Predator detection and avoidance by starlings under differing scenarios of predation risk

Practically all animals must find food while avoiding predators.An individual's perception of predation risk may depend on manyfactors, such as distance to refuge and group size, but it isunclear whether individuals respond to different factors ina similar manner. We tested whether flocks of foraging starlingsresponded in the same way to an increased perception of predationrisk by assessing three factors: (1) neighbor distances, (2)habitat obstruction, and (3) recent exposure to a predator.We found that in all three scenarios of increased risk, starlingsreduced their interscan intervals (food-searching bouts), whichincreased the frequency of their vigilance periods. We thenexamined how one of these factors, habitat obstruction, affectedescape speed by simulating an attack with a model predator.Starlings were slower to respond in visually obstructed habitats(long grass swards) and slower when they had their head downin obstructed habitats than when they had their head down inopen habitats. In addition, reaction times were quicker whenstarlings could employ their peripheral fields of vision. Ourresults demonstrate that different sources of increased riskcan generate similar behavioral responses within a species.The degree of visibility in the physical and social environmentaffects both the actual and perceived risk of predation.     

When are two heads better than one? Visual perception and information transfer affect vigilance coordination in foraging groups

Animals frequently raise their heads to check for danger. Ina group, individuals generally raise their heads independently.Earlier models suggest that all group members could gain bycoordinating their vigilance, i.e., each member raising itshead when others are not. We re-examine these suggestions, consideringgroups of different sizes, in light of empirical findings that:(1) animals can sometimes detect a predator without raisingtheir heads, and (2) when one member of a group detects a predator,the information does not always spread to other members of thegroup. Including these effects in models generally decreasesthe value of coordinated vigilance. Coordinated vigilance ishighly favored only when animals have a low probability of detectingpredators without lifting their heads but a high probabilityof being warned when another member of the group detects a predator.For other combinations, coordinated vigilance has little valueand may have a negative value. Group size has contrasting effectsdepending on how social information is obtained. Coordinationis favored in smaller groups when one or more detecting individualsprovide a constant amount of information to individuals unawareof the predator. On the other hand, coordination is favoredin larger groups if each detecting individual provides unawareindividuals with an independent source of information (i.e.,if the amount of information increases as the number of detectingindividuals increases). These results depend on the balanceof an escape due to social information and dilution of riskin groups with imperfect information spread. This frameworkcould be tested by examining species with different visual fieldsand in different environments.     

The group-size paradox: effects of learning and patch departure rules

In many species, foraging in groups can enhance individual fitness.However, groups are often predicted to be larger than the sizethat maximizes individual fitness. This is because individualforagers are expected to continue joining a group until thefitness in the group falls to the level experienced by solitaryforagers. If such a process were pervasive, social foraging,paradoxically, would provide little evolutionary advantages.We propose a solution to the group-size paradox by allowingforagers to learn about habitat quality and leave food patcheswhen their current intake rate falls below that expected forthe whole habitat. By using a simulation model, we show thatunder a wide range of population sizes, foragers using suchrules abandon under- and overcrowded patches, ensuring thatgroup size remains close to the optimal value. The results holdin habitats with varying patch quality, but we note that thelack of food renewal in patches can disrupt the process of groupformation. We conclude that groups of optimal sizes can occurfrequently if fitness functions are peaked and resources patchilydistributed, without the need to invoke relatedness betweenjoiners and established group members, group defense againstjoiners, or other mechanisms that were proposed earlier to preventgroups from becoming too large.     

Flock density, social foraging, and scanning: an experiment with starlings

Social foraging differs from individual foraging because italters both resource availability and the forager's behavior.We examined responses of starlings to the presence of conspecificsby manipulating foraging-group density experimentally, whileensuring that each subject's foraging opportunities were unchanged.To do this, we used individuals foraging simultaneously in fourbottomless enclosures placed at various separations in naturalforaging grounds. We measured foraging and scanning intensityand qualitative aspects of scanning of focal individuals. Additionally,we examined the temporal distribution of scanning between individuals.The focal individual analysis showed that (1) food-searchingactivity increased, while time spent scanning, time off theground and scanning bout length decreased with flock density;(2) food finding per unit of searching effort increased withdensity; (3) head orientation during scanning was sensitiveto companions' proximity: heads pointed away from the companionsat close distance, toward them at intermediate distance, andwas random farther away. The analysis of the (temporal overlappingin scanning) temporal distribution of scanning for the groupshowed that scanning was significantly synchronized when companionswere adjacent to each other but was not significantly differentfrom random at further separations. We conclude that behavioralresponses of individuals to the presence of others generateimportant changes in foraging performance even in the absenceof physical interference and, more generally, that assessingthe mechanisms that control the behavior of group members atdifferent flock densities offers a way to understand the functionaland ecological significance of foraging aggregations.