Intraflock variation in the speed of escape-flight response on attack by an avian predator

The benefits of flocking to prey species, whether through collective vigilance,dilution of risk, or predator confusion, depend on flock members respondingin a coordinated way to attack. We videotaped sparrowhawks attackingredshank flocks to determine if there were differences in thetiming of escape flights between flock members and the factorsthat might affect any differences. Sparrowhawks are surpriseshort-chase predators, so variation in the time taken to takeflight on attack is likely to be a good index of predation risk.Most birds in a flock flew within 0.25 s of the first bird flying,and all birds were flying within 0.7 s. Redshanks that werevigilant, that were closest to the approaching raptor, and thatwere close to their neighbors took flight earliest within aflock. Birds in larger flocks took longer, on average, to takeflight, measured from the time that the first bird in the flockflew. Most birds took flight immediately after near neighbors tookoff, but later flying birds were more likely to fly immediatelyafter more distant neighbors took flight. This result, alongwith the result that increased nearest neighbor distance increasedflight delay, suggests that most redshanks flew in responseto conspecifics flying. The results strongly suggest that thereis significant individual variation in predation risk withinflocks so that individuals within a flock will vary in benefitsthat they gain from flocking.     

Why some rails have white tails: the evolution of white undertail plumage and anti-predator signaling

Conspicuous plumage patches have evolved in birds as conspecific signals for mate attraction and assessment, intersexual competition or to signal alarm. Signals may alternatively be directed at potential predators to discourage pursuit. Rails (Family Rallidae) are ground-dwelling birds, many of which inhabit wetlands, while others occur in forests and grasslands. They are renown for their secretive nature and the tendency to flick their tails when observed. This behavior is more conspicuous in species with white undertail coverts that contrast sharply with darker body plumage. Using species comparisons and controlling for phylogeny, we investigated four hypotheses for the evolution of white undertail coverts in rails. We found little support for the hypothesis that white tails are sexually selected: white tails were not more common in species with polygamous as opposed to monogamous mating systems, species with sexual dimorphism, nor species that display their tails in courtship. Nor did our results support the hypothesis that white tail plumage evolved for intersexual competition during territorial interactions. Instead, we found that species that flock for at least part of the year and species found in open as opposed to concealing habitats were significantly more likely to have white undertail coverts. Rail species inhabiting concealing habitats are less commonly gregarious and more likely selected for crypsis. Using phylogenetically-controlled statistical inference we found that adaptation to open wetland habitats significantly precedes the evolution of white undertails, whereas gregariousness likely evolved later in some lineages. The inferred order of trait evolution suggests that this plumage characteristic could have been selected primarily for enhancement of an anti-predator signal rather than a social signal for conspecifics.     

Temporal and Contextual Consistency of Leadership in Homing Pigeon Flocks

Organized flight of homing pigeons (Columba livia) was previously shown to rely on simple leadership rules between flock mates, yet the stability of this social structuring over time and across different contexts remains unclear. We quantified the repeatability of leadership-based flock structures within a flight and across multiple flights conducted with the same animals. We compared two contexts of flock composition: flocks of birds of the same age and flight experience; and, flocks of birds of different ages and flight experience. All flocks displayed consistent leadership-based structures over time, showing that individuals have stable roles in the navigational decisions of the flock. However, flocks of balanced age and flight experience exhibited reduced leadership stability, indicating that these factors promote flock structuring. Our study empirically demonstrates that leadership and followership are consistent behaviours in homing pigeon flocks, but such consistency is affected by the heterogeneity of individual flight experiences and/or age. Similar evidence from other species suggests leadership as an important mechanism for coordinated motion in small groups of animals with strong social bonds.     

Immature gannets follow adults in commuting flocks providing a potential mechanism for social learning

Group travel is a familiar phenomenon among birds but the causes of this mode of movement are often unclear. For example, flocking flight may reduce flight costs, enhance predator avoidance or increase foraging efficiency. In addition, naive individuals may also follow older, more experienced conspecifics as a learning strategy. However, younger birds may be slower than adults so biomechanical and social effects on flock structure may be difficult to separate. Gannets are wide‐ranging (100s–1000s km) colonial seabirds that often travel in V or echelon‐shaped flocks. Tracking suggests that breeding gannets use memory to return repeatedly to prey patches 10s–100s km wide but it is unclear how these are initially discovered. Public information gained at the colony or by following conspecifics has been hypothesised to play a role, especially during early life. Here, we address two hypotheses: 1) flocking reduces flight costs and 2) young gannets follow older ones in order to locate prey. To do so, we recorded flocks of northern gannets commuting to and from a large colony and passing locations offshore and used a biomechanical model to test for age differences in flight speeds. Consistent with the aerodynamic hypothesis, returning flocks were significantly larger than departing flocks, while, consistent with the information gathering hypothesis, immatures travelled in flocks more frequently than adults and these flocks were more likely to be led by adults than expected by chance. Immatures did not systematically occupy the last position in flocks and had similar theoretical airspeeds to adults, making it unlikely that they follow, rather than lead, for biomechanical reasons. We therefore conclude that while gannets are likely to travel in flocks in part to reduce flight costs, the positions of immatures in those flocks may result in a flow of information from adults to immatures, potentially leading to social learning.     

