Behavioral Ecology Advance Access published online on June 17, 2008
Behavioral Ecology, doi:10.1093/beheco/arn063
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Predator-induced reductions in nest visitation rates are modified by forest cover and food availability
a Department of Ecology, Swedish University of Agricultural Sciences, PO Box 7044, SE-75007, Uppsala, Sweden b Population Biology, Department of Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, SE-752 36, Uppsala, Sweden c Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield, S10 2TN, UK
Address correspondence to S. Eggers. E-mail: sonke.eggers{at}ekol.slu.se.
Received 29 November 2007; revised 8 May 2008; accepted 15 May 2008.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Bird parents can alert predators to the location of their nest. One mitigating option is that parents reduce their nest visitation rate in exchange for a lower predation risk. Here, using field data and experiments, we show that Siberian jay Perisoreus infaustus parents adjust feeding visit rates depending on an interaction of 3 factors: predator activity, nest concealment, and food availability. The rate of nest visits increased with the degree of nest concealment; yet, this relationship was modified by the presence of corvid predators. As the vegetation became more dense, parents at sites with high corvid activity disproportionately increased their feeding visit rates when compared with birds at sites with low corvid activity. We experimentally assessed how nesting cover affects this response of parents to the presence of corvids by using an Eurasian jay Garrulus glandarius model. Parents nesting at open sites ceased nest visits, whereas those nesting in dense forest continued feeding, albeit at a lower rate. Cover may thus not fully compensate for the effect of predator activity on feeding visit rates. However, offspring exposed to high predator activity might still receive the same amount of food because parents may adjust load sizes to compensate. This idea was confirmed by an experiment showing that in areas of high predator activity, food-supplemented birds significantly decreased nest visits when compared with nonsupplemented birds. These results indicate that some bird species can employ multiple nest-defense strategies to reduce predator-attracting nest visits; yet, these strategies may carry fitness consequences through reduced offspring quality.
Key words: life history, nest activity, nest predation, parental care, phenotypic plasticity, provisioning strategy.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Under conditions where the delivery of food to offspring can alert predators to the nest location, bird parents are expected to trade the cost of reduced food delivery against the benefits of reduced predation risk (Skutch 1949
; Martin et al. 2000a, 2000b; Ghalambor and Martin 2001). Thus, nests that are more susceptible to predation should be characterized by lower parental nest visitation rates relative to nests in safer habitats. However, evidence for this predator avoidance hypothesis comes mainly from interspecific comparative analyses (Martin et al. 2000a; Strickland and Waite 2001) and experimental field studies on short-term responses of parents to the threat posed by predation (Ghalambor and Martin 2000, 2002). For contemporary bird populations, empirical evidence for predator-induced plasticity in overall nest visitation rates is scarce (Fontaine and Martin 2006).
One difficulty in finding evidence in support of this hypothesis is that the relationship between parental activity and nest predation is dependent on multiple factors including: nest type, the parents' capacity to deter predators, and the predator's search behavior (Collias NE and Collias EC 1984; Cresswell 1997; Weidinger 2002). For instance, nests may become relatively safe when burrows (e.g., sand martin Riparia riparia) or tree holes (e.g., most parrots, tits) are part of their construction making them relatively inaccessible compared with open cup–shaped nests (Lack 1968; Martin 1995). Moreover, larger species may often be able to deter predators thus making their nests less prone to predation (Strickland and Waite 2001). Also, the risk of nest predation is not static; mobile predators can create temporal variation in danger which could drive much of the variation in antipredator efforts throughout the day without influencing overall feeding visit rates (Lima and Bednekoff 1999; Eggers et al. 2005b).
For instance, Siberian jays Perisoreus infaustus reduce nest visits at times of the day when corvid nest predators frequently occur within sight of the nest but compensate for lost feeding opportunities by allocating more feeding effort to periods when predators are less active (Eggers et al. 2005b). Alternatively, reductions in nest visits as an antipredator strategy may be compensated for in species, which can increase the food delivery per visit, such as jays with their expandable throats (Strickland and Waite 2001). Choosing a nest site that provides better concealment of the nest (e.g., in dense vegetation) may also compensate for increased predator activity (Marzluff 1988; Eggers et al. 2006) although evidence for a link between nest concealment and nest fate is equivocal (Martin and Roper 1988; Howlett and Stutchbury 1996; Eggers et al. 2005a). It is important to recognize that these behavioral tactics are not mutually exclusive and may operate within a single system, making these antipredator behaviors difficult to detect without careful observation and experimental manipulation of these systems.
