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What The Flock? Birds Perform Intricate Aerial Ballets In Winter - But We're Not Sure Why

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Throughout the northern hemisphere, the dark cold months between November and February are the time when European starlings, Sturnus vulgaris, come together every evening into large flocks known as "murmurations". Prior to settling in to their chosen communal sleeping place for the night, these birds perform a series of intricate aerial manoeuvres, where the flock repeatedly circles, and changes shape and density above their roost site for roughly half an hour at dusk.

Also known as the common starling, these birds are native to the UK and Europe, and were introduced by humans throughout much of the world, where the birds have subsequently become established. Starlings are thrush-sized songbirds with short tails, pink legs and long yellow bills. Their iridescent black plumage features a cream-coloured speckle on the tip of each feather in winter (hence their name, "starling" and Sturnus, both of which which mean "little star"). In spring and summer, these birds form pairs and nest in cavities, but in autumn and winter, immature and adult birds form giant flocks that roost together at night. By sunset, these murmurations number between tens of thousands to more than a million individuals.

Since winter days are so short, birds have limited time to find food, so one would assume that starlings should conserve as much energy as possible to survive the long, cold nights. But instead of simply settling in to a safe place for the night, these vast flocks of starlings engage in a spectacular aerial ballet at sunset. Since starlings dedicate so much energy to murmurations at a time when resources are so limited, this must be an important activity for them. But surprisingly, a search of the literature reveals that we still don't know exactly why they do this.

Currently, several non-mutually exclusive ideas have been proposed that may explain this unusual social behaviour. One popular idea is that murmurations discourage predators. For example, one study reported that "waves" originate and propagate through a murmuration, away from the point of an attack by a hawk or falcon (doi:10.1016/j.anbehav.2011.07.006). It is possible that these waves may confuse a predator, thereby reducing the success of an attack. Another popular idea is the "selfish herd hypothesis", which proposes that starlings are all trying to get into the middle of the flock, where it is safest from attack by a hawk or falcon (doi:10.1016/j.anbehav.2008.08.034).

Although safety is probably a big part of the reason that starlings sleep together in large roosts, it doesn't explain why murmurations occur even in the absence of a predator.

Other hypotheses for why starlings form murmurations include the ability of flock members to assess each individual's short-term fitness, which could indicate how well each bird fed on any particular day. Individuals that appear to have been most successful finding food might be the ones to follow the next day (doi:10.1093/beheco/6.1.65). Another idea is that individual birds use murmurations, which are major athletic events that occur every evening, to assess the overall long-term health of potential mates (doi:10.1016/j.tree.2005.01.010).

Although we don't know the reasons for starling murmurations, we can still enjoy them. To find the murmuration that is nearest to you, simply contact your local Audubon Society if you live in the US, or if you live in the UK, you can contact your local RSPB for more information.

Video courtesy of freelance videographer & photographer, Paul Bunyard/Wild About Images.

Sources:

Andrea Procaccini, Alberto Orlandi, Andrea Cavagna, Irene Giardina, Francesca Zoratto, Daniela Santucci, Flavia Chiarotti, Charlotte K. Hemelrijk, Enrico Alleva, Giorgio Parisi and Claudio Carere (2011). Propagating waves in starling, Sturnus vulgaris, flocks under predation, Animal Behaviour 82 759-765 doi:10.1016/j.anbehav.2011.07.006

Claudio Carere, Simona Montanino, Flavia Moreschini, Francesca Zoratto, Flavia Chiarotti, Daniela Santucci and Enrico Alleva (2009). Aerial flocking patterns of wintering starlings, Sturnus vulgaris, under different predation risk, Animal Behaviour 77 (1) 101–107 doi:10.1016/j.anbehav.2008.08.034 ($)

Jennifer J. Templeton and Luc-Alain Giraldeau (1995). Patch assessment in foraging flocks of European starlings: evidence for the use of public information, Behavioral Ecology, 6 (1): 65-72 doi:10.1093/beheco/6.1.65 ($)

Sasha R.X. Dall, Luc-Alain Giraldeau, Ola Olsson, John M. McNamara, David W. Stephens (2005). Information and its use by animals in evolutionary ecology, Trends in Ecology and Evolution, 20 (4) 187–193 doi:10.1016/j.tree.2005.01.010 ($)

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