Contagious fear: Escape behavior increases with flock size in European gregarious birds

Flight initiation distance (FID), the distance at which individuals take flight when approached by a potential (human) predator, is a tool for understanding predator–prey interactions. Among the factors affecting FID, tests of effects of group size (i.e., number of potential prey) on FID have yielded contrasting results. Group size or flock size could either affect FID negatively (i.e., the dilution effect caused by the presence of many individuals) or positively (i.e., increased vigilance due to more eyes scanning for predators). These effects may be associated with gregarious species, because such species should be better adapted to exploiting information from other individuals in the group than nongregarious species. Sociality may explain why earlier findings on group size versus FID have yielded different conclusions. Here, we analyzed how flock size affected bird FID in eight European countries. A phylogenetic generalized least square regression model was used to investigate changes in escape behavior of bird species in relation to number of individuals in the flock, starting distance, diet, latitude, and type of habitat. Flock size of different bird species influenced how species responded to perceived threats. We found that gregarious birds reacted to a potential predator earlier (longer FID) when aggregated in large flocks. These results support a higher vigilance arising from many eyes scanning in birds, suggesting that sociality may be a key factor in the evolution of antipredator behavior both in urban and rural areas. Finally, future studies comparing FID must pay explicit attention to the number of individuals in flocks of gregarious species.     

Sleeping and vigilance in birds,II. An experimental study of the Barbary dove (Streptopelia risoria)

Sleep in birds involves a period of eye closure interrupted by short ‘peeks’ of eye-opening. These peeks allow a bird to scan the sleeping site for predators. Peeking and eye closure were recorded for eight focal birds in flocks of 1, 2, 3 and 6 doves. Half these doves were high risk (predator presented before the sleeping bout) and half low risk (no predator). Individual vigilance fell with increasing flock size, and was higher in high-risk birds. However, corporate vigilance increased with flock size. High-risk flocks were more vigilant than low-risk flocks. These results are discussed with reference to the costs and benefits of eye closure and peeking.     

Advantages of social foraging of Willow Tits Parus montanus

The mean number of Willow Tits Parus montanus in single-species flocks was significantly larger than in mixed-species flocks of Willow and Coal Tits P. ater. Both flock size and the tendency of Willow Tits to join mixed-species flocks were negatively correlated with ambient temperature, probably because each bird, when the metabolic rate of the birds increased, could allocate more time to foraging due to improved predator detection by many eyes. The vigilance time of Willow Tits decreased with flock size and was determined by the total number of individuals in a flock rather than by the number of Willow Tits in mixed-species flocks of Willow and Coal Tits.     

Modelling antipredator vigilance and flight response in group foragers when warning signals are ambiguous

The trade-off between feeding and vigilance in flocks of birds has been extensively studied and modelled. An assumption of many models is that if one bird spots the predator, it gives a signal and the rest of the flock takes flight. However, it has been observed that birds do not always respond to signals and in fact many signals turn out to be false alarms. Since taking flight is both costly in time and energy, it may be advantageous for birds not to respond to all alarm calls. A model is developed to show under what circumstances birds should respond to a signal. The model predicts that under most, but not all, circumstances, birds should respond to multiple detections but not to single detections. The model also predicts that if birds respond to all flights, they will have to compensate for the time lost to feeding and the greater energy requirement of spending more time in flight, by being less vigilant, and they have a lower probability of survival than birds which only respond to multiple detections.     

Evidence for a rule governing the avoidance of superfluous escape flights

When an imminent attack by a predator on a group of birds is signalled to non-detectors only by the departure of the detector, non-detectors may make time-wasting false-alarm flights in response to mistaken or non-predator-driven departures. The frequency of false-alarm flights might be reduced if group members assess the reason for single departures before responding. Immediate flights should only occur after multiple simultaneous departures, because these are only likely to be generated by an attack. The response delay between the detectors' departure and the next birds that respond should then be dependent on the number of detectors. On sparrowhawk attack, response delays in redshanks decreased significantly as detector number increased, controlling for raptor conspicuousness and proximity, and flock size and spacing. If response delay is modified because of risk dilution, it should increase with flock size and, consequently, the rate of alarm flights due to mistakes should decrease. However, response delay did not increase and flight frequency due to misidentification of non-raptors or non-predator-driven departures did not decrease with flock size. Significantly more feeding time was lost by birds in small flocks, suggesting that the dilution effect decreased the cost of each false-alarm flight rather than their frequency.     

Habitat use of Tibetan Eared Pheasant Crossoptilon harmani flocks in the non-breeding season

Habitat use by Tibetan Eared Pheasant Crossoptilon harmani flocks in shrub vegetation was investigated in the Lhasa area of Tibet during the non-breeding season of 19951996. Home range composition varied considerably among flocks, but stream belts were consistently used as foraging grounds. Slope direction, altitude and vegetation had little effect on habitat selection. In the absence of supplemental food, core range size was positively correlated with flock size, suggesting that food supplementation could support larger flocks. Flocks regularly roosted on the ground at midday at two or three relatively fixed sites within core ranges. At night they used patches of relatively tall, dense vegetation at the year-round sites in areas near cliffs or in hollows. The size of the night-roost site was related to flock size. Our results strongly suggested that both foraging and night-roosting habitats in the shrub environment are crucial to the birds.