May 12, 2013

In pursuit of the Rook

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

Any adventures about the more rural parts of the UK typically involve (for me, anyway) a lot of looking at the Rook Corvus frugilegus, a remarkable Old World corvid that occurs from the far western shores of the UK and France all the way east to Japan (it’s generally absent from the cold northern parts of Eurasia, only visits the Iberian Peninsula and most of southern Europe during the winter, and is absent from much of tropical Asia). The east Asian birds represent the distinctive subspecies C. f. pastinator (sadly, not the more awesome sounding dastinator*, as it says in one particularly famous book on crows). I’ve been loosely ‘collecting’ Rook images over the years and thought it high time to put a bunch of them together.

* That word really reminds me of the sort of thing Dr Doofenshmirtz would refer to in Phineas and Ferb.

Rooks are highly social birds that nest in large colonies called rookeries. They are, as would seem useful for a colonial nester, essentially non-territorial, only defending their nest and mate: all of this is remarkable and unique for a Corvus crow. It seems that Rooks rely heavily on the successful foraging of other members of the species, following them to fruitful patches within 2.5 km of the colony and massing together when rich food sources are located (Green 1996).

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To quote Franklin Coombs from his excellent 1978 The Crows, “Few bird colonies can have aroused as much interest as rookeries. The activities of rooks need to be watched from nest level as Lewis Harding (1848) did from his window, or better still from a hide among the nests” (Coombs 1978, p. 74). Harding’s story is an interesting one. He “returned from a distant climate … in a very imperfect state of health”, and was told by his doctor – Jonathan Couch (1789-1870) – to observe and record the habits of Rooks throughout the months of the year; in other words, to engage in what we now call occupational therapy. Harding’s notes cover both sides of more than 300 sheets of paper and his observations were both accurate and detailed, despite his lack of telescope or binoculars. See Coombs (1978) for a lengthy discussion of Harding’s work.

So far, I’ve personally only had the opportunity to watch rookeries from ground-level, but it’s obvious that lots of interesting stuff is going on up there. Like all corvids, Rooks are smart and display obvious problem-solving abilities. In one case, two captive Rooks in a laboratory co-ordinated their string-pulling movements such that they were able to slide a tray into their enclosure, and they performed this task as well as did captive chimpanzees (Marzluff & Angell 2012, and refs cited therein). Admittedly, single Rooks did not perform so well, however, since they started the string-pulling before their partner arrived (whereas chimps were more patient and seemed to understand the requirement of a two-part team). Rooks are also known to drop stones into water-filled containers so that floating food items can be raised up to within reach (validation of Aesop’s fable).

The Rook is a black crow (that is, a member of the genus Corvus), so unusual compared to the other Corvus species that Goodwin (1986) wrote that “its relationships within the genus cannot be deduced on present evidence” (p. 71): his phylogram depicts the Rook as the sister-taxon to the remainder of Corvus. However, genetic data indicates that the species is deeply nested within Corvus, closely related to the Eurasian raven C. corax and its kin (Ericson et al. 2005).

Rooks are famous for their bare, pale faces and bill-bases (the east Asian subspecies C. f. pastinator differs in having a feathered face and bill-base). The vernacular name Bare-faced crow was apparently used for the species in the past (Coombs 1978). Claims that the bare face only occurs due to the soil-probing behaviour practised by this species are not correct: you can read more about this issue in the Tet Zoo ver 2 article linked to below. A tall forehead and shaggy, loose flank feathers that give the bird a so-called ‘baggy trousered’ look are also distinctive. Males and females look the same (though males are slightly larger) and there are no seasonal changes in plumage.

Rooks are seemingly monogamous and pairs stay bonded for years and – supposedly – even for life (Madge & Burn 1999). As is typical (or ubiquitous) in ‘monogamous’ animals, extra-pair copulations are common and some are forced rapes, the male copulating (or attempting to copulate) with a female already incubating her own clutch (Goodwin 1986). Additional birds frequently intervene when such acts occur, though exactly what their motives are is uncertain. Mating occurs on the ground as well as in the rookery [image below from full sequence available here].

