It’s hot and sunny here in the UK right now, and elsewhere in the world too, I’m sure. In celebration of the current conditions, I figure now is a good time to recycle a Tet Zoo ver 3 article that’s already two years old: my brief review of sunbathing postures in dinosaurs. By which I mean birds. So, here it is again, with a few updates and new images. Enjoy!

To those academically interested, note that there’s a fairly extensive literature on sunbathing in birds and on the groups that practise it, on the postures they adopt, and on the function it might have (e.g., Hauser 1957, Kennedy 1969, Horsfall 1984, Simmons 1986).

We’ll start with gallinaceous birds. The adjacent photo shows a sunbathing Elliot’s pheasant or Bar-backed pheasant Syrmaticus ellioti with one wing slightly fanned out and the tail slightly spread. I’ve seen cracids, domestic chickens and other gamebirds do the same sort of thing (below, we see a cracid stretching one of its wings out in an apparent bit of sunbathing behaviour), and this got me wondering if waterfowl (close relatives of gamebirds within the clade Galloanserae) do the same thing – I’ve never seen a sunbathing duck or swan.

Having said that, geese at least (especially Canada geese Branta canadensis) sometimes sit in the sun with the wings hanging more to the sides than usual, thereby exposing the back to the sun. Various photos online labelled as showing ‘sunbathing’ ducks actually just shows ducks sitting in the sun, wings folded, in the normal resting pose. Swans sometimes swim along with one foot out of the water, toes stretched so that the webbing is fully exposed, and I’ve heard it said that this is to do with shedding heat, not collecting it. But, so far as I can tell, waterfowl don’t seem to stretch their wings out to ‘collect’ warmth – does anyone know otherwise? (if you’re thinking about the other members of Anseriformes – the screamers – let me give you a bit of advice and suggest that you don’t google ‘sunbathing screamer’). Since this article was first published in 2011, reports of sunbathing waterfowl have not come forward; indeed, an ornithologist specialising on waterfowl told me that they just don’t do it, so there.

Many birds that you really wouldn’t regard as sunbathers apparently are, if only they get the chance. Do owls sunbathe? As this photo – taken at Pook’s Hill Lodge in Belize and passed via entomologist Max Blake – shows, indeed they do. This is a captive Spectacled owl Pulsatrix perspicillata, contorted into a fairly weird posture and with its eyes shut, but apparently still enjoying the sun. In fact, google will show you that of owls of assorted species belonging to most lineages are very happy to sunbathe, sometimes doing so with fully outstretched wings, raised feathers, lifted tails and often with a raised, back-curved neck that makes the head point skywards or even somewhat backwards.

Because I tend to think of behaviours in a tree-based, phylogenetic context, I got wondering whether the same – or similar – postures are present in close relatives of owls, like mousebirds or the members of Picocoraciae. I’m not sure about mousebirds, but the sunbathing postures adopted by woodpeckers, bee-eaters and hornbills, at least, don’t seem especially unusual compared to other of other birds that perch and roost in trees.

Having mentioned perching, some birds will sunbathe while perched, fanning one or both of their wings while sitting well up off the ground. This Trumpeter hornbill Bycanistes bucinator spent a while with both of its wings fanned out to side.

On the subject of predatory birds, or raptors, if you will*, I was particularly happy to see this sunbathing Secretarybird Sagittarius serpentarius on its belly, wings fanning out to the side. It mostly kept both wings fanned out, but sometimes folded one wing away. Various hawk, eagle, vulture and condor species are well known for standing erect while facing the warmth of the sun and either holding their wings outstretched, or standing with their wings drooped to the sides in what’s known as a delta-winged posture. I’ve previously (Tet Zoo ver 2) used pictures of Andean condor Vultur gryphus and Lappet-faced vulture Torgos tracheliotos doing exactly this.

Certain storks and herons also adopt delta-winged and spread-eagle poses (Curry-Lindahl 1970, Kahl 1971): the adjacent image (kindly provided by Lars Thomas) shows a Grey heron Ardea cinerea. Poses that involve holding the wings out don’t necessarily show that the bird is trying to warm itself: they might also be used when the bird is trying to lose heat, dry its feathers, show off to rivals, or shade its eggs or babies. Cormorants, pelicans, anhingas and others are well known for adopting spread-winged postures when they need to warm up, cool down, or dry their feathers.

* Some book says that owls are included in the ‘raptor’ concept, others dispute this and limit it to hawks, eagles, vultures and falcons. On this occasion I’ll gloss over the fact that the term ‘raptor’ belongs to a guild of neornithines, not to dromaeosaurids.

Grebes sunbathe in a distinctive posture where the rump is raised toward the sun while the wings and feathers on the back are held slightly raised. This helps expose a dark-skinned patch on the bird’s back that’s presumably good at absorbing heat (Storer et al. 1976). Even white- or pale-feathered mutant grebes retain this dark patch, suggesting that it’s physiologically important (Jehl 1985).

