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Effects of Interference Competition on the Foraging Activity of Tropical Roseate Terns ’

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Effects of Interference Competition on the Foraging Activity of Tropical Roseate Terns ’ The Condor95~322-329 0 The Cooper Omthologml Society 1993 EFFECTS OF INTERFERENCE COMPETITION ON THE FORAGING ACTIVITY OF TROPICAL ROSEATE TERNS ’ DAVID A. SHEALERAND JOANNA BURGER Department of BiologicalSciences, Rutgers University,Piscataway, NJ 08855 Abstract. Although difficult to detect, interference competition among foraging birds may affect the fitness of an inferior competitor by lowering foraging success.We studied foraging flocks of Roseate Terns (Sterna dougallii) at Culebra, Puerto Rico, in 1990 to examine the effectsof small and large monospecificand mixed-speciesflocks on the foraging successand behavior of this threatenedspecies. Roseate Terns mostly foragedin small flocks with Brown Noddies (Anousstolidus). The frequency of foraging attempts in Roseate Terns decreasedwith increasingsize of mixed-speciesflocks, primarily due to passiveinterference by Brown Noddies, which foragedjust over the water surfaceand limited the area available to Roseate Terns. The size of monospecific Roseate Tern flocks had no effect on foraging attempt frequency. Roseate Terns also aborted more feeding dives and foraged more by dipping when in flocks with noddies, relative to flocks of only conspecifics.Proportionally fewer Roseate Terns than expected foraged in large, mixed-species flocks (>8 birds), sug- gestingthat they preferentially avoid large flocks that result in lower foraging success.Al- though Brown Noddies caused a reduction in Roseate Tern foraging attempt frequencies and an increasein aborted dives, we were unable to detect a negative effect on adult survival or reproductive successof this population. Key words: Competition:interference; foraging success;Roseate Tern; Sterna dougallii; Brown Noddy; Anous stolidus. INTRODUCTION Fish schools tend to be monotypic (Breder 1959, Interference competition among foraging birds Shaw 1970) suggesting that all birds in a flock may affect the fitness of an inferior competitor are vying for the same resource. Further, seabirds by lowering foraging success, particularly when use different prey-capture techniques (Porter and food availability is limited. Interference com- Sealy 1982, Hulsman 1989) creating the poten- petition occurs when one species impedes the tial for asymmetrical prey acquisition and pas- access of another species to a resource either sive interference between species. Competition through active aggression or territoriality (Schoe- probably does not lead to competitive exclusion ner 1983) or by passive means, such as creating (sensu Gause 1934) among breeding seabird spe- a barrier to the resource (Chamov et al. 1976). cies because the guild is only congregated during Maurer (1984) developed a model that predicts part of the year. Ashmole (1963) has argued that the ecological setting in which interference com- selection (in terms of mortality) in birds is stron- petition may occur. Despite Maurer’s model, only gest during the nonbreeding season when re- a few examples of passive interference compe- sources are more limited. Seasonal fluctuation in tition exist (Goss-Custard 1980, Waite 1984, food abundance may be a primary determining Poysa 1985), possibly due to the difficulty in de- factor in clutch size (Ricklefs 1980) and breeding tecting it (Maurer 1984) or in differentiating it success(Uttley et al. 1989). But competition may from exploitative competition (Anholt 1990). restrict breeding population size and reproduc- Aggregations of foraging seabirds provide an tive success as well, particularly when prey is opportunity to examine the effects of interference scarce (Ashmole 197 1). competition for prey resources. Seabirds often In 1990 we studied a guild of seabirds at Cu- congregate in mixed-species flocks over prey fish lebra, Puerto Rico, with particular emphasis on schools that are patchily-distributed and ephem- the foraging associations, behavior and success eral (Lack 1968, Safinaet al. 1988, Safina 1990). of the Roseate Tern (Sternadougallii). Roseate Terns historically nested on several cays in the Culebra Archipelago, but the breeding popula- ’ Received 15 October 1992. Accepted 15 January tion has remained consistently small (< 225 pairs; 1993. Fumiss 1983; Burger and Gochfeld, unpubl. data). I3221 INTERFERENCE COMPETITION IN TERNS 323 Nearby RoseateTern colonies at St. Thomas and scanning the flock with 7 x 35 binoculars until along the southwestcoast of Puerto Rico number the first Roseate Tern was encountered. We then up to 1,000 breeding pairs. The low numbers of timed the length of the bird’s foraging bout with Roseate Terns at Culebra may indicate that this a stopwatch and counted the number of com- site is