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Behavioral Ecology Vol. 14 No. 2: 246-250
© 2003 International Society for Behavioral Ecology
Nepotistic vigilance behavior in Siberian jay parents
Department of Population Biology, Evolutionary Biology Centre, Norbyvägen 18D, SE-75236 Uppsala, Sweden
Address correspondence to M. Griesser. E-mail: michael.griesser{at}ebc.uu.se.
Received 20 December 2001; revised 6 May 2002; accepted 25 June 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Many Siberian jay offspring (up to 50%) postpone independence and stay with their parents for up to 3 years. Parents offer such nondispersers a benefit in that they increase their vigilance while feeding together with retained offspring. In contrast, parents reduce their vigilance while in company of nonrelated flock members according to the "many eyes" principle. The preferential treatment offered by the parents provides an incentive for offspring to forego dispersal. Given evidence for mortality via surprise attacks by predators (goshawks), such nepotistic vigilance by parents could have a bearing on offspring survival and thereby promote delayed dispersal.
Key words: delayed dispersal, nepotistic vigilance, kin groups, Perisoreus infaustus, Siberian jays.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Kin societies form when mature offspring delay dispersal and remain on the natal territory instead of dispersing to search for a breeding opportunity. In most bird species with delayed dispersal, offspring forego personal reproduction and provide care to nondescendent offspring (Brown, 1987
; Emlen, 1995). Cooperative breeding requires that offspring first delay dispersal before they can help to raise nondescendent offspring (Brown, 1987), and delayed dispersal, which is the permissive factor for complex social interactions like cooperative breeding, is not well understood (Ekman et al., 2001a). Even if the offspring forego reproduction, it is not clear why they associate with their parents for prolonged periods, thereby giving up the possibility to search for a breeding opening in large areas. One reason the offspring postpone dispersal while waiting for a breeding opportunity could be that the natal territory offers benefits that cannot be found elsewhere.
Offspring represent an evolutionary asset to their parents. Parents therefore have an incentive to provide help to their offspring even after independence. Such help could involve behaviors that promote survival (Ekman et al., 2000) and help in acquiring a breeding position (Brown and Brown, 1984). Before producing grand-offspring, the offspring have to survive while waiting for a breeding opening, and delayed dispersal could be beneficial to the parents if their nondispersing offspring survive better on the natal territory than elsewhere (Brown, 1978). In the Siberian jay (Perisoreus infaustus), offspring have a higher first-winter survival rate when in company of their parents (Ekman et al., 2000). The higher survival of retained offspring suggests that the natal territory offers benefits. Parental nepotism could be one such benefit (Kokko and Ekman, 2002).
Nepotistic parents could promote the well being of retained offspring by providing them safer feeding conditions to enhance winter survival both in terms of predator protection and by having a predictable, less constrained access to resources. Parents tolerate retained offspring on a feeder, whereas they aggressively prevent immigrants from sharing a feeder (Ekman et al., 1994). Simultaneously, time is invested in vigilance behavior to reduce the predation risk while foraging. By grouping to capitalize on a "many eyes" effect, foraging individuals can reallocate time to feeding while enjoying greater protection against predators that hunt by surprise attack (Kenward, 1978; Pulliam, 1973). Although vigilance is sometimes a fundamentally selfish activity (Bednekoff and Lima, 1998), it could also be used to protect relatives.
In this study I tested the hypothesis that parents provide safer feeding conditions for retained offspring through nepotistic vigilance. The social system of the Siberian jay offers the possibility to compare the costs and benefits for individuals with prompt dispersal and delayed dispersal. About 50% of all offspring delay dispersal for up to 3 years (Ekman et al., 2002). Individuals with prompt dispersal usually join other flocks as immigrants. I used flocks of different composition (family flocks and nonfamily flocks) to test for differences in vigilance behavior of breeders while feeding with retained offspring as compared to while feeding with unrelated immigrants.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Study area and species
I studied Siberian jays in continuous taiga habitat close to Arvidsjaur, northern Sweden (65°40' N, 19°0' E). This study is based on data collected outside the breeding season, between mid-August and mid-October 1999. In addition, I included data for predation on Siberian jays collected between 1996 and 2001. All birds in the study population were ringed with a numbered metal ring and color rings for individual recognition. I took blood samples from all birds for sex determination (see Ekman et al., 2002
, for details; Griffiths et al., 1998; Ogawa et al., 1997). New flock members were aged from the shape of the outermost retricies, which are more rounded in first-year birds.
Siberian jays live in small flocks (ranging from two to seven individuals) in year-round territories with the breeding pair as a core. They show delayed dispersal typical of cooperative breeders, but extra birds (nondispersing offspring/immigrants) do not help at the nest (Ekman et al., 1994). Offspring show a large variation in the timing of natal dispersal. Dominant offspring delay dispersal for up to 3 years while they expel their subdominant brood mates. These individuals disperse in their first summer and become immigrants in other flocks (Ekman et al., 2002). Consequently, flock composition varies, with some flocks being composed only of family members, some being families associated by nonrelated immigrants, and others containing only nonrelated individuals as a result of breeding failure and immigration of unrelated individuals.
