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Obscure, extravagant tropical crows

The views expressed are those of the author and are not necessarily those of Scientific American.


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House crow (at left) by Priyanka Bansal; Tufted jay (at right) by Pete Morris.

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The word ‘crow’ typically conjures up an image of a reasonable large, typically black, typically unadorned passerine bird. Crows of this kind occur just about worldwide with the exception of South America and Antarctica – they’re very successful birds. But far less well known is that there is also a really interesting assortment of brightly coloured, often fairly extravagant, tropical corvids. Sure, everyone really interested in birds knows about the species concerned, but this article isn’t necessarily written for those people.

Here I want to focus very briefly on just three of the world’s obscure, extravagant tropical crows. All are American. Being mid-sized (for crows), crested, fairly long-tailed and with non-black plumage, they’re popularly known as jays. This is a vernacular term, not a taxonomic one.

Plush-crested, Curl-crested and Tufted jays

Photo by David Monniaux, from wikipedia.

We begin with the rather poorly known Plush-crested jay or Plush-capped jay Cyanocorax chrysops of south-eastern Brazil as well as north-eastern Argentina, Bolivia, Uruguay and Paraguay. It’s named for its dark, velvety head crest. Its violet-blue upperparts contrast with its creamy-white underparts. Like most forest-dwelling, tropical crows, its wings are very broad and its tail is long. The Plush-crested jay is best known for foraging in the canopies of rainforests, though it also descends to the ground, forages at intermediate levels in the forest, and can sometimes be found in forested islands on the pampas and in other wooded habitats. It moves around in small groups, sometimes associating with Purplish jays C. cyanomelas and Azure jays C. caeruleus (Madge & Burn 1999). It’s omnivorous, eating fruits, berries, bird’s eggs, arthropods and other items.

As is so often the case with bird species, some Plush-crested jay populations differ enough in appearance to perhaps warrant the naming of subspecies. C. c. tucumanus of north-eastern Argentina is apparently larger than the nominate C. c. chrysops while C. c. diesingii has a longer, more upright crest than C. c. chrysops and a somewhat different pattern of facial markings. A form named for a single specimen – C. c. interpositus – might be a hybrid between a Plush-crested jay and White-naped jay C. cyanopogon (Goodwin 1986).

Photo by Maria Bedacht, from wikipedia.

Another South American jay with an unusual feathery crest is the highly distinctive (but also fairly poorly known) Curl-crested jay C. cristatellus. It inhabits the tablelands of central Brazil and adjacent north-east Paraguay and is often imagined to be limited to a kind of woodland called ‘cerrado’. Here, short, gnarled trees grow within a savannah-style grassland. However, there’s at least one observation of a Curl-crested jay in dense riparian forest (Hardy 1969). Again, it moves in flocks: it’s almost certainly a social breeder but nothing much is known of its social behaviour or breeding biology (Goodwin 1986). Its crest consists of long, erect, stiff, curved plumes that grow amid shorter bristles. It possesses a wholly black ‘hood’ and, elsewhere on its body, a combination of pure white and violet-blue plumage.

Tufted jay, photo by Pete Morris.

One of the most striking and charismatic of tropical crows is the Tufted jay or Painted jay C. dickeyi of the Sierra Madre Occidental in western Mexico [adjacent photo from BirdQuest]. An inhabitant of both deciduous and evergreen woodlands in mountainous regions, it’s absolutely unmistakeable, with a black, fan-shaped crest of stiff bristles, white nape and white terminal half of tail. Given its remarkable appearance, it’s surprising that it was only discovered in 1934 (and named the following year).

Like other Cyanocorax jays, the Tufted jay is a generalist, eating fruits, berries, acorns, arthropods and occasionally bird’s eggs and even nestlings. It reportedly tears open bromeliads, though whether this is because it’s searching for animal prey or for stored food caches isn’t entirely clear (Goodwin 1986). Flocks consists of between four and 16 birds, with only a single pair nesting at a time and the rest of the flock helping by collecting nesting material as well as food for the incubating female and her chicks. It has an extremely diverse vocabulary, indulges in a great deal of mimicry of other bird species and has been reported to make all kinds of chattering noises as well as metallic, clicking and hooting calls. Its restricted range and social breeding make it vulnerable to habitat destruction and it has sometimes been referred to as “distinctly uncommon” (Madge & Burn 1999). “Intensive collecting” has also led to its local extinction in some places (Goodwin 1986). I wonder whether this ‘collecting’ was for taxidermy specimens or live ones destined for captivity.

A tree for New World jays

All three species covered here are members of Cyanocorax, and the Plush-crested and Tufted jay at least may be very close relatives within this clade. All Cyanocorax jays have fairly prominent ‘frontal tufts’ of some sort, are generally heavily built, and are mostly lowland crows with a co-operative breeding habit. Exactly how these striking Neotropical corvids are related to other members of the crow family, however, is open to debate, and various different positions have been proposed.