Here, we investigate whether Siberian jays employ multiple nest-defense behaviors. We combined field data and experiments in order to answer the following 3 questions. 1) Do Siberian jays reduce overall nest visitation rates in response to increased activity of visually oriented predators? 2) To what extent does variation in the amount of nest cover interact with predator activity to influence nest visits? The amount of forest cover is a key factor influencing the predation risk on Siberian jay nests (Eggers et al. 2005a; Eggers et al. 2006). Hence, we predict that overall nest visitation rates will decrease with the decreasing amount of nesting cover. A reduction in the amount of forest cover is also associated with reduced food abundance (Alaback 1982), increased adult mortality due to goshawk Accipiter gentilis predation (Griesser et al. 2006), and a warmer microclimate (lower thermoregulatory costs; Eggers et al. 2006). All these factors are therefore confounded with the effect of habitat-mediated risk of nest predation on nest visitation rates. To account for this, we explored the interaction between cover and corvid activity. 3) How does food availability affect predator-induced adjustments of feeding visit rates? If feeding visit rates are constrained by low food availability rather than the presence of visual oriented predators, extra food should allow parents to provision their offspring more often (Smith and Montgomerie 1991). Alternatively, experimentally provided extra food may allow jay parents to deliver the same amount of food by increasing load size and simultaneously reducing the overall rate of predator-attracting nest visits (Strickland and Waite 2001; Grieco 2002).
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METHODS |
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The study species
The Siberian jay is a sedentary bird species inhabiting northern boreal and subalpine coniferous forest in the high latitudes of Europe and Asia. Occupation of permanent all-purpose territories in this harsh environment is made possible by the bird's food storage behavior. The species is omnivorous, eating a variety of animal and plant foods (Andreev 1982
). Parental behavior and the nest are highly cryptic. A new nest is built each year, typically in spruce Picea abies (at our study site 93%; n = 112; Eggers 2002) or pine Pinus silvestris, so that it is situated on the sunny side of the tree trunk (direction [magnetic bearing], mean ± standard error [SE]: 154° ± 13, n = 14). The leaf area of the upper tree crown forms a cover around the nest. This reduces exposure to harsh weather and conceals the nest against visually oriented predators. At our study site in northern Sweden, the egg-laying period normally starts in the first half of April while the terrain is still snowbound. Ambient temperatures may occasionally drop below –20 °C, but nests are well insulated with lichens, feathers, and reindeer hair. Parents raise annually a single brood with a small but variable clutch size (range: 1–5, mean ± SE: 4.04 ± 0.7, n = 50; Lindgren 1975; Eggers 2002). Incubation typically lasts 19–20 days, and nestlings fledge approximately 3 weeks after the first chick has hatched. Renesting opportunities are restricted to incubation stages until late April (Lindgren 1975). This increases the value of a single nesting attempt, thus presumably promoting selection for antipredator behaviors (Eggers et al. 2006). Jay nests have a 46% probability (n = 185) of successfully raising at least one offspring to fledging (Eggers et al. 2005a), with approximately 70% of all failed nests, the result of predation. The primary nest predators are other corvid species (Eurasian jay Garrulus glandarius, hooded crow Corvus corone, and raven Corvus corax) that account for more than 80% of all predation events (Eggers et al. 2005a).
Monitoring reproduction
We monitored reproduction in a population of Siberian jays northwest of Arvidsjaur, northern Sweden (65°40'N, 19°0'E) between 1989 and 2004. Birds were individually banded with metal rings and a unique code of color bands. In March each year, female breeders were caught with mist nets and fitted with radio transmitters (total n = 277; Holohil Systems Ltd., ON, Canada), which allowed us to locate nest sites by radio tracking incubating females in April. Transmitters were glued to the base of the female's central tail feathers and were shed at the molt of these feathers in early June after the breeding season. Transmitter weight (1.85 g) at the time of attachment was 1.8–2.6% of a female's weight (mean ± SE: 81.7 ± 0.5 g; range: 70–98 g), and we detected no adverse effects of transmitters on the birds.
