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Thrilling encounters with freak passerine birds

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


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I photograph birds a lot – something that’s more possible than it was before due to the fact that I now own a half-decent camera (thank you, parents). On recent excursions I’ve succeeded in photographing a huge number of European passerines: something that inspires me to write at length about these animals… hey, stay tuned on that (famous last words). One thing that’s struck me in recent months is how many of the birds I see are freaks. That is, they possess developmental abnormalities, have missing or deformed body parts, or are weird in terms of the wear and tear they possess.

Poor, deceased male Common bullfinch (Pyrrhula pyrrhula). Waitaminute --- why can we only see one leg? Image by Darren Naish.

If we part the feathers... here's the malformed right leg. Up-close it's a bit gross, so I haven't uploaded a full-sized version of the image. Photo by Darren Naish.

We start with this unfortunate male Common bullfinch Pyrrhula pyrrhula, killed by collision with a car and kindly passed to me by Phil Budd of the Southampton Natural History Society. Phil noticed that the bird was one-legged and assumed that its right leg had been broken off at some point, or had perhaps never grown. As I parted the bird’s feathers, I was surprised to find a fully formed and complete, but gnarled and contorted, leg hidden beneath the plumage. Adjacent tissue from the body and matted feathers were partially adhered to the leg. Because the claws are not massively overgrown, it looks to me as if this condition was recently acquired in the bird’s life, and not present from early age. What could have caused this? You will note that the bird is, otherwise, is fine condition and (so far as we know) lived a normal adult life, despite its functional one-leggedness.

Incidentally, this is the second deceased Common bullfinch I’ve acquired over the years. The first one (a female, given to me in March 2009) was also a (probable) car victim. What’s curious is that both birds were affected with papillomatosis, a condition caused by papillomavirus infection. Cardueline finches are frequently papillomavirus carriers (Common chaffinches Fringilla coelebs are notorious for it); the bullfinches shown here were only affected lightly compared to some unfortunate individuals. The biology, diversity and genetics of avian papillomaviruses has recently become an area of major research interest; it’s a big and fascinating area that I can’t discuss further right now.

Foot of 2014 Pyrrhula at left; deceased female (from 2009) at right. Both individuals were infected with papillomavirus (note the gnarly growths on the feet). Photos by Darren Naish.

I’ve said in a previous Tet Zoo article that I often take the time to photograph Rooks Corvus frugilegus. Rooks are weird, bare-faced crows, unique to Eurasia, often associated with farmland and well known for their habit of probing into soil in quest of insect larvae and other prey (they were covered at length on Tet Zoo back in May 2013: link below).

Rook with left-side notching on its bill and slightly overlapping bill tips. Image by Darren Naish.

While on the way to Wales a couple of weeks ago, I photographed several Rooks that were sitting on a sign at a service station. One of these birds – featured above – had a distinctive oval-shaped notch on the side of its bill, and also slightly overlapping bill tips. Unusually worn bill tips and edges are quite often seen in Rooks and presumably tell you something about how the bird uses its bill when foraging or feeding. And I’ve seen exactly the same sort of thing in other birds before – some of you might remember the deceased Great spotted woodpecker Dendrocopos major (or Picoides major) I featured at Tet Zoo years ago and which also had an oval gap on the side of its bill. Exactly how these oval notches were caused is a mystery to me – might it show that the birds routinely pull plant stems or branches or something through the sides of their bills? Ideas welcome.

There's something weird about this pipit... Image by Darren Naish.

Finally, while at Lyme Regis recently for the Fossil Festival, I went and photographed pipits and wagtails on the big boulders at the sea’s edge. This Rock pipit Anthus petrosus was very obliging and I took many pictures of it, preening and foraging. But as I looked through the photos afterwards, I noticed that something was odd about its right foot – specifically, it was almost entirely missing. As you can see from the accompanying images, the bird possesses a fully complete right leg all the way down to the joint between the toes and the tarsometatarsus. Thereafter, almost nothing bar what might be the stumps of one or two of the toes. However, it still seems able to use its toe-deprived foot as a walking crutch.

Malformations, injuries and other abnormalities are generally not common in wild animal populations (example: bill deformities are present in less than 0.5% of the passerine population according to Pomeroy (1962), while Sharp & Neill (1979) reported a similar 0.6% of abnormalities within a single bird species). Abnormalities are, however, ubiquitous and might even be over-represented in the literature and perhaps even in our own perception of the world because we tend to notice and record them. Indeed, people have done a good job of recording some absolutely incredible examples (see Fox 1952). These might be caused by accidents, diseases, parasites and pollution, and there are some parts of the world where deformed or disabled animals are now more likely to survive than they were in the past since human feeding and/or waste disposal allows them to eke out a living.