Jaw articulation in (above) a magpie (Pica) and a New World jay (Aphelocoma). Note the special 'buttress complex' features in the New World jay. From Zusi (1987).

The Cyanocorax species share blue feathering, frontal crests and various voice characteristics with Cyanocitta (the Blue jay and Steller’s jay), and they’ve typically been linked with the scrub jays (Aphelocoma), the Andean Cyanoluca jays, the Central American Brown jay Psilorhinus morio, and the magpie-jays Calocitta, many of which are also bluish and crested. Together, these six genera form an assemblage known as the New World jays; the distinctive Piñon jay Gymnorhinus cyanocephalus is likely also a member of the group, but it’s regarded by some authors as closer to such Old World corvids as nutcrackers. All New World jays share a distinctive and specialised lower jaw morphology where an extra convexity near the jaw joint, a knob on the quadrate bone and a ‘chisel-shaped’  mandible tip form a functional complex that apparently allows them to exert extra force when breaking into hard objects with the jaw tips. This set of features was termed the buttress complex by Zusi (1987) and it isn’t seen in other corvids.

Because the majority of corvid lineages are Old World denizens, it has generally been thought that New World jays descend from a jay-like corvid that invaded the Americas via Beringia. The Perisoreus, which are ancestrally Eurasian but with one American species [the Siberian jay P. infaustus is illustrated below] , have been argued by some authors to be the most likely near-ancestors for New World jays, and indeed Espinosa de los Monteros & Cracraft (1997) recovered Perisoreus as the sister-taxon to the New World jay clade.

If this phylogenetic hypothesis is correct, a Perisoreus­-like jay presumably arrived in North America during the Early Miocene or so, its descendants spreading south and radiating in South American after the formation of the Panamanian isthmus. The idea of a Miocene invasion in part comes from Brodkorb’s (1972) description of the Late Miocene jay Miocitta galbreathi from Colorado. It shares various similarities with extant New World jays.

Siberian jay (Perisoreus infaustus), as illustrated in a 1905 volume by Johann Friedrich Naumann.

In an effort to resolve affinities within the New World jay clade, Espinosa de los Monteros & Cracraft (1997) examined cytochrome b and morphological data. They found Cyanocorax to form a clade with the magpie-jays. The Cyanoluca jays formed the sister-group to all other New World jays. Cibois & Pasquet (1999) analysed mtDNA from a diversity of crows and found Cyanocorax to group together with Cyanocitta (the Blue jay and Steller’s jay). However, this Cyanocorax + Cyanocitta clade was the sister-group to the rest of Corvidae with the exception of choughs (they occupied an even more distant position relative to the remainder of Corvidae), and it had no close affinity with Perisoreus or indeed any other jay-like corvids. This is a peculiar result as it means that New World jays diverged comparatively early on in corvid evolution; it also means that they have no close affinity with any of the Old World jays. You might like to wonder what the New World jay ancestor looked like in this scenario: they’re surrounded by choughs on the one hand, and the large clade that contains nutcrackers and true crows, and magpies and Old World jays, on the other.

Brown jay (Psilorhinus morio)

Saunders & Edwards (2000) found Cyanocorax to form a clade with the Brown jay and the magpie-jays [Brown jay in adjacent photo; image from BIRDseeN]. In this study, Cyanocitta formed a clade with the scrub jays. The idea that the Brown jay might be close to Cyanocorax isn’t as peculiar as it might seem – despite its brownish plumage it has sometimes even been classified as a species of Cyanocorax and at other times said to be especially close to Cyanocitta. In fact, there’s even an alleged Cyanocitta x Psilorhinus hybrid on record (Pitelka et al. 1956). An interesting peculiarity of the Brown jay is the bare, inflatable patch on its chest, purported to aid in temperature control. Incidentally, the phylogeny recovered by Saunders & Edwards (2000) suggests that co-operative breeding was likely primitive for the whole of the New World jay clade, but that it has been lost by Cyanocitta and in some taxa within Aphelocoma. As goes the origins of the New World jay clade, Saunders & Edwards (2000) again found Perisoreus to be the sister-group to the whole clade, and their results thus support the idea of a single invasion across Beringia.

Corvid phylogeny from Ericson et al. (2005).

