Tuesday, February 19, 2008

House Sparrow - "New" for North America?

[Major updates 25 Feb 2008, adding new information about the introduced Siberian populations]

You might think that the words "intriguing" and "House Sparrow sightings" are mutually exclusive, but here's a case where they go together! In October 2007 a flock of five House Sparrows showed up in the coastal village of Shishmaref in northwest Alaska. They were found and photographed by science teacher Ken Stenek, as you can read on his website here. Two males were still present as of mid-Feb 2008, demonstrating how hardy this species can be.

House Sparrow is a very rare visitor anywhere in Alaska, with only a few records in the state. Interestingly, one of the few prior Alaska records comes from Gambell, on St Lawrence Island, in mid-summer about 15 years ago. So the question is whether these birds, at the very western edge of Alaska, came from North America or Asia. North American birds originate from multiple introductions in the eastern states in the 1800s. They spread rapidly but they have not spread beyond southern NWT. In fact, in recent decades the species has been declining across most of its world range.

In contrast to the widespread declines in the developed world, House Sparrows have been expanding their range in East Asia, although this expansion is apparently man-made, involving deliberate transplants and releases of House Sparrows in Siberian cities (fide Dan Gibson). The spread of House Sparrows across North America was "accelerated" in the same way a century ago, and the success of the birds must also depend on development and human land-use changes and possibly also warming climate in recent decades. They are now apparently established in a number of settlements along the Russian coast of the Bering Sea at least as far east as Anadyr (Paul Lehman, Dan Gibson), and it would be very interesting to know whether or not these populations are increasing.

Above is an updated map (25 Feb 2008) of the current range of House Sparrow around Alaska (comments welcome). Shishmaref is indicated by the large red dot. Smaller red dots show Ketchikan in southeast Alaska where there have been five records of House Sparrow, two of those records involve pairs (fide Steve Heinl), and Gambell, on Saint Lawrence Island, where a single House Sparrow was found in summer 1993. Purple dots in Siberia show cities where House Sparrows are known to have been introduced - Magadan, Kamchatskiy, Anadyr, and Providenya (fide Dan Gibson), and presumably they have also been released in other cities and towns.

In North America the nearest population of House Sparrows is in southern NWT, about 1500 miles east-southeast of Shishmaref. In Asia the species occurs "naturally" only as close as the west edge of the Sea of Okhotsk, about 2000 miles west-southwest. Introduced populations are found along the coast farther east, including Anadyr, less than 500 miles west of Shishmaref, and Providenya, less than 100 miles from Gambell, where some were apparently released just weeks before the 1993 record at Gambell (fide Dan Gibson). My earlier assumption that these cities had been colonized naturally is apparently incorrect, and I don't know how healthy those populations are, whether they are increasing or not, or whether any persist in Providenya.

Still, the fact that Asian populations are so much closer argues in favor of an Asian origin for the Shishmaref birds. The water crossing, on the other hand, seems formidable for birds coming from that direction, and makes many people think the birds must have been "assisted" in their trip to Shishmaref.

If House Sparrows were coming to western Alaska from Canada I would expect them to colonize Fairbanks and other interior Alaska cities along the way. (They have not). Maybe these birds came up on a boat, especially given increased shipping traffic with the longer ice-free summers in recent years. If any bird has a chance of being transported by truck, train, or ship, it is the House Sparrow. But then I would expect them to be seen in Juneau, Haines, Anchorage, Fairbanks..., all cities with bustling transportation centers. Why Shishmaref?

The answer to the question of origin could possibly be found in a careful study of the birds' plumage, since North American birds must look slightly different from Asian birds, although all involve the nominate subspecies P. d. domesticus. Or perhaps stable isotope analysis of feathers could reveal the birds' origin. Ultimately, the next decade or so will probably give us the answer. If House Sparrows are increasing in eastern Siberia and begin to colonize other western Alaska towns (without appearing in southern Alaska) that will be strong evidence that they are invading from the west.

For now it's a very intriguing House Sparrow sighting, potentially adding a new species to the list of naturally-occurring birds in North America.


details of North American distribution from Audubon Christmas Bird Count data

Anderson, T. R. 2006. Biology of the Ubiquitous House Sparrow: from Genes to Populations. Oxford Univ. Press. available online here

Wednesday, February 13, 2008

Redpoll age variation and ID

The question of age variation has come up repeatedly in redpoll discussions, so I finally tried to find an answer to the question of just how important age-related variation in plumage might be. And based on published studies the answer is ... not very important.

Seutin et al. (1992) studied redpolls at Churchill, Manitoba using a character index, and found only one statistically significant age-related plumage difference (aside from the obvious red/pink breast of adult males) - flank streaking on adult males was lighter than on any other age/sex class. All other features - undertail covert streaking, rump color, forehead color, poll color, etc. were statistically the same on adult male, immature male, adult female, and immature female. This near-complete absence of age-related variation differs from what Knox (1988) reported - that in European redpolls immatures are darker than adults. I haven't read the Knox paper, so I don't know how different the results actually are, but regardless of what he found in Europe, the paleness of a redpoll in North America does not seem to be linked to the bird's age, and therefore ageing a redpoll is not helpful to the identification process.