suboptimal for breeding, possibly because pleted dives and aborted dives the bird made. of low food availability or becauseRoseate Terns Roseate Terns feed primarily by plunge-diving are inferior competitors to other breeding seabird into the water from variable heights, making speciesin the area. feeding attempts easy to quantify. We scored an Our objectives in this study were to quantify aborted dive when a tern began a dive but pulled the composition of foraging flocksat Culebra that up before contacting the water. Terns aborted included RoseateTerns, and to compare foraging dives both when other birds moved between them behavior and successof Roseate Terns between and the fish school and when their path from the large and small monospecific and mixed-species air to the water was clear. We were not certain flocks. A secondary goal of this study was to what causedterns to abort dives in the latter case, provide comparative data on the foraging be- but presumably it was related to the target fish havior of Roseate Terns in the Caribbean to the becoming inaccessible.Thus, for convention, we well-studied population in the northeastern hereafter refer to the two types of aborted dives United States. as being caused by birds and fish. We excluded all observations in which we could not determine STUDY AREA AND METHODS which situation caused an aborted dive. We studied foraging flocks of Roseate Terns in In this study we use the frequency of prey cap- the Culebra Archipelago, Puerto Rico. The study ture attempts as a relative measure of foraging site lies 37 km east of Puerto Rico proper and success.We were unable to determine capture 25 km west of the U.S. Virgin Islands (see Wet- successbecause we often observed flocks from a more 19 17, Kepler and Kepler 1978, Fumiss considerable distance and becauseprey fish were 1983, for detailed site descriptions). Roseate quite small (~20 mm standard length; Shealer, Terns nested on two small islands in the archi- unpubl. data). In 199 1, we observed R.oseate pelago (see Burger and Gochfeld 1988 for nest- Terns foraging in a shallow tidal lagoon in south- site characteristics).The total breeding popula- western Puerto Rico and found that capture suc- tion at Culebra in 1990 was 102 pairs. Other cessfrequency was highly correlated with capture breeding seabirds in the area included Brown attempt frequency (Fig. 1). Our 199 1 study only Noddies (Anous stolid@, Laughing Gulls (Larus validated this relationship up to four capture at- atricilla), Bridled and Sooty Terns (Sterna an- tempts per minute, whereas capture attempt fre- aethetus and S. fiscata), tropicbirds (Phaethon quenciesin the present study were always higher. aethereus and P. lepturus) and boobies (&da A possiblereason for this difference is that in the leucogaster,S. sula and S. dactylatra). 199 1 study, terns were foraging on larger fish On 14 days from 26 May to 1 July 1990, we (> 30 mm standard length) and in the absenceof observed foraging flocks of seabirds which in- predatory fish. Thus, search and handling times cluded Roseate Terns in the Culebra Archipel- were almost certainly greater,resulting in a lower ago. Sites were chosen opportunistically, wher- capture attempt frequency. Search and handling ever and whenever groups of birds were foraging times in the present study were presumably neg- over schools of prey fish, but all observations ligible since the surface thrashing of predatory were made from land. Data were collected only fish indicated the presenceof prey and the small- when predatory fish were visible at the surface, er fish were easier to swallow. Salt and Willard indicating that prey fish were readily available (1971) found that Forster’s Terns (Sterna fir- to foraging birds, and only when birds were ac- steri) swallowed small fish “almost immediately, tively foraging. sometimes while the bird was ascendingto hunt- When the above criteria were met, feeding ob- ing altitude.” Ulenaers et al. (1992) found that servationswere conducted on individual Roseate handling times of Great Crested Grebes (Podi- Terns until the feeding flock dispersed.A feeding cepscristatus) feeding on small fish did not limit observation consisted of first counting the total the number of prey eaten per unit time. The few number and speciescomposition of seabirds in times we observed Roseate Terns catching fish the flock. A test bird was randomly selectedby in flocks over predatory fish, we also noted that 324 DAVID A. SHEALER AND JOANNA BURGER 0 1 2 3 4 Attemptdmin FIGURE 1. Relationship between frequency of plunge dives and capturesin individual Roseate Terns. Study was conducted at a Roseate Tern colony in southwestern Puerto Rico in 1991. Points are individual birds followed for r4 min each. R2 = 0.911, df = 23, P < 0.01. fish were swallowed immediately upon the bird’s FORAGING BEHAVIOR AND SUCCESS emergencefrom the water. For the
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