Ericson et al. (2005) compiled phylogenies using data from several different genes. They mostly found Cyanocorax to group together with the magpie-jays – sometimes with the Brown jay as closer to the magpie-jays than to Cyanocorax. But, like Cibois & Pasquet (1999), they didn’t find Perisoreus to be at all close to the New World jays. Rather, they tended to recover a polytomy where New World jays, true crows and nutcrackers and Garrulus jays, magpies, and Perisoreus jays all diverged rapidly from an Old World ancestor. In many of these clades, the most basal positions are occupied by south-east Asian taxa. It seems that, while many of the crow groups radiated extensively in Eurasia from this south-east Asian area of origin, several moved to the Americas, with the New World jay clade apparently doing this earliest and most successfully.

I’ll definitely be coming back to corvid diversity, phylogeny and history some time. So much for a brief article on just those three species.

For previous Tet Zoo articles on crows and other ‘core corvoids’, please see…

And for more on passerines of all kinds, check out…

Refs – -

Brodkorb, P. 1972. Neogene fossil jays from the Great Plains. The Condor 74, 347-349.

Cibois, A., & Pasquet, E. (2008). Molecular analysis off the phylogeny off 11 genera off the Corvidae Ibis, 141 (2), 297-306 DOI: 10.1111/j.1474-919X.1999.tb07552.x

Ericson, P. G. P., Jansén, A.-L., Johansson, U. S. & Ekman, J. 2005. Inter-generic relationships of the crows, jays, magpies and allied groups (Aves: Corvidae) based on nucleotide sequence data. Journal of Avian Biology 36, 222-234.

Goodwin, D. 1986. Crows of the World. Trustees of the British Museum (Natural History), London.

Hardy, J. W. 1969. A taxonomic revision of the New World jays. Condor 71, 360-375.

Madge, S. & Burn, H. 1999. Crows & Jays. Christopher Helm, London.

Pitelka, F. A., Selander, R. K. & Del Torro, M. A. 1956. A hybrid jay from Chiapas, Mexico. Condor 58, 98-106.

Saunders, M. A. & Edwards, S. V. 2000. Dynamics and phylogenetic implications of mtDNA control region sequences in New World jays (Aves: Corvidae). Journal of Molecular Evolution 51, 97-109.

Zusi, R. L. 1987. A feeding adaptation of the jaw articulation in New World jays (Corvidae). The Auk 104, 665-680.

 

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Allen Hazen 6:44 pm 09/3/2011

    A recent popular book about oak trees (“Oak: The Frame of Civilization” by William Bryant Logan — a fun read, but it sounded as if a lot of the technical biology in it might be second-hand) suggested that there are a number of species of “jays” which specialize in acorns, and that they might have been essential to range-extension of Quercus after the ice age: they cache acorns, and fail to recover a fairly high proportion.

    1) Assuming this is right, and that it is right in both Eurasia and North America, I had guessed when I read it that there might be a clade of “oak planting corvids,” but it sounds as if this “symbiotic relationship” of oaks and medium-size corvids had to evolve separately in the two regions.

    2) But it DOES make sense of the “buttress”: for a bird the side of a jay to deal with acorns, it’s got to have some beefing up of the jaw area!

    Link to this
  2. 2. Andreas Johansson 8:54 am 09/4/2011

    Any idea why “standard” black corvids may be absent in S America?

    Link to this
  3. 3. Jerzy New 9:28 am 09/4/2011

    Next article about obscure, extravagant tropical mice: Capybara!

    Link to this
  4. 4. naishd 9:30 am 09/4/2011

    That’s a good question. I suppose one possibility is that members of the New World jay complex got there first and filled up the available niches. Any better ideas? There are Corvus crows in the Caribbean and in the northern part of Central America, so it does seem odd that they ‘stopped’ somewhere round about Nicaragua. Note that there aren’t any New World jays in the Caribbean, where there are Corvus crows.

    Darren

    Link to this
  5. 5. Jerzy New 9:34 am 09/4/2011

    @Allen Hazen
    Eurasian Jay is well known for spreading acorn since centuries. No doubt it helped oaks spread after the Ice Age. Here we come to interesting phenomenon: plants and animals spread northward after the Ice Age hundreds of times faster, than their dispersal abilities suggest. Not just oaks which may rely on Jays. Most probably, plant and animal dispersal is determined by freak, extreme events, like floods, gales etc., which occur less than once in human lifetime.

    @Andreas Johansson
    Dunno, but their niche is filled by eg. caracaras and medium-sized parrots.

    Link to this
  6. 6. BrianL 10:00 am 09/4/2011

    @Jerzy
    What parrots in South America take over a crow like niche? Not saying you’re wrong, but I can’t think of a single Neotropical parrot that is definately crow-like in ecology.

