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Lek Behaviour in Birds: Do Displaying Males Reduce Nest Predation?

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Lek Behaviour in Birds: Do Displaying Males Reduce Nest Predation? Anim. Behav., 1990, 39, 555-565 Lek behaviour in birds: do displaying males reduce nest predation? JOHN B. PHILLIPS Department of Biology, Indiana University, Bloomington, IN47405, U.S.A. Abstract. Male lek display in birds may reduce nest-related predation by decoying predators away from nests and alerting incubating females when a predator is approaching. The sentinel/decoy model predicts a region of decreased predator density just inside the maximum range at which predators are attracted by displaying males. The expected ring of successful nests is evident in data from three species of North American prairie grouse. Well-documented features of female mate choice in lekking species, including repeated visits to male display sites prior to mating, mate fidelity and mate copying, are consistent with females maximizing the proposed antipredator benefit. The sentinel/decoy model makes a number of unique predictions that will facilitate critical tests of the model. Lek mating systems have drawn the interest of males may arise because direct comparison of evolutionary biologists because it is assumed that males enables females to assess male quality more sexual selection and, in particular, female choice efficiently or more accurately (Emlen & Oring can be studied without the confounding influence 1977). Although there is no evidence to indicate of direct (or 'material') benefits that males provide whether mate choice in lekking species results in an to females (Borgia 1979; Bradbury & Gibson 1983). increase in the overall fitness of offspring, good In lek mating systems, males display conspicuously genes models remain popular because of the from small display territories, but do not control assumed absence of a direct benefit of mate choice resources that are necessary for reproduction or in lekking species (e.g. Borgia 1979). provide parental care, and do not control access to The 'hotspot' model (Bradbury & Gibson 1983; females (Bradbury 1981). This definition of lek Bradbury et al. 1986) proposes that leks form at behaviour includes both 'classical' leks in which local regions of high female density ('hotspots'). male display sites are densely clustered and According to the hotspot model, classical leks 'exploded' leks in which display sites are more should occur in species where there is considerable dispersed. variation in the density of females, while exploded Several recent models have attempted to explain leks should occur in species where the distribution various aspects of lek behaviour in birds such as of females is more uniform. Bradbury & Gibson dispersion of male display sites, variation in male provide data consistent with these predictions, mating success and location of nests relative to which suggests that classical leks organization male display sites (see Bradbury & Gibson 1983 for generally occurs in species with large, overlapping a critique of older ideas). female home ranges and diets (e.g. frugivory) that The 'male-avoidance' model (Wrangham 1980) promote female aggregation (see below). proposes that females select males that display The 'hotshot' model (Beehler & Foster 1988) away from nest sites to avoid costs associated with proposes that initial differences in male attractive- the presence of males (e.g. conspicuousness to ness to females, coupled with 'conservative' female predators, competition for food and wasted time mating patterns (e.g. mate fidelity and copy of other and energy resulting from unsolicited courtship). females), produces male 'hotshots' that are highly The most important prediction derived from successful in mating. Unsuccessful males associate Wrangham's model is that nesting success should with hotshots to gain access to females, but mating increase with distance from male display sites. is restricted by dominance interactions within the 'Good genes' models (e.g. Bradbury 1981) pro- group of males. The principal predictions of the pose that females select males with superior geno- hotshot model stem from the presumed influence of types that increase the genetic quality of the female mate fidelity and female copying on male females' offspring. According to the good genes mating success. These predictions will be discussed models, preference for mating within a group of in a later section (below). 0003-3472/90/030555 + 11 $03.00/0 1990 The Association for the Study of Animal Behaviour 555 556 Animal Behaviour, 39, 3 (a) 20 (c) >,15. ~, Io. (b) 7. r Distance from display site .::... "!