The lack of age-related variation does make sense given the lack of differentiation in redpolls at all. Going back to the species question: Seutin et al. analyzed their data on plumage variation by searching each age/sex class for any sign that the distribution of scores was bimodal; e.g., did each age/sex class show any hint of sorting into paler and darker groups on any of the measured plumage characters? Only adult males showed a tendency to sort into two groups, and that was slight. Immature males showed a suggestion of sorting into pale and dark groups only when sophisticated statistical tests were applied to the data. Attempts to sort females of either age class into pale and dark groups failed completely.

Combining this with Troy's similar finding that redpolls form a continuum from pale to dark, and Troy's data showing that intermediate-colored birds were also intermediate in skeletal measurements, implies that observers should expect to find lots of intermediate birds that cannot be confidently assigned to a species.

It's possible that characteristics not measured by Seutin or Troy - like scapular color, or forehead fluffiness, or voice, or something else (?) - might lead to better separation of two groups, but their inability to distinguish two clear groups does not bode well for identification of non-adult-male redpolls.

Monday, February 11, 2008

White-crested Elaenia - new for North America

[edited 12 Feb 2008]
From Texas comes the remarkable news of the discovery of an apparent White-crested Elaenia (Elaenia albiceps) found by Dan and Honey Jones on 9 Feb 2008. Photos, sound recordings, and ID discussion can be found at Martin Reid's website and some great photos at Erik Breden's website. The bird was found at the Sheepshead Woodlot sanctuary (preserved by the Valley Land Fund) on South Padre Island. [Update: after being seen by many on 10 Feb, the bird could not be found on 11 Feb; hopefully it will reappear]

Given that South Padre Island is a barrier island on the Gulf of Mexico, just north of the Mexican border, it has a distinct geographic advantage when it comes to attracting rare birds. But this particular species could have turned up almost anywhere. The fact that it landed just across the Mexican border would seem to be just a fluke (although one can't discount observer effects: birders on the Mexican border tend to be a little more open to these possibilities when an "odd" bird is found). White-crested Elaenia is a South American species, nesting in Chile and Argentina during the southern summer and migrating north to winter (during the southern winter) in the Amazon lowlands. [update: to clarify, this is referring only to the migratory southern "Chilean" subspecies, which may be a different species from the more or less sedentary populations in the Andes farther north. I'm assuming that the Texas bird is from the migratory southern population.] Fork-tailed Flycatcher is another austral migrant, and a more familiar vagrant in the US and Canada.

So let's speculate about some dates: If this is a young bird it most likely fledged sometime around January 2007, about one year ago. Then in April it migrated north to the lowlands of Brazil, where it stayed for the few months of the southern winter. In October 2007 it would have been ready to migrate back to its breeding range in southern South America, and presumably at that point it made a mistake and migrated north instead of south, bringing it to North America. Right now it should be on its breeding grounds, in mid to late summer. The urge to migrate should have waned in December, but maybe this bird found itself in some inhospitably cold place and was forced to move south for food, bringing it to South Padre Island. Or maybe it's been in that area since last fall. Or maybe this bird's clock, along with its navigation system, is screwed up and it's just off on a wild adventure. We'll never know, but given that the species has a range and migratory pattern like Fork-tailed Flycatcher [wrong - see note below], it is plausible that its patterns of vagrancy would be similar, and that suggests the scenario that this individual arrived in North America about three months ago.

The breeding range of this species is centered about 5000 miles (in a straight line) from South Padre Island, which is a remarkable distance. Small changes in direction of flight, or distance, would bring the bird to very different locations. It could just as easily have turned up at any of the Gulf Coast barrier islands, or a desert oasis like Big Bend, and traveling just 20% farther would bring most of the eastern US within its range! Something to watch for....

Note: Thanks to Chris Vogel for pointing out some flaws in my speculation, which reveals my ignorance of South American birds (and a lack of research). The range and migratory patterns of White-crested Elaenia and Fork-tailed Flycatcher are not very similar after all. Here are maps from Natureserve's InfoNatura database showing the ranges - Fork-tailed Flycatcher (left) and White-crested Elaenia (right)

both images © 2007 NatureServe, 1101 Wilson Boulevard, 15th Floor, Arlington Virginia 22209, U.S.A. All Rights Reserved.

White-crested Elaenia nests in the highlands of western South America, Fork-tailed Flycatcher in the lowlands of eastern South America. There is little overlap in breeding range and the migratory patterns must be quite different. Chris stresses the fact that altitudinal movements are a significant part of the Elaenia's migration, from colder mountains to warmer lowlands, while Fork-tailed Flycatcher is a lowland bird all year. Anyway, the main point I was trying to make is that both species are long-distance austral migrants (at least partly) and the Elaenia (like Fork-tailed Flycatcher) could have turned up almost anywhere in North America as easily as at South Padre Island.