    Link to this
  7. 7. vdinets 12:36 pm 09/4/2011

    Crows coexist with jays over much of N and C America, even though jay diversity is higher in C America than in S. It could be that crows’ presence in the New World is relatively recent, so they simply didn’t have time to colonize S America, but I doubt it since N American crows seem to be well-differentiated (the two Mexican spp. look very distinct).
    Another interesting thing is that Corvids seem to have crossed to the New World many times. Let’s count: the ancestor of New World jays, the ancestor of the Canadian Jay, nutcrackers (twice, the first crossing producing the Pinyon Jay, the second producing the American Nutcracker), and Corvus spp. (probably four times, as American crows seem to be polyphiletic and so do ravens). That’s eight times, and you have to add recent appearances of vagrant European species. It is, indeed, strange that only one group made it to South America.

    Link to this
  8. 8. Jerzy New 12:41 pm 09/4/2011

    @BrianL
    Crows Corvus are not only carnivores, much of their diet is in fact seeds and nuts. Medium sized parrots, like Aratinga conures adapted to open habitats, fill this niche nicely.

    Link to this
  9. 9. naishd 2:14 pm 09/4/2011

    Vlad (comment 7): yes, you have to posit several separate American corvid invasions, whatever the phylogeny. I can see evidence for at least seven migrations, but they’re not exactly the same as the ones you posit: (1) New World jays, (2) Perisoreus jays, (3) Pica magpies, (4) nutcrackers, (5) an early Corvus invasion that led to the endemic American species, (6) the Northern raven C. corax population that includes Californian and Chihuahuan ravens, and (7) the Northern raven population that includes all other North American Northern ravens. In addition to the articles cited above, see Omland et al. (2000) and Haring et al. (2007).

    The balance of evidence indicates that (as noted in the article above) the Piñon jay Gymnorhinus cyanocephalus is part of the New World jay clade, not a close relative of any of the Old World lineages.

    Refs – -

    Haring, E., Gamauf. A. & Kryukov, A. 2007. Phylogeographic patterns in widespread corvid birds. Molecular Phylogenetics and Evolution 45, 840-862.

    Omland, K. E., Tarr, C. L., Boarman, W. I., Marzluff, J. M & Fleischer, R. C. 2000. Cryptic genetic variation and paraphyly in ravens. Proceedings of the Royal Society of London B 267, 2475-2482.

    Darren

    Link to this
  10. 10. vdinets 5:52 pm 09/4/2011

    Oh, I forgot the magpies!

    Link to this
  11. 11. rafmarcondes 8:12 pm 09/4/2011

    Nice post! As a Brazilian ornothologist, I really enjoy when neotropical animals are featured in TetZoo.

    Let me add that Cyanocorax cristatellus used to be endemic to the vast Brazilian Cerrado biome, but with the widespread deforestation of the coastal Atlantic Forest, it has spread to areas formerly occupied by forest, expanding its distribution. It is now very common in almost the entire São Paulo state and is found even in Rio de Janeiro, both of which were mostly occupied by Atlantic Forest. Check out this map: http://www.wikiaves.com.br/mapaRegistros_gralha-do-campo (Cerrado in pink, Atlantic forest – or former forest – in green)

    Link to this
  12. 12. Allen Hazen 8:25 pm 09/4/2011

    Jerzy New (#5)–
    The Logan book made the point that “coopting” jays to plant their seeds had allowed the oaks to spread far more rapidly than they would have been able to without animal assistance. (As seeds go, acorns must be among the least adapted to dispersal by wind!)
    As i said, it’s a fun read, even if it has some weaknesses. Sort of tries to do for oak trees what Mark Kurlansky did for codfish, with I think a bit more scholarship.

    Link to this
  13. 13. Dartian 3:25 am 09/5/2011

    The question why the otherwise cosmopolitan Corvus is absent from the South American continent is indeed a good one. I am highly sceptical of the idea of competitive exclusion, at least on a continental scale (small oceanic islands might be another matter). The various ‘jays’ tend to be adapted to forested environments, whereas most Corvus species typically prefer more open habitats – I don’t really see much potential for niche overlap there. I also fail to see how New World caracaras would be significantly more serious competitors to Corvus than (say) Old World kites, which corvids are quite capable of coexisting with. Nor am I convinced that any Corvus species does/would really compete with parrots to any substantial extent; as is the case with the jays, most Neotropical psittacids too live mostly in forested areas. (In this context it’s notable that several species of Corvus are in fact present in the one place where psittacids have truly successfully colonised even savanna-semidesert habitats – namely, Australia.)

    To look at it from a different angle, Corvus species are able to coexist with other adaptable, mid-sized omnivorous birds in other parts of the world: for example, with hornbills in Africa, with currawongs & Australian ‘magpies’ in Australasia, and with gulls almost worldwide (including cities and other urban areas). If they can handle that kind of competition, surely they could handle caracaras too?