Monitoring predator activity
We estimated the activity of potential nest predators by counting the number of corvids that were seen or heard on our daily nest visits to nest sites during 30-min observation periods. To minimize the likelihood of the same predator being counted more than once, we removed observations of the same species made within 5 min of each other. Observations were balanced to yield an even sample throughout the day (0300–1800 h) at all sites. Habitat structure surrounding the nest had little impact on our estimates of predator activity because we only counted corvids that were heard and seen in the direct vicinity of nest sites (maximum 50 m radius around the nesting tree; see Eggers et al. 2005b).
Parental activity and nesting cover
We monitored parent activity throughout the nestling period at 28 nests with the help of video camera surveillance. We obtained continuous video recordings of activity at successful nests from the time we detected the nest, which was normally at the start of incubation and continued until fledging. This method excludes the possibility that differences in nestling age due to sampling effects could drive differences in nest visitation rates among territories. Cameras were triggered by passive-infrared-motion/heat detectors and placed in wooden boxes among the branches of a nearby tree (distance to nest 5–10 m). Each day between 0300 and 1800 h, we recorded the number of feeding trips to the nest by both parents. Virtually, all nest visits were feeding visits (>98%). These data were then converted into the number of trips per hour for each day and a mean for the entire nestling period.
Vertical stratification of spruce crowns is a forest attribute which strongly influences the amount of understory cover and, thus, forest openness in mixed boreal coniferous forest (Hunter 1990; Morrison et al. 1992). Corvids are predators that hunt using visual cues, and the risk that nest visits by parents should attract their attention to the location of the nest is a function of habitat openness (Eggers et al. 2005a; Griesser et al. 2007). Therefore, to test for the effect of protective cover on parental feeding activity, we assessed spruce density on a larger scale around our 28 monitored nests. All spruces within 1 m of four 50 m transects starting from the nest tree and heading N, E, S, and W were counted, with the exception of full-grown trees (typically >15 m) which had lost the branches on the lower half of their trunk (2–10 m). The census data for these four 100 m2 plots were pooled for each nest site, and the density of spruces per 100 m2 was taken to characterize local forest openness (see Eggers et al. 2005a). Such habitat characterization is valid for the Siberian jay because they normally build their nests in spruces at low height (2–10 m). Hence, as the amount of understory cover decreases, parents and their nestlings become more exposed.
Supplemental food provisioning
Of the 28 monitored nests, we provided 14 nests daily with 20 g ground beef (10% fat content) per nestling during the nestling stage. This additional food corresponded to the daily amount of food required by 10- to 17-day-old nestlings (Kokhanov 1982). Because acquisition of high quality territories associated with low predation threat is linked to the disperser's phenotype (Ekman et al. 2002), we stratified supplemented and nonsupplemented parents by equalizing the overall reproductive success between treatment and control territories to control for the effect of parental phenotype on provisioning efforts. To express the long-term reproductive success for each territory, we calculated a nest success index that consists of the sum over all years (1989–2004) of the difference between the actual reproductive success within a given territory (0, failure; 1, success) and the probability of nest success in the population in general (proportion of nests that were successful) for each year (see Ekman et al. 2001). We located feeding tables 100 m from the nest on the main stem of a spruce tree to minimize the risk of attracting predators and conspecifics to the immediate vicinity of the nests (mean ± SE distance to nest position of nearest breeding pair: 868 ± 147 m). On all occasions, food was depleted by the parents from the feeder within 15 min and stored in the forest canopy with no interference from other species.
Analysis of nest visitation rates
Because of repeated sampling of territories across years, we used a general linear mixed model (proc mixed, SAS 9.1 service pack 3; SAS Institute, Cary, NC) with territory identity as a random factor to examine the relationship between nest visits per hour and the following fixed effects: 1) brood size, 2) the activity of corvid nest predators (low activity in remote forest areas: 
1 corvids/h, mean ± SE: 0.6 ± 0.1/h and high activity nearby human settlements: >1 corvids/h, mean ± SE: 1.7 ± 0.3/h) (see Eggers et al. 2005b), 3) the amount of additional food sources available to parents (extra food: 1, no extra food: 0), and 4) the amount of protective nest cover (continuous variable). Nest visits were log(10) transformed to normalize the data. We explored first-order interactions between the explanatory variables to test our predictions on how predator activity and food availability should modify the effect of nesting cover on overall nest visitation rates, given that feeding visits impose a cost by alerting predators to the location of the nest.
