There were three words in particular that stuck out from Eno’s North West weather forecast last night, Gloomy, Damp and Heavier. The first two words are self-evident with the third relating to the intensity of droplets falling from on high. Hence I find myself sat at the PC again, thinking and blogging about birding without actually picking up my bins or pliers. In fact after recent weeks of seeing huge Swallow roosts and finding for the first time a Goldfinch roost, I found myself reflecting upon birds and their sleeping arrangements, in particular their communal roosts.
Lots of species roost communally, mostly outside of the breeding season. We have all seen evening flights of gulls heading out to sandbanks, watched massed waders on the tideline or marvelled at the acrobatic formations of thousands of Starlings or Red-winged Blackbirds at dusk. Maybe we have watched a procession of Wrens making their way into a garden nest box to spend the night, or disturbed the Blackbirds from the thick Holly bush near the front door. As a birder and a bird ringer I have been interested in roosts for a many years and both watched and worked communal roosts of Swallow, Chaffinch, Brambling, Linnet, Tree Sparrow, Greenfinch, Goldfinch, Redwing, Blackbird, Fieldfare, Pied Wagtail, Yellow Wagtail, Long-eared Owl, and various mixed wader flocks.
What all those gatherings have in common is that the birds are resting, roosting and sleeping, but it is not quite the slumber we humanoids know of eight hours tucked up in a warm bed. A bird’s disturbed and restless sleep that appears to us more like a quick nap is in fact a product of their innate ability to distribute sleep between the right and left hemispheres of their brain and effectively remain awake in the other; hence the roosting bird with one eye open, head tucked in, but always alert to predators. Also, birds that roost off the ground have specially evolved tendons in their legs so that when they are on a perch and relax their legs, their feet automatically curl around the perch and hold on with a tight grip so they can sleep without falling off. It’s easy to test that function out by allowing a Sparrowhawk or Kestrel to grip a finger or other suitable part of your anatomy.
Studies into communal roosting in birds theorise three main benefits: Guy Beauchamp, The evolution of communal roosting in birds: Behavioural Ecology (1999).
1. A reduction in thermoregulation demands, (Thermoregulation is the ability of an animal to maintain body temperature within certain boundaries, even when the surrounding temperature is very different). The presence of nearby birds in communal roosts may reduce the energy demands for thermoregulation by for instance huddling together and thereby lessening the cooling effect of wind speed or direction. My experience of winter finch and thrush roosts has shown how they favour thick evergreen bushes in sheltered spots to spend the night, but conversely Fieldfares alone amongst the thrushes seem to favour rough grassland in which to pass the dark hours. The whole idea of warding off the chill by crowding together makes absolute sense when thinking about a tightly packed flock of waders on a windswept winter beach.
2. A decrease in predation risk. For instance, the presence of more eyes in a communal roost may increase predator detection. In addition, the sheer number of individuals using a communal roost may decrease the risk of predation to an individual bird through the dilution effect. However, the greater conspicuousness of communal roosts may mitigate against the dilution effect, i.e. we have all witnessed how large roosts attract several predators. The geometric structure of the communal roost is also thought to provide increased predation avoidance; e.g. individuals that occupy central positions in a communal roost may be buffered from predators to a greater extent than birds sleeping on the edge or higher up. Does the Sparrowhawk dive into the Swallow roost and carefully select a bird from the centre or take the first one it happens upon near the top stems or the outer margins of the reed bed?
3. Increased foraging efficiency. Increased foraging efficiency is often considered the main advantage of communal roosting in birds, whereby a roost acts as an information centre where unsuccessful foragers can follow more successful or experienced companions to good feeding areas. Communal roosts have been hypothesized to be an accumulation of foragers spending the night as close as possible to good feeding areas, i.e. birds join large roosts to reduce commuting costs from the daily centre of activity or simply join in as they pass on migration. Observations from local roosts of Swallows and finches support this theory with the whole roosting process from start to finish taking a couple of hours as birds arrive in small groups from all compass directions before settling down for the night. Ringing recoveries from the same periods also support this theory with quite local recoveries but also birds from further afield or indeed highly migrant birds like Bramblings recruited into a large Chaffinch roost, or on other occasions Sand Martins and Yellow Wagtails joining a Swallow roost.
Well it’s now 2pm but still peeing down and whilst the above makes food for thought, creates a post for Another Bird Blog and passes an hour or two, I’m now tired and going for a quick snooze.
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