    I would guess that the real reason why Corvus is absent from South America is because of dispersal barriers in the form of unsuitable habitat in the northern parts of the continent. Tropical rainforest does not seem to be ideal habitat for Corvus; the taxon is almost or completely absent from African and Asian rainforests. Perhaps this is what has prevented these birds from spreading to more suitable habitats in the semitropical and temperate southern parts of South America? (I realise that this explanation isn’t entirely satisfactory either; the extent of tropical rainforest coverage has varied a lot throughout the Neogene and surely there must have been times when suitable dispersal ‘corridors’ existed? Still, I much prefer this explanation to one involving inter-taxon competition.)

    Link to this
  14. 14. naishd 4:51 am 09/5/2011

    Response to Dartian: we should be sceptical about any claims of competitive exclusion among these corvid groups, for sure. But it’s not true that New World jays won’t overlap with Corvus crows because the former are forest-adapted – on the contrary, New World jays are unlike most Old World jay-like forms in including open-habitat forms. As per comment 11 above, note the spreads of the Curl-crested jay following the removal of dense forest and its replacement by cerrado. On the spread of Corvus, these birds have done fine at colonising tropical forest in the Old World – these are various forest-dwelling Corvus crows in India, south-east Asia, the Philippines and Australasia (e.g., Jungle crow C. levaillantii, Grey crow C. tristis on New Guinea). And it’s not an ‘Old World only’ thing, since various of the New World Corvus species (e.g., those in the Caribbean) have also done fine at penetrating tropical woodlands.

    Anyway, some data suggests that Central America and parts of Amazonia were not heavily forested during parts of the Pleistocene but instead dominated by savannah – if this is true, it makes it even harder to understand why Corvus crows didn’t make it at all the way to South America. In short, it’s hard to believe that habitat requirements somehow prevented Corvus from making it all the way south.

    Darren

    Link to this
  15. 15. Dartian 6:09 am 09/5/2011

    Darren: For the record, I didn’t say that there is no niche overlap between New World ‘jays’ and crows. I said that there doesn’t seem to be much overlap (as in: not enough overlap between jays and crows for the former to prevent the latter from colonising). Those are two rather different things, at least from an ecological point of view.

    Also for the record: ‘tropical forest’ is not necessarily the same thing as ‘tropical rainforest’. Ecologically speaking, there is a big difference between, say, an Asian monsoon forest and an Amazonian old-growth rainforest.

    Incidentally, are any Corvus species known in the South American fossil record?

    Link to this
  16. 16. naishd 6:32 am 09/5/2011

    Sure, I know all that :) But not only are Corvus crows apparently capable of penetrating tropical rainforest (e.g., Grey crow on and around New Guinea), the range of forested habitats available across Central and northern South America is diverse – there’s everything in the Atlantic Forest from low coastal forests, dry forests and deciduous forests to mangroves and moist, evergreen forests. Again, I can’t see an obvious biome-related reason why Corvus crows couldn’t have invaded some or all of southern Central America and South America.

    I checked, and can’t find any records of fossil Corvus from South America. Things would be more interesting if they had successfully invaded in the past :)

    Darren

    Link to this
  17. 17. Zoovolunteer 6:50 am 09/5/2011

    Are there any recent records of potential colonising Corvus sp in South America? At least some species have spread via ship-assisted passage well outside their natural range (e.g. House Crow C.splendens in the Netherlands). I would have thought at least some American Crows might have travelled south on shipping.

    Link to this
  18. 18. naishd 7:15 am 09/5/2011

    House crows have reached Chile, but they failed to breed there. See…

    Nyári, Á., Ryall, C. and Peterson, A. T. 2006. Global invasive potential of the house crow Corvus splendens based on ecological niche modeling.  Journal of Avian Biology 37, 306 -311.

    Darren

    Link to this
  19. 19. vdinets 10:54 am 09/5/2011

    Dartian: the northern coast of Colombia and Venezuela, where Caribbean crows would have arrived, is mostly dry tropical forest or savanna; some places can even be called desert. It is extremely similar to Caribbean islands themselves.
    I can’t think of any ecologic barriers for southward spread in Mexico, either, especially considering that there’s plenty of mountains and a lot of northern species have followed them all the way down to Honduras or even Colombia.
    The only possible explanation I can think of is some unknown pathogen.
    On the other hand, a lot of things are missing from all or parts of South America for no obvious reason. Why are there no shrews or Caenolestids in the Amazon, for example?

    Link to this
  20. 20. Dartian 3:10 am 09/6/2011

    Vladimir: “the northern coast of Colombia and Venezuela, where Caribbean crows would have arrived, is mostly dry tropical forest or savanna

    Yes, but note that in comment #13, I was focusing on the absence of Corvus from the semitropical and temperate, seemingly-ideal-for-crows southern parts of South America. In order to get there, the crows would still have to have gotten through or by the ecological barrier that is the Amazonian rainforest.