One-footed Rock pipit launches into flight. Photo by Darren Naish.

It’s been said several times that deformities and other abnormalities are becoming more common in birds and other animals, presumably as problematic parasites and other organisms are becoming more widespread due to pollution, ecosystem disruption and climate change (e.g., Blus et al. 1998, Johnson et al. 2003, 2004). There are good reasons for thinking that some animal groups – frogs are the ultimate example – really are being negatively affected by this combined onslaught. But is this true for birds? Abnormalities in some birds have been convincingly linked to DDT and other forms of pollution (e.g., Sharp & Neill 1979), but I don’t know if a consistent picture has emerged yet: some sources say that bill deformities are becoming more common, but others report that they have declined relative to the situation of a few decades ago. Craves (1994) said that “fewer reports of birds with deformed bills are appearing in the literature; whether this is due to a real decline in occurrence or a lack of interest in reporting is not known”.

Amazing abnormal California thrasher (Toxostoma redivivum), from Fox (1952).

I have one final thing to say on the subject. Many animals with malformations are competitively inferior to their contemporaries and are less likely to do well in the survival stakes (several deformed passerines, for example, were noted as bearing heavy lice infestations). But this is not universally true. ‘Freaks’ with sickle-shaped bills, crossed bill-tips and hooked bill tips have been recorded in species where those features might (theoretically, at least on occasion) have conferred a competitive advantage. Several wild crows have been recorded with enlarged, raptor-like hooked bills, for example. If the timing is right – if those crows were to pass on their weird genes and do well against competitors – herein lies part of the evolutionary process. And, remember, we are always seeing ‘evolution in action’.

For previous Tet Zoo articles relevant to some of the issues covered here, see…

Refs – -

Blus, L. J., Melancon, M. J., Hoffman, D. J. & Henny, C. J. 1998. Contaminants in eggs of colonial waterbirds and Hepatic Cytochrome P450 enzyme levels in pipped tern embryos, Washington State. Archives of Environmental Contamination and Toxicology 35, 492-497.

Craves, J. A. 1994. Passerines with deformed bills. North American Bird Bander 19 (1), 14-18.

Fox, W. 1952. Behavioral and evolutionary significance of the abnormal growth of beaks of birds. Condor 54, 160-162.

Gochfeld, M. 1972. Avian abnormalities and the scientific literature. American Birds 26, 705.

- . 1975. Developmental effects of Common terns of western Long Island, New York. Auk 92, 58-65.

Johnson, P. T. J. & Chase, J. M. 2004. Parasites in the food web: linking amphibian malformations and aquatic eutrophication. Ecology Letters 7, 521-526.

- ., Lunde, K. B., Zelmer, D. A. & Werner, J. K. 2003. Limb deformities as an emerging parasitic disease in amphibians: evidence from museum specimens and resurvey data. Conservation Biology 17, 1724-1737.

Pomeroy, D. E. 1962. Birds with abnormal bills. British Birds 55, 49-72

Sharp, M. S. & Neill, R. L. 1979. Physical deformities in a population of wintering blackbirds. Condor 81, 427-430.

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. BilBy 10:32 pm 05/5/2014

    I see a lot of magpie larks (Grallina cyanoleuca) in Australia with messed up feet – toes missing, bunched up feet – which I have been told is bumblefoot: bacterial infections due to slight wounds in the feet. They ‘seem’ to be doing fine, and I don’t know if the urban ones (magpie larks do very well in urban areas) are more likely to be bumblefooted than rural ones, or if they are more likely to have lice infestations. As there are lots of them and they are quite easy to individually identify it might be worth doing a little behavioural study on them – foraging rates, flight initiation distance etc.

    Link to this
  2. 2. irenedelse 3:01 am 05/6/2014

    Phil noticed that the bird was one-legged and assumed that its right leg had been broken off at some point, or had perhaps never grown. As I parted the bird’s feathers, I was surprised to find a fully formed and complete, but gnarled and contorted, leg hidden beneath the plumage. Adjacent tissue from the body and matted feathers were partially adhered to the leg. Because the claws are not massively overgrown, it looks to me as if this condition was recently acquired in the bird’s life, and not present from early age. What could have caused this?

    I can picture an encounter with a predator (a cat?) who lunged upwards and grabbed the leg in its teeth as it was taking flight, but failed to make a kill? If the bird escaped with a messed up leg, it may have more or less healed in a retracted position.

    Link to this
  3. 3. vdinets 3:31 am 05/6/2014

    The first thing that comes to mind is that predation in urbanized habitats is lower than in more natural ones, despite the artificially increased cat and rat populations. I’m pretty sure somebody must have studied this.