Confounded effects of microclimate, food supply, and nest concealment
Nesting cover may not only provide protection from visually oriented predators but also increase thermoregulatory costs through a colder microclimate in the denser vegetation (Eggers et al. 2006). Also, food availability might be reduced in open habitat because of lower food abundance and/or increased predation risk, with feeding visit rates becoming lower as the amount of cover decreases. To examine the extent by which variation in nest visitation rates could be attributed to differences in corvid activity, we therefore explored the interaction between nest cover and corvid activity and estimated regions of significance (O'Connor 1998). A significant reduction in the magnitude of the predicted effect of cover on feeding visit rates in response to higher predator activity would provide evidence for predator-induced reductions in feeding visit rates.
Experimental test of cover-dependent response to nest predators
To experimentally test the effect of forest cover on the probability that parents would perceive a nest predator as a threat and adjust nest visits accordingly, we presented a mounted Eurasian jay for 30 min at nest sites characterized by a dense forest structure (>6 low spruces/100 m2, n = 16 nests) and a more open forest structure (<4 low spruces/100 m2, n = 16 nests) between 1000 and 1500 h. We chose these 2 classes of spruce density because it allowed us to place the predator model 30 m from the nest, which is close enough to ensure that parents would detect it as well as perceive it as a threat to their nestlings. In the denser vegetation, the predator model remained concealed from parents when feeding their young at the nest (mean visibility range ± SE: 10 ± 3 m; using a Leica Disto laser distance meter, 8 measures per nest site obtained at 45° steps using a 360° circle with vernier scale at 2-m height), whereas it was more visible in a more open forest structure (25 ± 1 m). During the preexperimental period, the predator model was covered by a black plastic bag and attached to the main stem of a spruce tree 2 m above ground. To minimize the impact on subsequent nest activity of our activities around the nest site and the time since last feeding, we exposed our predator model after a 30-min period after the first feeding trip was observed. To avoid habituation of parents to our stuffed predator model, we turned its skull and lifted its tail feathers every 5 min with a remote control. If parents attacked the predator, we removed the model immediately to minimize possible negative effects of mobbing activities on feeding visit rates.
We compared the number of parental feeding visits recorded during the experimental period (T2) with those observed by video cameras 1 h prior to the preexperimental period (T1) and explored the interaction between time (T1 and T2) and spruce density (repeated design, proc GLM, SAS Institute). This analysis was to evaluate whether cover modified the perceived risk of nest predation and thus influenced the antipredator response in terms of nest visitation rates.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Cover-dependent effects of predator activity on overall nest visitation rates
Nest visitation rates increased significantly with increasing levels of nest cover at sites with high and low activity of corvid nest predators, when the effects of brood size and extra food were controlled for (Table 1, Figure 1). There was also a tendency toward lower overall nest visitation rates associated with higher corvid activity (high corvid activity (mean ± SE): 1.1 ± 0.1, low corvid activity: 1.2 ± 0.1, P = 0.08, Table 1). The effect of corvid activity on nest visits depended on nesting cover. Corvid activity in open habitat (<6 spruces/100 m2) did not explain variation in nest visitation rates among nest sites. However, as the amount of nesting cover increased, parents exposed to high corvid activity had relatively lower increases in their feeding visit rates when compared with parents exposed to low predator activity. As a result, high corvid activity was estimated to reduce overall nest visitation rates in denser habitats significantly from 1.5 ± 0.1 visits/h to 1.1 ± 0.1 visits/h (mean ± SE; region of significance: >6 spruces/100 m2, P < 0.05; Table 1, Figure 1).
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Cover-dependent responses to predator model presence
The experimental presentation of a Eurasian jay model affected parental nest visitation rates of Siberian jays unequally in dense and open forest. Nest visitation rates declined significantly between control and experimental periods irrespective of spruce density but to a larger extent at nest sites associated with a more open forest structure (repeated-measures analysis of variance, time x spruce: degrees of freedom [df] = 1, F = 14.8, P < 0.001; time: df = 1, F = 76.2, P < 0.0001, spruce density: df = 1, F = 10.8, P < 0.01; Figure 2). Pairs nesting at sites with higher levels of nest cover (i.e., dense forest) maintained nest visitation in the presence of the model predator (mean ± SE: 0.8 ± 0.2 visits/h), whereas parents in open habitat did not (no visitations: 0.0 visits/h). This was not simply a consequence of parents in denser forest failing to detect the predator model. In most trials, parents detected and moved toward the predator (open forest: n = 13, mean distance ± SE: 60 ± 10 m; dense forest: n = 13, 30 ± 10 m) and gave alarm calls at a distance (open forest, n = 9; dense forest, n = 6) but rarely approached and attacked the model aggressively (open forest: n = 2; dense forest: n = 3).