    (Of course, the absence of Corvus from northern South America admittedly remains unexplained. Unless, of course, Corvus was present there once but then went extinct – because of loss of suitable habitat during Pleistocene rainforest expansion, perhaps?)

    Darren: Since you seem to prefer sticking to the idea that Neotropical crows couldn’t compete with Neotropical jays, I’ll badger you about that a bit more… How do you explain that the various magpies and treepies (Cissa, Urocissa, Dendrocitta, etc.) that live in the tropical parts of Asia have not been able to prevent Corvus from dispersing widely in that region? I find it rather hard to believe that crows can’t compete with jays if they can compete with magpies.

    Link to this
  21. 21. Jerzy New 7:17 am 09/6/2011

    @Recent news of Philippine salty
    http://www.bbc.co.uk/news/world-asia-pacific-14800269

    What about a review of tales of 10-m crocodiles?

    Link to this
  22. 22. naishd 7:42 am 09/6/2011

    Dartian (comment 20): ha ha, I’m not “sticking to the idea that Neotropical crows couldn’t compete with Neotropical jays”, I’m merely pointing to the fact that there are problems in relating Corvus absence in South America to ideas about biomes or habitats. As I said above, “we should be sceptical about any claims of competitive exclusion among these corvid groups, for sure”. And I’m wary of seeming to promote the idea that competitive interactions might occur across clades – interactions really occur, of course, at the species or population level.

    Nevertheless, why (potentially) could Corvus crows manage alongside Old World magpies, treepies etc. but not against New World jays? Well, here are two hypotheses. (1) Old World Corvus crows evolved at about the same time and in about the same place as magpies, treepies etc. – the respective groups thus differentiated eco-morphologically right from the start to avoid competition (note that, as a generalisation, magpie-like corvids generally do more arboreal foraging for arthropods than Corvus crows, with more terrestrial Pica being a bit of an exception). (2) Unlike magpies, treepies etc., New World jays may have a competitive advantage when it comes to the handling of hard-shelled mast, thus making it more difficult for Corvus crows to eke out a living in those habitats where other resources are not widely available.

    These are just ideas, and they’re iffy, but this has been fun to think about.

    Darren

    Link to this
  23. 23. Dartian 10:23 am 09/6/2011

    Darren: “this has been fun to think about

    Agreed. Thanks for the responses.

    One final(?), general thought on the global distribution pattern of Corvus:
    It would be interesting to know to what extent human beings and their activities have facilitated the dispersal of these birds over the last centuries and millennia. Many crow species willingly, or even preferably, associate with people (even though the affection is often decidedly one-sided) and thrive in human-altered habitats such as farmland, pastures, cities, etc. Some, such as the jackdaw Corvus monedula and the rook C. frugilegus, rarely even live “in the wild” anymore, at least not in the Western Palearctic. Other crow species may be less intimately associated with people but even so, I think it’s safe to say that many Corvus species have greatly benefited from human activities. Thus, it may be that Corvus, generally speaking, is today more widespread than it’s ever been throughout its evolutionary history.

    Link to this
  24. 24. John Harshman 11:41 am 09/6/2011

    So here’s one I’ve always wondered about: why are there many Turdus species all over the Old World, and many likewise all over Central and South America, but only one north of Mexico?

    Link to this
  25. 25. naishd 12:09 pm 09/6/2011

    John – have you seen…

    Voelker, G., Rohwer, S., Outlaw, D. C. & Bowie, R. C. K. 2009. Repeated trans-Atlantic dispersal catalysed a global songbird radiation. Global Ecology and Biogeography 18, 41-49.

    Darren

    Link to this
  26. 26. vdinets 1:07 pm 09/6/2011

    Dartian,
    “In order to get there, the crows would still have to have gotten through or by the ecological barrier that is the Amazonian rainforest.”
    That’s what the Andes are for. Note also that the Andes above the timberline form an almost-unbroken chain of Corvid-free habitat from Venezuela to Terra del Fuego.
    I don’t think Pleistocene rainforest expantion eliminated arid coastal habitats in Venezuela and Colombia. There’s plenty of plant and animal endemism in the area.
    Jerzy: the issue of maximum croc size has been discussed to death on croc forums. Some people still say that in good old pre-hunting days there were 10-m monsters, but I find the lack of any hard evidence hard to explain. Even fast-growing porosus x siamensis hybrids don’t exceed 7 m. I think 7 m is the maximum.

    Link to this
  27. 27. John Harshman 1:36 pm 09/6/2011

    Re: Voelker et al. 2009.

