Individual variation in the competitive ability of interference-prone foragers: the relative importance of foraging efficiency and susceptibility to interference

1. Individual variation in the competitive ability of foraging animals aris es from variation in their intrinsic foraging efficiency and in their susce ptibility to interference from competitors. Empirical and theoretical studi es have concentrated on quantifying the latter and examining its role in de termining the distribution and dynamics of animal populations, but have sel dom considered the role of variation in foraging efficiency. Using the freq uency of occurrence of oystercatchers in supplementary feeding habitats as an index of their competitive ability, we assessed the relative importance of foraging efficiency and susceptibility to interference in determining th e overall competitive ability of an individual. 2. Individual mussel-feeding oystercatchers varied in their tendency to sup plement their low-tide intake by feeding on other prey on upshore tidal fla ts and in fields. Individuals that opened mussels by stabbing occurred on u pshore flats more often than birds which hammered mussels, whereas young bi rds were more likely to visit the fields than older birds. This difference between habitats emphasized the need to understand the individual differenc es underlying these class effects. 3. Foraging efficiency increased with age and differed between feeding meth ods. Dominance also increased with age, but did not differ between feeding methods. An individual's foraging efficiency was not related to its dominan ce. 4. Individual variation in the usage of either upshore flats or fields, and of each habitat separately, was related to individual variation in foragin g efficiency, but not to variation in dominance. Individuals of poor intrin sic foraging ability made greater use of supplementary feeding habitats tha n did more efficient foragers. 5. Our results show that, even in an interference-prone system and across a wide range of circumstances, individual variation in foraging efficiency i s the major determinant of overall competitive ability. We believe, therefo re, that this source of individual variation is of greater importance in de termining variation in mortality risk within a population than the effort i nvested in its study hitherto would suggest. We suggest that a modelling ap proach is necessary to establish the competitive conditions under which sus ceptibility to interference might become the more important determinant of competitive ability and, hence, whether such conditions are ever likely to occur in natural populations. We argue that greater emphasis needs to be pl aced on identifying the determinants of foraging efficiency and its variati on between individuals.