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  4. 4. irenedelse 4:10 am 05/6/2014

    @ vdinets:

    That certainly must factor in. Plus, in a city, predators like cats and dogs are more likely to be kept inside or closely monitored and prevented from attacking birds. In addition, urban areas offer a regular supply of food, both from garbage bins and from people intentionally feeding birds. I can see what happens with pigeons here in Paris, where pigeon overpopulation is a problem: individuals with deformities abound. Especially foot injuries or tumours, from what I’ve seen.

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  5. 5. DavidMarjanovic 4:56 am 05/6/2014

    I can see what happens with pigeons here in Paris, where pigeon overpopulation is a problem: individuals with deformities abound. Especially foot injuries [...], from what I’ve seen.

    Same in Vienna.

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  6. 6. Jerzy v. 3.0. 5:10 am 05/6/2014

    From my experience, missing legs in birds are most commonly caused by entanglement in torn plastic bags, human hair, strings from torn fabric, fishing line and similar. It cuts blood flow and the part of the leg dies and falls off. Yet another reason not to litter. Anglers in my area are especially bad, leaving lots of trash: tangled fishing line, plastic bags, whatever.

    I am not sure, but I suspect that in urban birds, another problem may be also infected cuts to the feet from pieces of glass, metal etc.

    @3
    I was just going to comment that predation in recent years is not low in urban areas. Crows, jays, birds of prey, owls, foxes etc now commonly colonized cities, even densely built up areas.

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  7. 7. naishd 5:23 am 05/6/2014

    Thanks for comments so far. Pigeons with missing or deformed feet seem to be ubiquitous in the urban areas of the world… there are several competing ideas as to why they end up this way, the most likely (in my opinion) being that they get tangled in anti-pigeon netting and so on. I might devote an article to this subject one day.

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  8. 8. barndad 7:13 am 05/6/2014

    Reminds me that pre-extinction, the Norfolk island Kaka (Nestor products) was supposed to have a high prevalence of bill deformity. http://en.wikipedia.org/wiki/File:Nestor_productus.png

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  9. 9. Pristichampsus 7:56 am 05/6/2014

    BilBy- I have encountered 2 or more Magpie Larks with legs badly tangled in string, so much as to impair their walking. I agree with Jerzy v 3.0 that this bumblefoot may be exacerbated by entanglement in string and plastic. I have a suspicion that magpie larks might get tangled legs more often, I don’t know why. Perhaps the relatively fine beaks of magpie larks are not strong or dexterous enough to peck or pull away any offending string, I could imagine a corvid quite easily disentangling its food by using its strong bill, and problem-solving intelligence.

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  10. 10. LeeB 1 8:03 am 05/6/2014

    In Auckland both pigeons and seagulls with deformed legs or missing feet are noticeable, and also the occasional blackbird with an injured leg.
    Especially for the seagulls tangling in fishing line seems a likely cause for the loss of feet.

    As long as they can still fly birds can avoid terrestrial predators even with severely damaged hind limbs, and birds of prey capable of taking aerial prey are non-existent in the city here.
    And people feel sorry for the injured birds and may preferentially feed them, contributing to their continued survival.

    LeeB.

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  11. 11. John Scanlon FCD 11:02 am 05/6/2014

    Magpie Larks are a largely terrestrial, giant species within a small arboreal lineage, so – while they’re incredibly successful in terms of abundance and habitat diversity – their feet are exposed to very different conditions than in most of their ancestors over recent millions of decades (think of humans and bad backs). We had a pair nesting (elegant bowl-shaped mud nest on a horizontal branch) in the backyard in Mount Isa from 2004-07 (the male had one really messed-up foot), fledged 1 to 3 young per year and as far as I could determine, not one of the young survived to adulthood. Adults are very good at dodging cars, but juveniles not so much. Not sure if they’re so long-lived that this was a normal success rate, or if towns act as a sink for exurban populations with fewer mechanical threats.

    Oval notch on one side of the bill: sure sign of a pipe smoker. I think this may be illustrated somewhere in the Narnia books.

    One time at Maroubra beach, I spent a while watching a flock of Silver Gulls who were paying close attention to my chips. One of the birds had no feet at all, and seemed to be particularly skilled at hovering and catching flung morsels. A couple of times it settled for a rest, plugging its stumps into the sand. Probably not so useful on other surfaces.

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  12. 12. keesey@gmail.com 1:00 pm 05/6/2014

    I recently mounted a very unusual starling — the lower bill is much, much longer than the upper: https://www.flickr.com/photos/keesey/13914342969/

    Makes me wonder if skimmers evolved by saltation.