A complex and diverse repertoire of postural displays are present in these birds, and if you know what to look for you can usually get some idea of the sorts of signals they’re conveying. A series of movements that involve bowing, drooping the wings and fanning and drooping the tail signal various stages in assertiveness and/or submissiveness. So-called ‘full-forward’ or ‘forward threat’ poses signal aggression associated with nest defence or squabbles over food, and ‘flight intention’ movements of the sort seen throughout corvids (this is where the wings are repeatedly flipped and tucked away again) seem to signal uncertainty or nervousness.

Mating flights (sometimes involving groups of birds numbering 10 or 12) feature specific stiff-winged wing flaps where the wings are raised at an especially high angle on the upstroke. Like many colonial nesters, Rooks sometimes engage in ‘dreads’ or ‘outflights’. The colony goes quiet and nothing much seems to be happening. All of a sudden, there’s a burst of cawing and most or all of the birds take off in unison. This behaviour might be initiated by concern about a local predator (or perceived predator).

There seems to be some sort of relationship between Rooks and falcons, with Common kestrels Falco tinnunculus and Lesser kestrels F. naumanni both being well-known for their habit of sometimes nesting right within a rookery. Even better, breeding in Red-footed falcons F. vespertinus and Amur falcons F. amurensis is (sometimes) seemingly dependent on the presence of rookeries and these species – which are colonial nesters themselves – don’t start their nesting activities until the Rook nesting season is underway. They then move in and either take over disused Rook nests, or else evict Rook adults, nestlings and eggs from their nests before claiming them as their own (Madge & Burn 1994). Long-eared owls Asio otus and Ospreys Pandion haliaetus have also nested within active rookeries on occasion, and rookeries are sometimes mixed with active nests belonging to herons, cormorants and ibises.

Numerous anecdotes describe unusual bits of behaviour in this species, most of which can be interpreted as sensible (and plausible) responses to stimuli. Burton (1978) described an instance in which a flying Rook dove at a Sparrowhawk, forcing the hawk to release a live Starling it had captured, and another in which a breeding pair seemingly enlisted helpers to quickly re-build a nest a few hours after the original one had been destroyed by a person.

Rooks are terrestrial foragers that feed in grasslands and agricultural fields, though they also forage on shores, tidal mudflats and garbage dumps if those occur nearby. Again, they’re highly unusual compared to other Corvus crows in that they aren’t generalists that habitually feed on anything, but specialists that probe into the ground for worms, insect larvae and other arthropods, and seeds and edible roots. Earthworms seemingly form the bulk of their diet followed by tipulid fly larvae (known popularly as leatherjackets). During the breeding season, the male’s bill becomes shortened due to extensive probing behaviour while that of the female (she is solely responsible for incubation) increases due to relative lack of wear (Green 1996).

Whether Rooks are beneficial or damaging to agriculture has been much-discussed. Their avid consumption of tipulids surely makes them highly beneficial, since these larvae are significant crop pests. Food intended for transport back to the nest is collected in a buccal pouch that, due to the lack of feathering at the base of the bill, is especially obvious relative to that of other corvids.

I’ve watched Rooks a lot. At wildlife parks and other outside attractions they’re typically abundant and also relatively unafraid of people, hence some of the close-range photos you see here. However, I really want to spend appropriate time properly observing the goings-on at an active rookery. And one day I will, in my extensive and copious spare time.

For previous Tet Zoo articles on crows and other corvoid passerines, see…

Next: ICHTHYOSAURS!!

Refs - -

Burton, M. 1978. Just Like An Animal. Biddles, London.

Coombs, F. 1978. The Crows. B. T. Batsford, London.

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

Green, P. 1996. The communal crow. BBC Wildlife 14 (1), 30-34.

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

Madge, S. & Burn, H. 1999. Crows and Jays: A Guide to the Crows, Jays and Magpies of the World. A & C Black, London.

Marzluff, J. & Angell, T. 2012. Gifts of the Crow: How Perception, Emotion, and Thought Allow Smart Birds to Behave Like Humans. Free Press, New York.