- Top of page
Nest predation is one of the major selective agents shaping avian life histories. Anti-predator strategies evolved by birds include direct effects of parental behaviour (nest defence, sensuMontgomerie & Weatherhead 1988) as well as indirect ones, such as decision where (nest site selection) and when (timing) to breed. Nest defence may be active, including direct attacks, mobbing, distraction display, nest guarding and vocalization, or passive through sitting on the nest (nest attentiveness), thereby hiding the eggs from view and serving to camouflage the nest and to discourage some of the potential predators (Montgomerie & Weatherhead 1988; Martin 1992a). The effectiveness of different kinds of nest defence depends on the bird’s capacity to perceive risk associated with the particular predator (McLean & Rhodes 1991; Burhans 2000; Veen et al. 2000) and capacity to deter or distract it (Blancher & Robertson 1982; Komdeur & Kats 1999; Schmidt et al. 2001a). Apart from cost through allocation of time between nest defence and competing activities (Martin 1992a; Komdeur & Kats 1999), parental activity associated with nesting can be costly because it can attract predator’s attention to the nest (Martin et al. 2000a; Martin, Scott & Menge 2000b). The relative strength of the positive (‘nest defence’) and negative (‘nest disclosure’) effects of parental activity on nest survival in particular bird species is likely to depend on the type of nest predators involved and their foraging behaviour.
The lack of consistent patterns in nest predation studies has traditionally been attributed to spatial/temporal variation in abundance of nest predators, or to interactions among the effects of nest site, parental and predator behaviour (Götmark et al. 1995; Hogstad 1995; Murphy, Cummings & Palmer 1997; Söderstrom, Pärt & Rydén 1998; Rangen, Clark & Hobson 1999; Schmidt 1999; Dion, Hobson & Larivière 2000; Martin et al. 2000b; Schmidt et al. 2001b). For example, Cresswell (1997) ascribed the lack of uniform (across species) effect of nest concealment to a compensating effect of parental nest defence operating on the within-species level. This hypothesis predicts that species capable of efficient nest defence will show a weaker relationship between nest concealment and survival than species that cannot deter predators and must rely on passive protection through nest concealment. Assumption implicit in this explanation is that on the within-species level, parent birds can respond to nest site associated risk of predation by altering their behaviour (McLean, Smith & Stewart 1986; Hobson, Bouchart & Sealy 1988; Burhans & Thompson 2001). Ricklefs (1977) found some support for an interspecific relationship between mean nest concealment and intensity of nest defence. Yet, sound interpretation of the observed patterns of predation is often limited by unknown identity of principal nest predators in the particular study system (Söderstrom et al. 1998; Pietz & Granfors 2000; Willson & Gende 2000).
Figure 1. Model relating nest survival to possible effects of parental behaviour and nest concealment, as reflected in survival of natural (with parents; solid lines and filled points) and experimental (without parents; dashed lines and open points) nests. Points indicate the mean survival rate across all levels of nest concealment. The net effect of parental behaviour can be: (A) neutral – either no effect or antagonistic effects that offset each other; (B) positive –‘nest defence’; (C) negative –‘nest disclosure’. Effect of nest concealment unconfounded by parental behaviour can be: (a, d) neutral; (b, c) positive. Effects of parental behaviour and nest concealment can be: (a, c) independent; (b, d) interacting. Each subplot is identified by column (A–C) and row (a–d) coordinates.
Download figure to PowerPoint
It is assumed that survival of experimental nests reflects effect of nest site (e.g. concealment) regardless of parental behaviour, whereas survival of natural nests reflects combined effects of nest site and behaviour. The positive (nest defence) or negative (nest disclosure) effects of parental behaviour are broadly defined as any effects due to presence of parents at the nest that have positive or negative effect on nest survival. These positive and negative effects may combine, thus yielding the net effect of behaviour that can be neutral (positive and negative effects offset each other), positive or negative. This net effect can be inferred from comparison of the mean survival rates between natural and experimental nests: no difference (neutral effect; Fig. 1A); higher survival of natural nests (positive effect; Fig. 1B); lower survival of natural nests (negative effect; Fig. 1C). Effect of nest concealment unaffected by parental behaviour can be inferred from the relationship shown by experimental nests: neutral effect (Fig. 1a,d); positive effect (Fig. 1b,c). The null model is that neither behaviour nor concealment effect is present (Fig. 1Aa). Any of the behaviour effects may combine with any of the concealment effects, both effects being either independent (Fig. 1a,c) or interacting (Fig. 1b,d). Interaction means that the effect of behaviour is not constant across different levels of nest concealment and can be of two opposite directions. First, parental behaviour can compensate for the effect of nest concealment, yielding no relationship between concealment and survival in natural nests (Fig. 1b). Alternatively, parental behaviour can generate relationship between concealment and survival of natural nests in cases, where effect of nest concealment per se is neutral (Fig. 1d).