    Link to this
  13. 13. irenedelse 1:05 pm 05/6/2014

    On the subject of deformed bills: that California thrasher looks like it’s trying to become a sickle-bill…

    Link to this
  14. 14. naishd 1:09 pm 05/6/2014

    Comments 12 and 13: given the similarities these malformed individuals have with specialised taxa that evolved (we presume) via natural selection, you really do have to wonder if odd and sporadic mutations could, as Mike says, lead to remarkable things via saltation…

    As I said in the article, the mutants would have to be extremely ‘lucky’ (in terms of timing) in order to get that competitive edge.

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  15. 15. imhennessy 9:12 pm 05/6/2014

    Now that speciation has been raised,I’m reminded of John Conway’s frequent podcast complaints that birds are overly prolific. I’m thinking mostly in terms of sexual selection and mutual sexual selection, but bill shape raises the possibility that natural selection is also at play.

    Anyway, if feathers and beaks can exhibit variation at a lower energetic cost, and/or with a smaller impact on fitness, they lower the bar for innovation.

    Assuming that is true, flight, and especially migration and high density mating grounds, provides birds with huge pools of potential mates. In that environment, standing out would seem, intuitively, to be the way to go.

    I think birds may be speciose because they can be.

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  16. 16. Dartian 2:15 am 05/7/2014

    Irenedelse:
    I can picture an encounter with a predator (a cat?) who lunged upwards and grabbed the leg in its teeth as it was taking flight, but failed to make a kill?

    Except that cats don’t grab birds with their teeth – they use their clawed forelimbs for that.

    More generally speaking, the feet and legs of small passerines are very unlikely body parts for any predator to grab. (Flying passerines tuck their feet and legs against their bodies – they don’t protrude while in flight.) Unsuccessful predator attacks are probably a negligible cause of missing limbs and digits in passerines. Far more likely reasons, as has already been pointed out, are injuries caused by human-produced materials such as strings, pieces of plastic, etc.

    John:
    Magpie Larks are a largely terrestrial, giant species within a small arboreal lineage

    I like to think of the magpie lark as the passerines’ attempt to evolve a shorebird look-alike. (When I visited Australia and saw my first magpie lark my initial thought was: “What an odd-looking plover.”)

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  17. 17. naishd 4:25 am 05/7/2014

    Cats vs birds: I agree with Dartian’s comment (# 16) that cats are likely not responsible for the missing toes and feet we see in birds… in virtually all occasions. Note, however, that there are those cases where cats partially consume and/or mutilate a prey animal, only for the still-living prey to escape and survive, despite its missing limbs, tail, or whatever.

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  18. 18. Jerzy v. 3.0. 5:06 am 05/7/2014

    @11
    Yes, collisions and accidents are under-appreciated threat to birds. I was surprised to learn that they are enough that whole bird populations may decline and go extinct (eg. some little owl and barn owl populations in Europe).

    @15
    Birds can live with pretty surprising handicaps. I saw once white Meadow Pipit on spring migration in Poland. So this normally cryptic little bird must have hatched, raised and survived autumn migration, wintering and journey back.

    Link to this
  19. 19. BilBy 5:43 am 05/7/2014

    @18 Jerzy – do you have refs for those owl population studies? Thanks

    Link to this
  20. 20. Tayo Bethel 5:54 am 05/7/2014

    Isit possible that Lanidae evolved from a small corvoid ancestor withasimilarbill deformity as that foundin some crows? Right now I’m reading a large guide to shrikes of the world, and it mentions that the hooked bills so characteristic of shrikes dont developuntilquite late in ontogeny. One can imagine a harsh desert environment where a hooked bill was selected for dealing with larger prey than thattaken by similar-sized passerines. Who knows–one dayin the distant future we might have shrikelike crows. Scary.

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  21. 21. SteveKuntz 9:58 am 05/7/2014

    Several years ago I saw a chickadee in upstate New York with that same downcurved bill. It had become locally famous, and birdwatchers would travel to a backyard feeder to see it. The bill was relatively as long as that thrasher photo, which would seem to be a huge handicap, but it looked healthy and well fed, probably thanks to bird feeders. It would be interesting to figure out if species which feature this unusual bill shape appeared all at once due to a freak mutation, or if the bill slowly lengthened over a long period of time.

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  22. 22. irenedelse 1:07 pm 05/7/2014

    @ Dartian, Darren:

    There goes my exciting scenario! ;-)
    Although to be fair, I was thinking about the bird with the deformed leg in the photo, not the ones with missing toes. That leg makes me think of gruesome medical cases where a broken limb wasn’t properly set and healed in a retracted position.

