Category Archives: Diversity

Bill overhang in corvids

I hardly notice it when I’m in the field because the bird is either too far or too fussy for me to examine the finer details of its beak, but looking at pictures like this I can’t help but notice that little extra bit of bill that sweeps over this raven’s bottom mandible like an awning. It leaves me wondering, what is this overhang for? A why is it more noticeable on some individuals than others?

When we think of birds with curved upper mandibles we are most primed to think of raptors, which have the most conspicuously curved overbite of any group of birds. This shape give them the tearing power they need to breakdown and consume their prey. But upon closer inspection we can see that actually most birds, even some hummingbirds, have a slight* overbite known as maxillary overhang. Among corvids, common ravens have one of the most noticeable overhangs, though they can be quite visible in crows as well. Unlike raptors though, for most other birds this overbite has less to do with food consumption and more to do with hygiene.1

While they can be good at hiding it under those sleek feathers, wild birds are constantly contending with a suite of exterior parasites like gnats, feather lice, and mites. The presence of these parasites, and the speed at which they can cause real harm if left unchecked, is part of the reason birds dedicate so much time and energy into preening. To remove and kill these pinhead sized parasites, birds carefully comb through their feathers, individually plucking up offenders with their bill tip. Even a small (1-2mm) overhang has been show to provide a significant advantage to birds by providing a backdrop against which the lower mandible can pin and damage parasites. Without such an overhang, parasite loads can be as much as 4x higher. By contrast, the overhang doesn’t seem to play any measurable role in foraging success or efficiency.1

Given the largely carrion based diet of common ravens, it’s possible their more visible overhang provides some advantage when it comes to digging into a carcass. Since it hasn’t been tested, however, I can only guess that if some feeding advantage does exist here, it’s slight. This is supported by looking at other carrion specialized corvids, which don’t seem to show a particular bias in longer overhangs relative to their omnivorous or seed eating corvid cousins, and the fact that even within common ravens overhang length is variable.

This common raven doesn’t have a particularly big overhang

To an extent, the longer the overhang the more effective it can be. As anyone who has tried to maintain naturally long nails can attest though, long keratin based appendages are prone to damage and breakage. Bill tips are no different, which explains why there can be so much variation between and within species. The overhang is playing an evolutionary balancing act to arrive at the length that is the most effective and the least vulnerable to damage. That means that not only will some species have shorter or longer overhangs, but individuals within species may, at any point, exhibit more or less obvious tips as due to recent damage. As a result of this variability, I don’t recommend looking to the level of bill overhang as a diagnostic tool for identifying ravens. While it’s certainly useful for crushing bugs, you can’t bank on it being useful for winning that next round of #CrowOrNo.

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* Some corvids have more than simply a slight overbite. Unusually long or misshapen bill tips can be the result of a viral infection called avian keratin disorder. Check here to learn more about AKD in crows.

Literature cited

  1. Clayton DH, Moyer BR, Bush SE, Jones TG, Gardiner DW, Rhodes BB, and Goller F. 2005. Adaptive significance of avain beak morphology for ectoparasite control. Proc. R. Soc. B 272: 811-817

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Filed under Corvid health, Crow curiosities, Diversity, Ravens

A tale of two crows: northwestern vs. American

If you search nearly anywhere along the west coast from California to southern Alaska, you will find our most persistent avian neighbors: crows.  Cloaked in their Gothic outfits and uttering that all too familiar harsh caw, most people—even many experts—might not register that the neighbors in the north are not exactly like their counterparts in the south.  While it’s the American crows (Corvus brachyrhynchos) that have staked their claim to the contiguous states, it’s the northwestern crow (Corvus caurinus) that calls the coast home from British Columbia to southern Alaska. That is, at least as far as the field guides have been telling us since northwestern crows were first described scientifically in the mid 19th century.  Despite this early recognition that one of these things was not like the other, however, differentiating American crows from northwestern crows on the basis of phenotypic features like size, voice, and behavior has since proven to an almost impossible challenge; especially in places like Washington where the two ranges meet.  This has resulted in questionable hand waving by people like me about what those crows in Seattle really are.  I’ve always called them American crows without any qualifiers, but are they really? Might they be northwestern crows? Or something in between?  Fortunately, a new study by Slager et al. (2020) lays bare the reticulated evolutionary histories of the two crows of the Pacific Northwest.