Based on my massive sample size (n = 2), it seems that pigeons like to recline on their sides during sunbathing, with the wing on the opposite site raised up such that the underwing is fully exposed to the sun. In fact, this is known to be typical for pigeons and doves (Hauser 1957, Nicolai 1962, Johnston 1965). The bird shown in the adjacent image - a Mindanao bleeding-heart dove Gallicolumba crinigera - stayed on its right side for a while, sometimes with its left wing raised and sometimes with the wing held loosely at its side. The Pink pigeon Nesoenas mayeri shown below also reclined with one wing raised (this time, reclining on its left side), but also fanned the right wing out flat on the ground. It fanned its tail feathers out at the same time.

Sunbathing passerines of all kinds typically lie on their fronts, on the ground, their wings and tails spread out as much as possible. I photographed the sunbathing Greater necklacked laughingthrush Garrulax pectoralis and Snowy-crowned robin-chat Cossypha niveicapilla shown below; Michael O’Sullivan provided the image of the sunbathing Superb starling Lamprotornis superbus and Natee Himmapaan took the photo of the European starling Sturnus vulgaris. Note that the robin-chat has raised the feathers on its back and rump, and across much of its body in fact. Spread-eagled postures like this are seen widely across Telluraves – the great neognath clade that includes owls, woodpeckers, parrots and passerines.

The birds shown here are all in alert poses, even when lying recumbent. That is, their heads are up and they’re keeping an eye on the surroundings. It’s been noted in the literature that sunbathing is frequently risky: the birds are in a prone position and are relatively incautious and easy to approach. There are some indications that birds know this, in cases preventing (or trying to prevent) their mates from engaging in sunbathing (Kilham 1981). If birds are in a particularly safe location they’ll sometimes rest the head on the ground. A particularly neat image of a Peregrine Falco peregrinus doing exactly this – it was photographed on its nest, many metres above the ground – can be seen here.

Everyone familiar with birds knows that sunbathing behaviour is common, widespread and easy to spot, and various authors have wondered what function it might serve (e.g., Goodwin 1967, Kennedy 1968, Mueller 1972, Horsfall 1984). Some passerines have been seen sunbathing when already heat-stressed (they were gaping, projecting their tongues, and reclining in areas where the ambient temperatures were already higher than those generally thought to be ideal for small birds) (Blem & Blem 1992). This supports ideas that sunbathing is sometimes carried out for some function unrelated to the need to warm the skin or body: maybe it helps dislodge parasites, helps maintain feather condition (the easiest way to straighten a slightly damaged feather it to hold it in a stream of hot air), or helps ease the discomfort associated with moulting.

And in the spirit of All Yesterdays (go here for the Tet Zoo article, here to buy the book), I'll finish by saying… where are all the sunbathing non-avian dinosaurs? (said with Aves used for the avialan crown). If you feel inspired to illustrate a sunbathing dinosaur (of any sort), feel free to send it along - if I get enough such images, I'll do a follow-up article. For previous articles on other aspects of bird behaviour, see...

Refs - -

Blem, C. R. & Blem, L. B. 1992. Some observations of sunbathing in swallows. Journal of Field Ornithology 63, 53-56.

Curry-Lindahl, K. 1970. Spread-wing postures in Pelecaniformes and Ciconiiformes. The Auk 87, 371-372 .

Goodwin, D. 1967. Some possible functions of sun-bathing in birds. British Birds 60, 363-364.

Hauser, D. 1957. Some observations on sunbathing in birds. Wilson Bulletin 69, 78-90.

Horsfall, J. 1984. Sunbathing: is it for the birds? New Scientist 103 (1420), 28-31.

Jehl, J. R. 1985. Leucism in Eared Grebes in western North America. The Condor 87, 439-441.

Johnston, R. F. 1965. Sunbathing by birds. The Emu 64, 325-326.

Kahl, M. P. 1971. Spread-wing postures and their possible functions in the Ciconiidae. The Auk 88, 715-722.

Kilham, L. 1981. Sunbathing Vermilion-crowned flycatchers repulse mates. The Auk 98, 839.

Kennedy, R. J. 1968. The role of sunbathing in birds. British Birds 61, 320-332.

- . 1969. Sunbathing behaviour of birds. British Birds 62, 249-258

Mueller, H. C. 1972. Sunbathing in birds. Zeitschrift für Tierpsychologie 30, 253-258.

Nicolai, J. 1962. Uber Regen-, Sonnen- und Staubbaden bei Tauben (Columbidae). Journal of Ornithology 103, 125-139.

Simmons, K. E. L. 1986. The Sunning Behaviour of Birds. Short Run Press, Ltd., Exeter (UK).

Storer, R. W., Siegfried, W. R. & Kinahan, J. 1976. Sunbathing in grebes. Living Bird 15, 45-58.