Abstract
Pre-roost murmuration displays by European starlings Sturnus vulgaris are a spectacular example of collective animal behaviour. To date, empirical research has focussed largely on flock movement and biomechanics whereas research on possible causal mechanisms that affect flock size and murmuration duration has been limited and restricted to a small number of sites. Possible explanations for this behaviour include reducing predation through the dilution, detection or predator confusion effects (the “safer together” hypotheses) or recruiting more birds to create larger (warmer) roosts (the “warmer together” hypothesis). We collected data on size, duration, habitat, temperature and predators from >3,000 murmurations using citizen science. Sightings were submitted from 23 countries but UK records predominated. Murmurations occurred across a range of habitats but there was no association between habitat and size/duration. Size increased significantly from October to early February, followed by a decrease until the end of the season in March (overall mean 30,082 birds; maximum 750,000 birds). Mean duration was 26 minutes (± 44 seconds SEM). Displays were longest at the start/end of the season, probably due to a significant positive relationship with day length. Birds of prey were recorded at 29.6% of murmurations. The presence of predators including harrier Circus, peregrine Falco peregrinus, and sparrowhawk Accipiter nisus was positively correlated with murmuration size (R2 = 0.401) and duration (R2 = 0.258), especially when these species were flying near to, or actively engaging with, starlings. Temperature was negatively correlated with duration but the effect was much weaker than that of day length. When predators were present, murmurations were statistically more likely to end with all birds going down en masse to roost rather than dispersing from the site. Our findings suggest that starling murmurations are primarily an anti-predator adaptation rather than being undertaken to attract larger numbers of individuals to increase roost warmth.
Highlights
Collective animal behaviour, where multiple individuals of a species act in a highly-coordinated manner, is both taxonomically and ecologically widespread [1]
One spectacular example of collective behaviour is the pre-roost aerial displays undertaken by European starlings Sturnus vulgaris [9,10]
Using a citizen science approach, we gathered more than 3,000 records of starling murmuration behaviour from across the UK and covering two years
Summary
Collective animal behaviour, where multiple individuals of a species act in a highly-coordinated manner, is both taxonomically and ecologically widespread [1]. One spectacular example of collective behaviour is the pre-roost aerial displays undertaken by European starlings Sturnus vulgaris [9,10]. These murmurations (so-called because of the sound produced by multiple wingbeats) can involve thousands of individual birds forming a coherent three-dimensional murmuration “cloud” within which the movement of each individual bird is highly cohesive and synchronized. This synchronised movement means that the group forms a range of different shapes [11,12] including spheres, planes and waves [13,14] while remaining more-or-less static with respect to a focal point on the ground (generally the roosting site). Murmurations generally end with the birds descending en masse to roost
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