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By examining differences in both nuclear DNA from 62 specimens and mitochondrial ND2 markers from 259 specimens collected across North America, the team was able to evaluate when these two likely initiated speciation, the process of becoming distinct species from a shared ancestor. What they found is that American and northwestern crows likely split some 440,000 years ago when late Pleistocene glaciers really made mess of things by geographically separating formally intact populations.  Isolated in their respective pockets of livable habitat (called glacial refugia) the formally united species did what all organisms do in the face of new selective pressure: they changed, and from one species emerged two.  Well, kind of.

Eventually of course, the Pleistocene ice ages came to a close and the glaciers that had divided their ancestors receded away.  Although their time apart made a lasting impression on their genome, it did not appear to make a lasting impression on their taste in sexual partners.  The team found extensive genomic admixture (the presence of DNA in an individual that originated from a separate population or species), suggesting pretty pervasive hybridization between the two species.  In fact, along their shared 900 km range from coastal Washington to British Columbia they found not one “pure” individual.  For just how long American and northwestern crows have been hybridizing remains unknown, but the evidence suggests that it’s been happening since well before colonial landscape changes.

These revelations beg two important and contradictory questions.  The first is the answer to just what the hell crows in Seattle are.  The answer appears to be option C: a hybridized mix of American and northwestern crow, but with slightly more all-American genes.  That seems to be true throughout the Washington coast.  Once you hit Oregon though, you’re getting almost all ancestral American crow.  The opposite pattern is true moving from British Columbia north: what starts as hybrids with a stronger northwestern crow bias, are “pure” northwestern crows once you hit Juneau.

SLager

Figure shows the extent of hybridization between American crows (red) and Northwestern crows (blue) along the PNW coast. Image from Slager et al. (2020).

The second question, however, is whether those distinctions are really of any biological value. After all, what appears to have happened is that while these two “species” may have gotten started on a path to different destinies, a changing climate brought them back together before any firm reproductive isolating mechanisms (i.e. physical features, behaviors, or physiology that prevent different species from breeding with one another) could take hold. While that kind of genetic evidence is already pretty damning, the phenotypic evidence that they might be different has likewise eroded.  When closely examined, the features that appeared to be diagnostic in the 19th century like the northwestern crow’s smaller size and intertidal habitat use, and a difference in vocalizations, seem to simply be reflections of local adaptions and individual differences present in both species. So, while the guidebooks might still call them different things, the fact that neither the crows themselves nor the ornithologists can really tell them apart warrants serious consideration of whether northwestern crows should be officially absorbed into the American crow.

As it happens, the authority on such things, the American Ornithological Society’s North and Middle America Classification Committee, is currently examining that very proposal as apart of the 2020 Proposal Set C.  Expect an official ruling soon.  While I don’t know for sure what they will decide, the evidence out of this current study does not bode well for the continued recognition of Corvus caurinus as a speciesSo if you want to see a northwestern crow, my advice is to do it sooner rather than later.

Literature cited

Slager DL, Epperly KL, Ha RR, Rohwer S, Wood C, Van Hemert C, and Klicka J. 2020. Cryptic and extensive hybridization between ancient lineages of American crows. Molecular Ecology doi: 10.1111/mec.15377

 

 

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Filed under Corvid diversity, Crow life history, Diversity, New Research, Science, Taxonomy

Corvid of the month: Rooks

In honor of last week’s #CrowOrNo photo, I wanted to spend some more time spotlighting a corvid perhaps less well known to my fellow North Americans, the rook (Corvus frugilegus).

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Photo c/o Pam P.

Among corvids, adult rooks might be one of the most unmistakable species. Their naked, chalk colored chin, lores, and bill base gives their face an appearance resembling no other bird.  The grey crow, or bare-faced crow (Corvus tristis), shares a similar facial pattern but is easily distinguished by its rather blushing appearance and blue eyes.  In addition, whereas the grey crow is known mainly to the peoples of Papua New Guinea, rooks have one of the widest distributions of any corvid species, breeding from Sweden all the way to China.

(Hover over tiled photos for captions)

Of course, what made last week’s #CrowOrNo submission such a challenge was that the photo was not of an adult, but rather a first year bird.  With their nasal hairs intact, first-year rooks look something like a crow/raven hybrid. Although bill shape is, I think, the best tell, one other field marker to look for are their notoriously shaggy “pants” (belly and leg feathers) in contrast to crows and ravens.

