160 [.April•"• FLOCKING BEHAVIOR IN BIRDS BY JOHN T. EMLEN, JR. MOVERN studies of social behavior in birds have concentrated on two problems: 1) the mechanismsof integration of the organized socialunit, and 2) the effectsof the socialenvironment on the activity, fecundity, and survival of the individual. The first of these has been brought to your attention by Dr. Collias in his discussionof the devel- opment of socialbehavior (Auk, 69: 127-159, 1952). The secondwill occupy the attention of the two remaining papers in this symposium, thoseby Dr. Davis and Dr. Darling (Auk, 69: 171-191, 1952). In the interimI wouldlike to bringup for yourconsideration a third aspect of socialbehavior--that of flockingresponses, gregariousness, and the various factors which determine the size and density characteristics of bird flocks. The actuality of flocking behavior in birds does not need to be proved. It is everywhere in evidence, indeed it is difficult to find situations and specieswhich do not show at least a trace of it. Large and dense bird flocks are familiar to the most casual observer. Chat- tering hordesof migrating blackbirds, swirling cloudsof swallowsin a pre-roostingflight, jostling crowds of sea-birdson a rocky islet; such scenesprovide someof the most thrilling spectaclesto be seenin bird life. The term flock, while commonlyassociated with such spectacular phenomena,will be applied in this discussionto any aggregationof homogeneousindividuals, regardlessof size or density. The word homogeneousas used here is not to be interpreted in too strict a manner, but is employedin order to excludethe specialheterogeneous groupingsof sex and age categoriesoccurring in the breedingpair and the parent-young family group. A flock in this broad sensemight result simply from a convergenceof independent individuals at a common, localized sourceof attraction such as a patch of shade or a feeding station. It might, on the other hand, arise as a result of a mutual attraction betweenindividuals. In many bird flocks it is probable that both of these factors operate, the relative r5les of each varying with the speciesand with the circumstances. SOCIAL FoRcES IN BALANCE The convergenceof birds in responseto external physical factors presents,in itself, no great problemsto the student of bird behavior. Social responses,on the other hand, are highly complexand fraught with challengingproblems. Vol.1952 69•J ]•MI,I•N,Flocking Behavior in Birds 161 The tendencyof birds to respondpositively to the presenceof others of their kind, commonly referred to as gregariousness,is little under- stooddespite its conspicuousnessand widespreadoccurrence. Various writers have comparedit with hunger, a craving or sensationof dis- comfortwhich arisesin the absenceof a physicalrequirement. Trotter (1916:30) describedgregariousness as an impulsein individualsto be in and remain with the flock and to resist anything which tended to separatethem from it. Craig (1918) classifiedit as an appetite, which he definedas "a state of agitation which continuesso long as a certain stimulus--is absent," and which is resolvedas soon as the appeted stimulusis received. Wheeler (1928:11) comparedit with the appe- tites of hungerand sexand notedits persistentnature and its striking effects on segregatedindividuals. Various psychologistshave re- garded it as a condition of responsivenessto social stimuli which, if blocked, leads to frustration activities. Illustrationsof gregariousbehavior are not hard to find. Nearly everyone has watched stragglersfrom a flock of Starlings hurry to join their confreres,or has seen passingCrows respondto a flock of their kind on the ground. Duck and goosehunters are thoroughly familiar with the effect that a groupof decoyshas on their quarry. Alverdes(1927:108) noted how the artificial isolationof a socialanimal suchas a dogproduces numerous signs of discomfortwhile the presence of a companion,even one belonging to anotherspecies, will quiet these "socialcravings." Stresemann(1917) discussedthe fascinationwhich a flock of birds holdsfor a segregatedindividual. While few would deny this positive social reaction among the membersof a flock, Allee (1931) and others have pointed out that there is another factor operatingin the formation and regulationof aggregations, the factor of tolerance of social encroachment. Socialtolerance may be consideredas promotingflocking behavior by permitting the membersof a populationto convergein response to either environmental or internal (gregarious)factors. For our purposes,however, it is convenientto considertolerance in its negative aspect as intolerance, an expressionof independenceor self-assertion acting in oppositionto forceswhich tend to bring birds togetherinto flocks. Socialintolerance is functionallythe