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  23. 23. BrianL 1:09 pm 05/7/2014

    @Jerzy:
    Years ago, when I was just a child, my father and I spotted a group of White Wagtails that included not one but two truly white individuals. It was impossible to say if they were siblings or parent and offspring or something else, but it was truly an interesting phenomenon and these two must have performed the same feat as your pipit did. The encounter also comedically involved both of us saying ‘it’s over there!’ simultaneously when we still thought there was just one white individual in the group!

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  24. 24. irenedelse 2:39 pm 05/7/2014

    Interesting fact about unusually coloured passerines: the rare but real instances of white individuals in species with a normally black plumage has inspired popular expressions in several languages: a “white crow” in Russian (белая ворона, or ‘belaya vorona’) means a person who doesn’t quite fit. In French, a “white blackbird” (‘un merle blanc’) means someone exceptionally lucky or gifted.

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  25. 25. Chabier G. 3:26 am 05/8/2014

    Deformities are very common in white storks in Spain, one major cause of concern are discarded straw bale nylon strings,adult storks take these strings to use them as nest lining, chicks get tangled when they are small, and, as chick size increases, the string knots strangulates limbs, results are really horrible. Indeed, most of these young storks don’t survive long enough to leave the nest. Deformities caused by chemical pollution are also present, I’ve seen a fledging white stork with one single bone instead of ulna-radius, and a single bone bar instead of normal metacarpus (in both wings). It’s not surprising, as our surveys have detected levels of mercury and other heavy metals astonishing high in stork chicks from some areas.

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  26. 26. Jerzy v. 3.0. 4:01 am 05/8/2014

    @Chabier G.
    Straw bale nylon strings, in particular, are also a danger to white stork chicks in Poland. Storks are helped a little by their popularity. Entangled chicks are often spotted and rescued by local people, and farmers care a little not to litter specially to protect storks from strings.

    @BilBy – sorry, cannot find redferences now. For Barn Owl in Britain you should find easily citations about vehicle collisions. Little Owls were in Germany I think.

    Link to this
  27. 27. Dartian 4:41 am 05/8/2014

    Irenedelse:
    the rare but real instances of white individuals in species with a normally black plumage has inspired popular expressions in several languages

    Interestingly, in Europe in Ancient and Medieval times, ‘black swan’ meant something/someone very rare or unusual – or even something that doesn’t exist. Of course, the metaphor quickly became obsolete when Australia was discovered…

    Link to this
  28. 28. Chabier G. 6:00 am 05/8/2014

    @Jerzy
    Here, at the rescue center we try to do the best with entangled chicks, but it’s often impossible to save them. There are many people concerned about storks, but farmers seem to be very different from those of Poland, almost nobody worries about nylon strings littering.
    And, about owls and traffic, we receive all the animals belonging to protected species, found dead (injured, orphaned, ill, etc) or alive, from the whole region of Aragón. I’ve been checking our files from 1994 to 2013, traffic casualties are the main cause of dead/injury in owls, 890 out of 2.717 birds (33%).Little owls are the most affected, 44% of 619 individuals, in barn owl 26 % of 768 birds were struck down by cars, in eagle owl also 26 % of 661 birds (but 36% were electrocuted).But the main threat to both barn owl and little owl is the lack of suitable places for nesting, as old country buildings are decaying and modern ones haven’t got cavities. Tawny owl and eagle owl populations are increasing, in spite of all their problems.

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  29. 29. Gizz47 7:11 am 05/8/2014

    Hi Darren.
    I noticed a few pigeons with missing second digits in Portsmouth recently. The first time I saw it was at one of the train stations: both its second digits were entirely missing. So much so that it looked as if they’d never been there at all. A few days later I saw another two with exactly the same deformity on both feet (and no, it wasn’t the same pigeon!) I wonder if they are all losing them the same way.

    I remember reading ages ago that pigeons lost toes by roosting on/in objects which freeze during the night in cold weather, causing severe frost bite. I used to think it was a bit odd (surely that would be uncomfortable enough to make the pigeons pick more appropriate spots to have a kip?). But apparently they don’t really notice, until it’s too late. Because human-constructed environments offer ideal winter-time shelters, the birds have become accustomed to mild conditions (a little too accustomed). This is causing them to change how the blood flows through their feet and it becomes inadequate to deal with freezing temperatures when they arise. It was hypothesised that the bird instinctively assumes its naturally cracking thermoregulatory blood flow system will cope, but ….it doesn’t. Bye bye toes! I don’t know where I read this unfortunately: it would be interesting to see if an actual study has been done on it.

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