The transition to bare-faced adult occurs during the bird’s first complete body molt when they’re around 10-15 months old.  This process can take as little as 25 days but for most birds occurs over the course of several months1.  The evolutionary reason for this loss may have something to do with their foraging habits, which consists largely of probing for worms, though this remains unclear.

Unlike many Corvus species which are more general with respect to their diet, rooks are fairly specialized to feed on the small worms that live among the roots of plants.  One consequence of this diet is that there are distinct boom and bust seasons.  In April and November, wet conditions make worms plentiful, but in other times of years drier conditions drive worms deeper and out of reach.  Since access to food can be precarious, rooks have adopted a rather unusual incubation strategy compared to most birds.  Rather that commencing incubation when the entire clutch is laid, which promotes the same hatch date, rooks start incubating the first egg as soon as it is laid.  This chick is born earliest, giving it a clear advantage over its future siblings.  If food becomes sparse only this chick will survive. If food remains abundant, the parents can provision enough to supply the larger, more dominant chick, and its younger siblings.2

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Photo c/o Danny Chapman

Rooks are distinct from other corvids with respect to their behavior as well.  In contrast to crows or ravens, rooks are essentially non-territorial.  During the non-breeding season they are most commonly found in large foraging groups (much to the chagrin of local agricultural farmers, I imagine).  During the breeding season they nest in colonies, rather than individual territories, though they will defend the area around their nest and their mate as necessary.  A nest is often reused by the same pair year after year until it is razed by weather, or the pair is forced for some other reason to construct a new nest.  Like many other Corvus species, they maintain a socially monogamous life-long mate.3

Cognitively, rooks demonstrate many of the same skills that have brought some of their peers into the global spotlight. For example, when in the care of humans, rooks have demonstrated an astounding alacrity for tool use, though they are not known for manufacturing tools in the wild. For example, captive rooks have been shown to bend wire into hooks to extract food out of a tube like New Caledonian crows, or work together to solve problems like chimps (though unlike chimps, they do not appear to understand when cooperation is necessary or how it works).4,5

Taken together, these snippets of their biology and behavior demonstrate what unique members rooks are to the Corvus genus.  I envy my counterparts across the Atlantic and Pacific and encourage them to take a second look at the rook whenever opportunities present themselves.

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Photo c/o Paul Wilson

Literature cited

1.  Dunnet GM, Fordham RA, Patterson IJ. (1969).  Ecological studies of the rook (Corvus frugilegus) in North-East Scotland.  Proportion and distribution of young in the population.  British Ecological Society 6: 459-473

2.Green P. (1996). The communal crow.  BBC Wildlife 14: 30-34

3. Coombs CJF. (1960). Observations of the rook Corvus frugilegus in southwest Cornwall Ibis 102: 394-419

4. Bird CD, and Emery NJ. (2009).  Insightful problem solving and creative tool modification by captive nontool-using rooks.  PNAS 106: 10370-10375

5.  Bugnyar T. (2008).  Rooks team up to solve a problem.  Current Biology 18: R530–R532

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Filed under Birding, Corvid diversity, Corvid of the month, Diversity, Uncategorized

15 of the prettiest corvids from around the world

When most Americans think of corvids, the color palette that probably comes to mind is black, grey, blue, white and iridescent.  Together, these colors have certainly assembled a handsome collection of birds, but there’s an awful lot more pigments in the corvid family than that.  Even among those colors, some of the more topical corvids exploit them in dazzling ways.  Unlike American or New Caledonian crows though, these birds have not gotten their fair share of the corvid limelight and it’s time we change that.  The following is a sample of just some of the corvid diversity more folks should know about.  If you really want to have a little extra fun, ignore the scientific names and see if there are any birds you would group together in the same genus (reminder: Family>Genus>Species).  Then you can go back and look at the first part of the scientific name to see if you were right!

These photos were primarily sourced from some of the generous and talented photographers that can be found on Flickr.  Please click on their names to see more of their great work.

1. Common green magpie (Cissa chinensis).  Found in parts of India, China and Indonesia.  A flocking corvid often found low in the canopy where they can be very difficult to spot among the vegetation.  According to the International  Union on the Conservation of Nature (IUCN) their populations are considers stable and of least concern.

