PREDATION AND COLONIALITY IN CLIFF SWALLOWS (PE TR 0 CHELID ON PYRRHONO TA) GERALD S. WILKINSON 1 AND GREGORY M. ENGLISH-LOEB 2 •Department of Biology, University of California at San Diego, LaJolla, California 92093 USA, and 2Department of Biology, San Diego State University, San Diego, California 92182 USA ABSTRACT.--The hypothesis that the time required to detect an approaching predator varies inversely with colony sizewas tested. Ninety-five attacks by Loggerhead Shrikes (Lanius ludovicianus) and American Kestrels (Falco sparverius) were simulated at six Cliff Swallow (Petrochelidon pyrrhonota) colonies. Colony size, which ranged from 18to 320 nests, did notexplain a significant portion of the variance in thetime it took to detect approaching predatormodels, even when effects due to wind velocity,breedingstage, nest exposure, andair temperature were removed. Onlyair temperature proved tobe a significant predictor of detection time. To determine whetheror not large colonies might reduce the relative amount of predation more effectively than smallcolonies, we conducted periodicpredator censuses around each colony and observed actual predator attacks. Relative predator density increased only five-fold, whilecolony size increased by a factor of 20. Further, we found no relationship between attack rateandcolony size.Consequently, we suggest thatlarger col- onies may dilutethe effect of local predators and thereby suffer less nestling predation on a per nestbasis.Received 24 April 1981,accepted 1 September 1981. MOST birds must detect and subsequently evadepredators if they are to survive. Birds that live in groups may havean advantage over solitary individuals, because they can ex- change information about a predator's location and thereby reducethe risk of predation on other group members. Although the evolution- ary advantages of such alarmcalling behavior arestill in dispute (see Harvey and Greenwood 1978,for review), many workers believethat one major evolutionary advantage to being in a large groupis early detection of approaching predators. In this paperwe describe field ex- periments on the Cliff Swallow(Petrochelidon pyrrhonota) thattest the effect of colony size on the time it takesto detect avian predators, and that demonstrate the ability of Cliff Swallows to discrminate predator from nonpredator models. In addition, we evaluate an alternative advantage to largecolony size, that of diluting an individual's chances of being captured (Ber- tram 1978). The dilution hypothesis states that any in- dividual'sprobability of being captured by an attacking predator decreases as the number of his neighborsincreases, assuming all have an equal chanceof being captured. Obviously, this dilution effect will always occur unless predation increases with colony size in com- pensation. Thismayoccur in two ways. Either the numberof predators may increase, or the attackrate of eachpredator may rise. If nest- lings are being preyed upon, then the magni- tude of the dilutioneffect will be greatest when the colony is highly synchronous. This idea is quite similar to the predator swampingor sa- tiation hypothesis(Lloyd and Dybas 1966, Clark and Robertson 1979), which states that groups satiate predators by synchronizing vul- nerability in time or space, thusdecreasing the probability that any given individualwill be preyed upon. Accordingly, we assess the va- lidity of the dilution hypothesis by using predator censuses, directobservations of at- tempted predation on swallows, andbreeding synchrony estimates at Cliff Swallow colonies of different sizes. METHODS Study subject.---Cliff Swallows were chosen as subjects of this investigation because they live in colonies of different sizes, emit high-pitched, single- note alarm calls(Samuel 1971a), and show a stereo- typeddisplay once an approaching predator is de- tected. They may build from 15 to several thousand of their gourd-shaped mud nests beneath a bridge, overhanging cliff, or building eave (Mayhew1958, Samuel 1971b).Consequently, only avian predators cangainaccess to theirnests easily. American Kes- trels (Falco sparverius) often attack Cliff Swallow col- onies (pers. obs.), asthey doBank Swallow (Riparia riparia) colonies(Freer 1973, Windsor and Emlen 1975). They attemptto grab perched adults(Bonnot 459 The Auk 99: 459-467. July1982