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Food-dependent effects of predator activity on overall feeding visit rates
Supplemental feeding of parents affected overall nest visitation rates unequally among nest sites (Table 1). At sites with low predator activity, parents that received extra food did not visit their nest more often (mean ± SE: 1.3 ± 0.3 visits/h) than parents that did not have access to extra food (1.2 ± 0.1 visits/h, F = 0.41, df = 1, P = 0.53). However, at nest sites associated with high predator activity, food-supplemented parents visited their nests at significantly lower rates (0.86 ± 0.1 visits/h) when compared with parents without extra food (1.4 ± 0.2 visits/h, F = 6.7, df = 1, P = 0.01, Figure 3). These results are consistent with the idea that extra food allows Siberian jay parents to reduce predator-attracting nest visits by increasing food load sizes.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Nest predation provides a logical selective force shaping avian life-history traits, which are thought to reflect fitness trade-offs between the costs and benefits associated with providing care (Clutton-Brock 1991
; Roff 1992). However, parental provisioning efforts in birds have been attributed mainly to the food requirements of nestlings (Wright et al. 1998; Briskie et al. 1999; Kilner et al. 1999), whereas attempts at explaining differences in rates of parental feeding visits in response to predation threat are rare (Ghalambor and Martin 2001, 2002; Eggers et al. 2005b; Fontaine and Martin 2006). This is surprising because food appears often to be a poor predictor of variation in reproductive strategies within and among species (Martin 1995; Ferretti et al. 2005).
Cover-dependent effects of predator activity on feeding visit rates
The results of our study show plasticity in the nest attendance of Siberian jay parents which is adjusted to the risk posed by nest predators. Our predator exposure experiment confirmed that habitat structure is an important factor that influences the vulnerability of nests toward visually oriented nest predators (Eggers et al. 2005a, 2005b): It was only in dense forest that parents continued to deliver food to their young in the presence of a model predator. Thus, it appears that parents account for the effect of cover on the probability that a nest visit could reveal the location of the nest to the predator. In a more open forest stand, where the predator can be seen from the nest and the predator thus easily could spot the nest, parents ceased all activity around the nest in the presence of a predator model. These results are consistent with our field data which show that rates of nest visits decline as the nesting habitat becomes more open (Figure 1). However, although nest cover is the key driver of parental behavior, it is unclear to what extent cover-dependent nest predation risk alone causes this pattern. This is because forest cover is likely to affect food supply (Alaback 1982), the risk of adult predation (Griesser et al. 2006) and microclimate (Eggers et al. 2006) on feeding visit rates. Therefore, it was crucial to explore the interaction between nest cover and corvid activity to test if corvid predators contribute to reduce overall nest visitation rates in the jay. The fact that the magnitude of the effect of nest cover was lower at nest sites associated with high corvid activity supports the idea that predators influence overall nest visitation rates (Figure 1). However, only parents nesting in dense vegetation (region of significance: >6 spruces/100 m2) reduced feeding visit rates in response to high predator activity, whereas parents nesting in open sites show little response to corvid activity. One possible explanation for this pattern is that visitation rates in open habitats are already low as reductions in the vertical stratification of tree canopies due to forest thinning both reduce the amount of food supply and increase the risk of predation to parents (Alaback 1982; Griesser and Ekman 2004; Griesser et al. 2006). The adverse effects for offspring nutritional status from a reduction may thus override the effect of corvid activity on nest visitation rates (Eggers 2002). The requirements of the nestlings may simply not allow parents to further reduce their provisioning efforts in response to elevated predator exposure because both growth and quality of the offspring would deteriorate.
Effect of thermoregulatory costs on feeding rates
Forest structure not only affects the exposure to predators but also the microclimate around the nest. An open habitat structure in the nest vicinity creates a warmer microclimate, thereby reducing the thermoregulatory costs, thus mitigating the need to keep up feeding visits in open forest stands (Eggers et al. 2006). Regardless, as the vegetation became denser, parents exposed to low corvid activity did increase their nest visitation rates to a larger extent than parents subject to high corvid activity. This difference cannot be attributed to changes in food availability and microclimate but confirms our prediction that the activity of nest predators constrains rates of feeding visits in denser habitat. It also suggests that the increased amount of cover in denser vegetation does not fully compensate for the effect of increased corvid activity on nest predation risk.