2. Taiwan Magpie (Urocissa caerulea).  Endemic to Taiwan, these are flocking corvids that feed mostly from tree canopies.  Their populations are stable.

magpie

Photo c/o Dave Irving

3. Lidth’s jay (Garrulus lidthi).  The only jay endemic to Japanese islands of Amami-oshima and Tokunoshima.  In the fall they can gather in groups as large as 100 birds where they feed mainly on acorns.  Habitat loss and predation have this bird listed as declining and vulnerable.

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Photo c/o Mark Curley

4. Unicolor jay (Aphelocoma unicolor).  Ranges intermittently from western Mexico to El Salvador and may have as many as 5 subspecies! They are considered of least concern, though populations are declining.

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Photo c/o Maynor Ovando

5. Green jay (Cyanocorax yncas). These jays can be found from southern Texas throughout central America and as far south as Peru.  Although they are a solitary breeder, they are very social in the non-breeding season.  Populations are increasing and they are considered of least concern.

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Photo c/o Wade Strickland

6. Spotted nutcracker (Nucifraga caryocatactes).  Native across Europe and Asia, theses birds are monogamous and generally hang out only with their mate.  Like the unicolor jay they are considered of least concern but trends point towards decline.

eurasian-nutcracker

Photo c/o Dave Irving 

7. Ceylon magpie (aka Sri Lanka blue magpie; Urocissa ornata).  Another Sri Lankan endemic.  These birds can be seen in pairs or small flocks where they noisy work the tree canopy.  They are considered vulnerable and declining.

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Photo c/o Dave Irving 

8. Black-throated magpie jay (Calocitta colliei).  This bird is limited to northwest Mexico where it can be found in open woodlands chatting loudly and waiving their tails.  They are common and populations are stable.

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Photo c/o Ahmed Eldaly

9. Turquoise jay (Cyanolyca turcosa).  Can be found in the humid forests of Ecuador, north Peru, and south Columbia.  Overall this species is poorly documented.  Right now it is considered a least concern species but it’s possible that’s due to lack of data.

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Photo c/o Dave Irving

10. Rufous treepie (Dendrocitta vagabunda).  Found across India and southeast Asia.  These birds are generally wary of people but have been known to enter houses to look for geckos.  Would love to find one in my house!  Populations are stable and they are considered least concern.

11.  Gold-billed magpie (Urocissa flavirostris).  Endemic to Taiwan, these birds keep their tails erect when on the ground, possibly to prevent the feathers from being damaged. Populations are stable and they are considered of least concern.

12. Bornean green magpie (Cissa jefferyi).  Found only in Indonesia and Malaysia, these birds look (and act) similarly to the common green magpie.  Relatively little is known about this species as they are hard to access high in the mountain forests.  It’s listed as of least concern.

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13. Eurasian jay (Garrulus glandarius) Found throughout Europe, Asia and Northern Africa.  This bird is common to most birders outside of North and South America.  Although these birds remain with their mates for extended periods they do not keep much contact throughout most of the year. Eurasian jays are in no danger of global population decline.

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Photo c/o Simon Forster

14. Azure-winged magpie (Cyanopica cyana).  Native across Asia and as far north as Mongolia and southern Siberia*. They form small family parties during the breeding season but amass into large groups during the nonbreeding season.  Populations are increasing and they are of least concern.

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Photo c/o Crotach

15. Finally we have, literally, the beautiful jay (Cyanolyca pulchra). Found only in the Andes of Colombia and north Ecuador.  They can be found alone or in pairs but are not believed to interact in large social groups.  Little known about reproduction, social behavior or predators.  Populations are declining and near threatened.

beautiful-jay

Photo c/o Andrew Spencer

Did you pick out a favorite bird?  Let me know in the comments and I’ll try to dedicate a post to it!

Literature cited

*A previous version of this post incorrectly stated that the Azure-winged magpie spread as far west as Spain and Portugal.  Although this was true at the time the reference guide used to write the post was written, since 2002 these western birds have been splint into their own species called the Iberian magpie1.

1Fok KW, Wade CM, Parkin DT (2002). “Inferring the phylogeny of disjunct populations of the azure-winged magpie Cyanopica cyanus from mitochondrial control region sequences.”. Proc. Roy. Soc. B Biol. Sci. 269 (1501): 1671–1678.doi:10.1098/rspb.2002.2057.

All other species descriptions were based on: Madge, S. and Burn, H. 1999.  Crows and jays.  Princeton, New Jersey: Princeton University Press 1999.

 

 

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Filed under Birding, Diversity