> Figure 1. (a) Predators (solid dots) were assigned random locations and directions of movement. Outside the radius ('r') at which predators are attracted to the male display site (' +'), the predators' movements (arrows) were assumed to be independent of the displaying males. For predators located within the radius of attraction, however, the arrows are the average of the 'spontaneous' (randomly assigned) directional preference and a 'male-directed' directional preference of twice its strength. (b) The location of each predator is shown after an arbitrary distance of movement (indicated by the length of arrows in a). The average density of predators at progressively greater distances from the male display site is plotted in (c). Just inside 'r', the density of predators is lower than the average density in the habitat (---), creating an optimal zone (~II)where nests and incubating females will experience lower predation. Common to all four models is the assumption and 80-90% in tropical species (Snow 1963; Lill that males in lekking species contribute only their 1974a; Pitelka et al. 1974; Wittenberger 1978). gametes to the next generation. This assumption is Although males do not directly defend nests or virtually universal in the recent literature on lek incubating females against predators, a majority of behaviour (e.g. Emlen & Oring 1977; Wittenberger nests are located within the range of male displays 1978; Bradbury & Gibson 1983; Beehler & Foster (see below). Hence, the behaviour of displaying 1988). In this paper, I propose a direct benefit of males may affect the likelihood of predation on lek display to nesting females: a reduction in nest nests and incubating females. predation due to the presence of conspicuously Males may benefit females by serving as decoys. displaying males. If predators are drawn toward the males' display Lekking birds are subject to high levels of nest site, a region of decreased nest predation should predation. Typically 40-60% of nests are dep- occur just inside the maximum distance at which redated in temperate zone, ground-nesting species these predators are attracted (Fig. 1). Nests located Phillips: Nest predation in lek-breeding birds 557 in this 'optimal' zone should be exposed to a lower NESTING SUCCESS density of predators because predators drawn away frofn nests by the displaying males will be replaced The male sentinel/decoy model makes several at a lower rate by predators entering the optimal testable predictions about the influence of display- zone by chance from the region beyond the range of ing males on the relative spatial and temporal male displays. Predators may also be less effective distribution of successful and unsuccessful nests. in searching the area within range of the displaying (1) Nests located in the hypothesized region of males, either because they focus their attention on reduced predator density (Fig. 1) should experience the displaying males or alter their behaviour to lower levels of predation relative to nests located stealthily approach male display sites. both closer to and farther from display sites than Males may also serve as sentinels. If incubating this optimal distance. Therefore, successful nests females can detect male responses to a predator should occur at approximately the same mean approaching the display site (e.g. an alarm signal, distance from the nearest display site, but exhibit escape response, cessation of display or shift in the less variance in that distance, than do nests and location of display), they can use displaying males incubating females that are preyed upon. as sentinels. A female that is forewarned to the approach of a predator can behave cryptically to This prediction contrasts with the male-avoidance decrease the likelihood of being located. In contrast model (Wrangham 1980), which predicts that suc- to the decoy function of males, the value of males as cessful nests will on average occur at greater dis- sentinels will be greatest for females nesting close to tances from male display sites, because at greater the mate display site and will decrease gradually distances, there will be fewer of the costs associated with increasing distance until females are no longer with the presence of the conspicuously displaying able to detect the male response. males. The maximum benefit to incubating females (2) Nest sites at the optimal distance from male should be derived from males that serve as both display sites should be occupied preferentially. If decoys and sentinels. Predators attracted by dis- there is competition for nest sites, subordinate playing males will be drawn away from nests located females should be displaced closer to, or farther at an optimal distance from the male display site from, the display sites than the optimal distance. (Fig. 1), but will again pose a threat as they leave the vicinity of the displaying male(s). However, if male This expectation of the male sentinel/decoy model behaviour alerts incubating females to nearby pre- also differs from Wrangham's model, which pre- dators, females will be able to minimize the danger dicts that nests located beyond the influence of from departing predators by behaving cryptically. displaying males should be occupied preferentially. The sentinel function of males may cause the opti- (3) If displaying males are removed or cease dis- mal distance for nests to be closer to male display playing while females are incubating, nest success sites than would occur if only the decoy benefit was should decrease and the predicted clustering of operating (and may moderate to some degree the successful nests should be eliminated. impact of high predator densities near the male display site).
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