antithesisof gregarious- ness. If we follow Craig in callingthe cravingfor companionshipan appetite,this second,negative factor is an "aversion,"a state of agi- tation whichcontinues so long as a certain stimulusis presentbut which ceaseswhen that stimulusis withdrawn (Craig, 1918). This negativereaction of individualsto crowdingis of widespread occurrencein animal populationsof all kinds and needslittle elabora- 162 Sm,•N,Flocking Behavior inBirds [April[Auk tion. It is particularly conspicuousin birds and is nowhere better illustrated than in the territorial behavior of both colonial and non- colonial species. It underlies most of the situations which lead to conflict between individuals in nature and is considered basic in our modern conceptsof the mechanismsof population regulation. Prob- lems of social tolerance and the r81e of aggressionin the integration and regulation of bird flocks have recently been reviewed by Collias (1944). We thus have two opposingforces, a positive force of mutual attrac- tion and a negativeforce of mutual repulsion,interacting in the forma- tion of bird flocks. The positive force initiates the processand acts centripetally in drawing membership; the negative force serves a regulatory r81e, limiting the size of the flock and preventing dose crowding through its centrifugal action. Such a concept may be criticized by those who, encounteringdifficulties in elucidating emo- tions in subhumansubjects, object to the word "force" as applied to bird behavior. In the present ease, however, I am not referring to any stored or penned-upenergy but simply to the causeof the centri- petal or eentifugalmovements observed, whatever that might be (see Webster's unabridged dictionary, 2nd edition, definition No. 15). The responsesmight be regardedas essentiallytropistie and compara- ble to the prototaxesproposed by Wallin (1927). No matter what terminology we choose, there seems little doubt that positive and negativesocial responses occur and interact. Craig observedthis interaction in the behavior of his caged Ring Doves as they settled on their roostsfor the night. Each bird, he sayssought a perch close to friendly companions,but ,tot too close,and the diffi- culties involved in satisfying both the appetite for companionshipand the aversionfor crowdingoften kept the birds busy for more than an hour. I have observedsimilar performancesin roosting crows and in loafing flocks of starlings and swallows. By way of illustration, I would like to relate some observations which I made on Cliff Swallows, Petrochelidonpyrrhonota, during the past summer. Cliff Swallowsnear a group of nestingcolonies at Moran, Wyoming, spent much of their time loafing on spansof telephonewires. Positive social forces were immediately apparent in this behavior for the dis- tribution of the birds over the available percheson the roostswas far from random. Of the thousands of linear feet of wire to be found in the area only a few relatively small sectionswere used at any one time. One or two birds, alighting apparently at random, typically served as the nucleus for a potential gathering. Other birds followed until an aggregation of 100 or more had accumulated, all within a space of 100 to 150 feet. Vol.1952691 J E•xL•N,Flocking Behavior in Birds 163 Negative socialforces were also apparent, for in spite of the overall compactnessof the group no bird ever held a perch closerthan about four inchesfrom its nearest neighbor. Birds were constantly arriving and leaving, and an unstable situation occasionallyarose when a bird attem )ted to securea perch too closeto another, already-settledbird. 32 /o •O•o a 20 o/ o•ø _e•" ß ß 0'• • • I I I I I I I I I I I I I I I I I 0 • • • 8 I0 I• I• I• ,18 TI•E IN •INUTES FIOgR• 1. Sample curve showingin.ease in numb• of Cliff Swallowsp•ching on the •n•al thkd (open ckcles), east tMrd (solid ckcl•), and west thkd (half ekcles) of a sectionof telephonewkes ne• Moran, Wyoming, Au•st 4, 1950. Shuffling and reshufflinginevitably followed until the proper spacing had been reestablishedall down the line. Ten-inch gaps were filled centrally and without trouble, a six-inch spaceon the o•er hand was usually avoided and, if chosen,was invaded only with the accompani- ment of aggressivedisplays and a local reshufflingof perches. Swal- lows move their feet but little when once settled and this tolerance limit of four inches may be related to the maximum reach of a bird from a fixed perch. In an accumulating aggregation of this sort the een•al portion of the perching area tended to fill more rapidly than did the peripheral portions, and shifts from peripheral to central positionsoeecred more frequently than did shifts in the other direction. Thus when the perching area was optically divided by the observerinto