The model shows that each of the two basic patterns exhibited by survival of natural nests (Fig. 1a,b vs. c,d) can be explained by several alternative mechanisms. First, lack of relationship between concealment and survival of natural nests (Fig. 1a,b) can result either from no effect of concealment combined with independent effect of parental behaviour (Fig. 1a), or from positive effect of concealment masked by an interacting effect of behaviour (Fig. 1b). Second, the positive relationship between concealment and survival of natural nests (Fig. 1c,d) can result either from positive effect of concealment combined with independent effect of parental behaviour (Fig. 1c), or may be due to effect of behaviour interacting with no effect of concealment (Fig. 1d). The above alternatives generate different predictions for survival of artificial nests relative to that of natural nests, and thus can be evaluated experimentally.
This study was designed to test these alternative hypotheses using data on four open nesting passerines characterized by different body size (presumed correlate of defence potential) and different nest size and placement (correlates of overall nest conspicuousness, accessibility to predators and risk of disclosure through parental activity). The survival of natural and experimental nests of each species was concurrently monitored to separate the relative effects of parental behaviour and nest site. Cresswell’s (1997) hypothesis generates the following predictions for the species under study. Song thrush (Turdus philomelos, Brehm) and blackbird (T. merula, Linnaeus) – relatively high nest defence potential and conspicuous nests: natural nests should survive on average better than experimental nests; there should be no relationship between concealment and survival for natural nests but a positive one for experimental nests (Fig. 1Bb). Blackcap (Sylvia atricapilla, Linnaeus) and yellowhammer (Emberiza citrinella, Linnaeus) – relatively low nest defence potential and concealed nests: there should be no difference in mean survival rates between natural and experimental nests, survival should increase with concealment in both natural and experimental nests (Fig. 1Ac).
- Top of page
The null model (Fig. 1Aa) was rejected – apart from multitude of other suspected factors, nest survival was influenced by parental behaviour and/or nest concealment in all species and in most years. The basic prediction was supported; the principal nest predators and the relative importance of behaviour and nest site effects differed among species. The observed patterns included: positive effect of parental behaviour combined with neutral effect of nest concealment (Fig. 1Ba; thrushes); independent positive effects of behaviour and concealment (Fig. 1Bc; yellowhammer); neutral effect of behaviour combined with positive effect of concealment (Fig. 1Ad/Ac; blackcap). These species-specific patterns are consistent with the hypothesis (Ricklefs 1977; see also Murphy et al. 1997; Willson et al. 2001) that relatively larger species with more conspicuous nests should engage in more vigorous nest defence compared to smaller species with less conspicuous nests. The question of whether the observed species-specific effects of behaviour result more from differences among species in the intensity of nest defence or from differences in its efficiency remains open to further study. The interspecific pattern was further complicated by the within-species interactions between nest site and type of predator, which effects may prove to be quite common (Rangen et al. 1999; Dion et al. 2000). Moreover, predation risk associated with particular nest sites may be influenced by interactions among several different predators, both within and between trophic levels (Schmidt et al. 2001b).
Experimental control of parental behaviour enabled the mechanisms behind the observed effects of nest concealment to be distinguished. The positive relationship between concealment and nest survival resulted either from effect of nest site per se (yellowhammer) or from the effect of parental behaviour (blackcap), which changed from negative (nest disclosure) to positive (nest defence) between poorly and well-concealed nests, respectively. The alternative view, that the lack of within-species relationship between nest concealment and survival (thrushes) results from parental behaviour compensating for the predation risk associated with poor nest concealment (Cresswell 1997), was not supported.
The evolutionary patterns in life-history traits associated with the risk of nest predation depend on the proximate mechanisms of predator–prey interactions (Martin et al. 2000b). The accumulating evidence in support of multiple and interactive effects on nest predation is reflected in a growing feeling that further studies should integrate, among others, effects of nest site, parental behaviour and foraging behaviour of predators in a single conceptual framework (Murphy et al. 1997; Schmidt 1999; Martin et al. 2000b; Willson & Gende 2000). Potential explanatory power of mechanistic approach has been demonstrated by Schmidt et al. (2001a,b). Highly variable and even conflicting results of this study demonstrated the importance of scale in nest predation studies. When considered separately, the apparently clear-cut results of partial analyses (single years/species), would lead to different conclusions, which might be the case with small-scale experiments.