear is highly unlikely to entrap seabirds, though I have known lobstermen who reported finding Common Eiders in their traps from time to time, so never say never.