    Nice. Doesn’t resolve the question, but it’s a partial explanation. Quote: “The lack of North American colonization from the Central American–Caribbean clade is also relevant here, in that many regions of southern North America were not substantially affected by glacial cycles. This indicates that colonization and speciation should have been possible for millions
    of years, at least in the southern United States. Yet, excepting the very recently derived but distributionally ubiquitous migratorius, North American colonization is simply not evident in Turdus.” So apparently T. migratorius is a very recent immigrant, which might explain why it hasn’t diversified. But what kept Turdus out of the region until so recently? Dispersal from the Caribbean to the mainland (but not the north) is apparently ancient. Still a puzzle.

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  28. 28. vdinets 7:49 pm 09/6/2011

    John: may be they did colonize North America, but we call them Catharus and, recently, Hylocichla?

    Link to this
  29. 29. Dartian 2:56 am 09/7/2011

    Vladimir: “Note also that the Andes above the timberline form an almost-unbroken chain of Corvid-free habitat from Venezuela to Terra del Fuego.

    And how do you suppose to reconcile that fact with your “unknown pathogen” explanation (comment #19) for the absence of Corvus from South America?

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  30. 30. vdinets 12:30 pm 09/7/2011

    Dartian: easily. There is plenty of available habitat south from Northern Mexico, but the only tropical area that Corvus managed to colonize was the islands. The most obvious explanation is that there is something on the mainland that prevents them from spreading south. Jays won’t do: jay species on the plateaus of Central Mexico are pretty much the same spp. as within the range of Tamaulipas and Sinaloa crows. What else could it be?

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  31. 31. John Harshman 2:06 pm 09/7/2011

    vdinets:

    A fine hypothesis, but unfortunately none of those thrushes is phylogenetically a Turdus; in fact they’re quite distantly related. One might hypothesize, though, that the incumbency of Catharus has resulted in competitive exclusion of Turdus from the north until recently. Why just “until recently” would be another puzzle.

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  32. 32. naishd 3:08 pm 09/7/2011

    Guess what: Turdus thrushes were in N. America during the Pliocene at least – there’s a fossil one from California and a few from Florida. These records aren’t of T. migratorius, so seem to represent taxa that later went extinct.

    Darren

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  33. 33. vdinets 5:24 pm 09/7/2011

    Darren: so, may be the first Turdus in North America were tropical lowland species; they went extinct during the glacials, and then after a while the ancestor of T. migratorius moved in?

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  34. 34. Allen Hazen 11:37 pm 09/7/2011

    (Seriously offtopic, but don’t twitter.)
    You tweeted: “How sucky that Tet Zoo ver 2 @ #ScienceBlogs – dead since July – gets more than twice as many hits as ver 3.”
    —One thing that gets ver 2 a daily hit from me is its usable and comprehensive blogroll.

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  35. 35. Dartian 4:45 am 09/8/2011

    Vladimir: What I’m getting at is that I’d wish you’d be more specific about this supposed crow-killing pathogen. How do you suggest that it spreads? Via some vector organism? If so, which South American species fit(s) would that be? If the same pathogen is able to prevent Corvus from establishing itself both on the lowlands of northern South America as well as high up in the Andes, it (or its vectors) must be exceptionally flexible ecologically. Or do you envisage that this hypothetical super-pathogen acts as a gatekeeper that kills any unfortunate vagrant crow or raven as soon as it lands on South American soil? Furthermore, what is it that prevents this pathogen from spreading north and wiping out all crows and ravens in North and Central America and the Caribbean?

    Finally, as your scenario pretty much requires this supposed pathogen to be dramatically lethal, how has it then remained unknown to science? Is there any known pathogen that kills crows (and crows only) and has it ever been found in South America? Can Corvus species be kept in captivity in South America or do they mysteriously die off?

    In other words: To me, your explanation seems to raise at least as many questions as it supposedly answers.

    Link to this
  36. 36. Dartian 4:50 am 09/8/2011

    Me: “If so, which South American species fit(s) would that be?

    Agh, a brain fart happened. Ignore the “fit(s)” in that sentence.

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  37. 37. vdinets 1:00 pm 09/8/2011

    Dartian: The patogen might be benign in native species, but lethal to crows. In this case the chances of discovering it are slim, unless you try introducing crows into the wild, which is probably not a good idea. If, for example, it is transmitted by mosquitoes, it might well be present throughout South and Central America, but not in Northern Mexico – there are a few mosquito species with ranges like that.
    Would be interesting to know why House Crows introduced to Chile failed to get established.
    I know the pathogen idea doesn’t have much supporting evidence, but do you have a better explanation?

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  38. 38. naishd 5:17 pm 09/8/2011

    That Chilean House crow pair not only failed to breed in their one Chilean summer; they also died during the winter. But it’s assumed that this was due to poor cold tolerance.

    Darren

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  39. 39. vdinets 9:04 pm 09/8/2011

    That would be weird. They happily breed in Holland where winters are much colder than in most of Chile (I assume they didn’t land in Patagonia?)

