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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Thrilling encounters with freak passerine birds

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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.

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

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