Effect of load size on feeding visit rates
An interspecific comparative analysis of nest visitation rates by Strickland and Waite (2001) indicated that small corvid species such as the Siberian jay maximize food load size as a mechanism to reduce the number of nest visits, which could attract predator attention. Our results are consistent with their proposition because food-supplemented parents reduced feeding visit rates as compared with parents that received no extra food. Furthermore, the effect was only evident in environments containing relatively high numbers of visually oriented corvid nest predators. Increased food availability in denser forest could also facilitate parents to reduce nest visitation rates through larger food loads (Figure 1). However, a critical test to show that increased load size contributes to reduce feeding visit rates would require the assessment of food loads. Unfortunately, this is not possible in the Siberian jay because newly hatched chicks receive a well-digested "soup" of food, whereas older young are given partially digested or undigested food items that are carried in the expandable throats of their parents and therefore cannot be seen.
Furthermore, the observed response to extra food under high corvid activity raises the question why nonsupplemented birds do not simply provide fewer loads of larger size to reduce offspring predation risk. Increased food availability has been shown to reduce foraging time, which in its turn allows more time for antipredator activities, such as nest guarding (Martin 1987; Ward and Kennedy 1996; Lima 1998). Nonsupplemented parents, however, may not be able to allocate more time to antipredator activities through increased foraging efficiency. As a result, larger food loads will only help to reduce nest activity but could still involve a net cost when benefits in terms of antipredator defense do not outweigh costs associated with reduced feeding visit rates (see Eggers et al. 2005b)
Costs of predator avoidance
Siberian jays prefer high-productive territories located away from human settlements, where they escape exposure to higher numbers of corvid nest predators (Ekman et al. 2001; Eggers et al. 2005a). Despite antipredator behavior, high rates of nest failure in open forest stands close to human settlements suggest that nest visitation rates can not be reduced sufficiently by the parents to compensate for the increased exposure to predators in these habitats (Eggers et al. 2005a, 2005b). However, our results indicate that even jays that occupy relatively safe nest sites close to human settlements may suffer from elevated predator activity through lost feeding opportunities. Larger food loads and compensatory spells of feeding visit rates will not necessarily compensate for reduced nestling growth during times of poor nutrition (Metcalfe and Monaghan 2001). Hence, increased efforts to avoid nest predators could result in poor provisioning conditions at the nestling stages and prevent an individual from developing its optimal phenotype despite its genetic potential (Simons and Martin 1990; Richner 1992; de Kogel 1997; Griesser et al. 2006).
The results presented here demonstrate that nest visitation patterns in open-nesting bird species can be directly affected by predator presence, whereas breeding at denser nest sites might partially mitigate the risk posed by predators. Predator-induced reductions in overall nest visitation rates have so far not been demonstrated in other bird species; yet, it is very likely that many open-nesting bird species are able to strategically trade off the nest predation risk against feeding visits (Skutch 1949, Ghalambor and Martin 2000, Fontaine and Martin 2006). Moreover, our results suggest that the visitation rate must balance the nutritional status of nestlings against the need for antipredator efforts. Poor condition early on in life may affect important life-history parameters such as juvenile survival (Richner 1989, Griesser et al. 2006) or recruitment potential (Eggers 2002). This implies that the effect of nest predators on reproductive success can be larger than expected from predation rates alone. This has important conservation implications and provides another example for how predators could exert an important influence on the evolution of parental care behavior.
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FUNDING |
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This study was supported by grants from Kungliga Vetenskapsakademin, Verner von Heidenstams fond (KVA-1373502); Stiftelsen för Zoologisk Forskning (1999–2001, Uppsala University), The Swedish Research Council, FORMAS (2003-0719); and Konung Carl XVI Gustaf 50-års jubileumsfond (2004-4 to S.E.).
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ACKNOWLEDGEMENTS |
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We are grateful to Karl Evans, Janet Gardner, Matt Low, and Tomas Pärt for careful reading and constructive comments on the manuscript and A. Laurila and K. Räsänen for advice on the statistical analysis of the data presented in the manuscript. All manipulations described in this study including banding, radio-tagging, supplemental feeding, as well as predator exposure experiments were performed under the license of the responsible ethics board (Umeå djurförsöksetisk nämnd; license no. A80-99 and A4504).
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