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  40. 40. Dartian 3:50 am 09/9/2011

    Vladimir: “do you have a better explanation?

    Not really, if you by ‘explanation’ mean something that’s currently supported by actual scientific data. (Note that I’m not suggesting that the idea of pathogens playing a part in preventing the establishment of crows in South America is somehow inherently impossible; it’s just that I think it’s unlikely that this would be the only or even the main reason. In the natural world, biotic interrelationships are usually more complex than that. Diseases, for example, rarely wipe out their host species completely.)

    But, if you absolutely want me to offer another explanation (which I freely admit is only speculative), I’ll still prefer my original climate/vegetation-related idea, combined with an element of stochasticity:
    1) Unsuitable lowland rainforest habitat on the Panamanian Isthmus has prevented North American Corvus species from reaching more favourable habitats in the northern parts of South America via land. The Corvus populations in adjacent parts of Central America are presumably relatively low at these distributional limit areas, so historically there has been no strong population pressure that has ‘forced’ individuals to disperse. 2) Caribbean crows have not colonised because they, being now island-adapted, are more sedentary than their mainland ancestors, and thus unlikely to disperse across long stretches of open sea. 3) Sheer chance may have played a part too. Corvus is not the only bird taxon that’s notably absent from South America; cranes, for example, are found on all other inhabitable continents except South America. Gruids are an old lineage of mostly migratory, strong-flying species; surely they have had both the time and the opportunity to disperse to South America? Well, as it happens, they didn’t. As with the crows, there may well be some concrete reason as for why cranes are absent from South America (although, given the differences in morphology, ecology, behaviour, etc. between crows and cranes, we should at least expect the exact reasons to be somewhat different), but AFAIK we have no good evidence of any such reason.

    Which brings me to my final comment: At the end of the day, we need to keep in mind that as long as there are no actual, relevant data to examine on this matter, all our scenarios amount to little more than just-so stories.

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  41. 41. vdinets 1:59 pm 09/9/2011

    Dartian: that’s very nice… but crows didn’t stop in Panama. They are missing from all of Central America and southern Mexico. Even Mexico City area doesn’t have them, despite being an arid plateau with cold winters, not separated from Northern Mexico (where Chihuahuan Raven occurs) by any ecological barrier. So rainforests have nothing to do with it.

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  42. 42. CS Shelton 11:58 pm 09/9/2011

    Hey does anyone know if Corvus brachyrhynchos in North America sometimes has patches of light gray coloring in uniform areas (symmetrically under the wings, or on the chest, etc)? I’ve seen it but I’m not sure if it might have just been a trick of weird lighting, since it wasn’t obvious like pure white would have been.

    I conjectured from my customary position of total ignorance that maybe my local crows have a tiny bit of magpie blood in them, since in addition to that peculiar trait, I noticed an unusually strong iridescence in one’s tail recently. Then again, I’ve read that crows are iridescent, though I’d never noticed it before that day (a very sunny one).

    I can’t construct a concise sentence tonight. Sorry about that. :-P

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  43. 43. Dartian 3:20 am 09/10/2011

    They are missing from all of Central America and southern Mexico.

    That’s not true. The raven Corvus corax is found (or at least was found until a few decades ago) as far south as Nicaragua.

    rainforests have nothing to do with it

    “Nothing”? That’s a strong assertion. Can you back it up with equally strong evidence?

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  44. 44. vdinets 1:43 pm 09/10/2011

    Proves my point even better. The southern limit of Corvus does not match the northern limit of rainforests. Sinaloa crow penetrates the rainforest zone in Western Mexico, and raven does so in Nicaragua, but no species occurs in dry forests of Yucatan or northwestern Costa Rica, or in highlands of central Costa Rica which are connected to the mountains of Nicaragua.

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  45. 45. CS Shelton 9:31 pm 09/10/2011

    vdinets- Northern limit of *tropical* rain forest anyhow. In my home state, Corvus corax is more likely to be found in a rain forest than in the cities.

    Link to this
  46. 46. Dartian 4:03 am 09/11/2011

    Vladimir “Proves my point even better.

    Let me remind you that your original point was that it’s some sort of pathogen that prevents Corvus from dispersing to South America. And that thus far, you haven’t produced any evidence that such a pathogen even exists! Not even the merest anecdote, never mind citing solid, peer-reviewed research. Anyone can have opinions and anyone can make assertions, but to do science you need actual data. Unless and until you produce such data, your pathogen idea remains squarely in fantasyland. And any claims that you’ve “proved” your point are grossly premature.

    Link to this
  47. 47. vdinets 12:49 pm 09/11/2011

    I was referring to my point that Corvus’ supposed inability to penetrate rainforests is not a good explanation.

    Link to this

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