Nestlings were initially given only metal bands and were banded with color rings if they then remained in the study population. I considered unbanded immigrants to be unrelated to the breeding pair. In five territories where I did not follow breeding, I assessed relatedness by aggressive behavior by parents (for six retained offspring and two immigrants). This assessment was done before data collection and confirmed by an unbiased observer. Aggressive interaction between breeders and extra birds were examined using the following classification: 0 = no aggression, 1 = subordinate allowed to be close, 2 = subordinate is displaced, 3 = subordinate chased (see Ekman et al., 1994, for details). Parents are not aggressive toward their own offspring but show a high frequency of aggression toward immigrants. This method has previously proven to be a reliable indicator of kinship, either when compared with individuals of known origin (born in the territory and banded as nestling: N = 62; immigrants in territories with failed reproduction or successful breeding: N = 47) or when controlled against DNA finger printing (N = 13; see Ekman et al., 1994, for details).
Data collection
I compared the foraging behavior in six flocks with nondispersing offspring and six flocks with immigrants. The presence of retained offspring reflects successful reproduction, as retained offspring were found from all successful broods. Hence, whether groups contained retained offspring or not should therefore not be confounded with phenotype quality or territory quality. Flocks were studied on feeders of standardized design that allow five individuals to feed simultaneously. I placed feeders according to a standardized protocol in the middle of small forest openings to control for effects of habitat structure. Feeders are likely to mimic conditions under which Siberian jay behavior has evolved. Under natural conditions, the Siberian jay is likely to have foraged as a scavenger. Although seemingly artificial, my experimental setup should therefore be a good proxy for a natural situation where a flock exploits a carcass. Family flocks contained three (n = 3 flocks), four (n = 2), and five (n = 1) members. One family flock with four and one with five individuals included a nonrelated immigrant. Flocks without retained offspring contained three (n = 3), four (n = 2), and five (n = 1) members. In total I studied 24 breeders (12 males and 12 females), 8 retained offspring (5 males and 3 females), and 13 immigrants (4 males and 9 females). Every flock was observed three times for about 15–20 min (44–59 min observation time in total) between mid-August and mid-October 1999.
I recorded the behavior of the jays at feeders with a Hi8 video cam from about a 10 m distance. This distance allowed me to read the color bands with binoculars without disturbing the behavior of the jays. All of my comments (identity and comments on behavior of all individuals on the feeder) were recorded on the video soundtrack. From the videotape I extracted the following data for all individuals using The Observer 3.0 for Windows: length of visits to feeder; overall proportion of time on feeder; initial behavior on arrival on feeder (feeding = lowering head directly, vigilance = head up or turning head); way of leaving feeder (aborted by other individual or spontaneous); group size (numbers of individuals on the feeder or at a maximum 1m off on the ground). Group size here stands for the number of individuals in feeding association, whereas flock size accounts for the number of individuals associating in the territory.
Data analysis
I sampled vigilance rates with time-point measurement (Altman, 1974) from the videotapes. For every breeder/parent and retained offspring on the feeder, I recorded the position of the head with 2-s intervals. Feeding jays have to lower their head to peck at the suet. To swallow collected food, jays raise their head in horizontal position. The position of the head is rarely over horizontal position. The only head movement that is not mandatory for feeding is turning the head horizontally. I therefore only recorded turning the head horizontally as vigilance.
To test for the effect of kinship on vigilance rates, I focused on the response of breeders/parents when feeding together with an offspring/immigrant, controlling for group size (number of individuals on the feeder). In feeding associations of two, the parent was sharing the feeder either with its offspring or with an unrelated individual in a nonkin association. The composition of kin associations of three individuals is more complex. Such associations are either composed of the two parents and their offspring or they contain one parent, one nonrelated immigrant, and the offspring. Pooling the data for associations with both parents and those with only one parent and a nonrelated immigrant is based on the assumption that parents behave toward each other as nonrelated immigrants (i.e., there is no protection of the other partner in increased vigilance rate). This assumption seems justified, as I could not find any heterogeneity on vigilance rate among feeding associations composed of mated pairs and where a nonrelated immigrant substituted one partner. The composition of nonkin associations of three birds is more straightforward. There is either one adult and two nonrelated immigrants or two adults and one nonrelated immigrant. I tested for the response in vigilance rates of parents to presence of offspring and vice versa.
All statistical tests were performed using SAS 8.2 for Windows. To test for differences in mean time spent on the feeder, the initial behavior on the feeder, and the way of leaving the feeder I categorized all observations using:
- kinship: retained offspring versus immigrant; comparing behavior of immigrants and retained offspring only (vigilance rates, way feeding bout was terminated). For median time on feeder, total time proportion on feeder and initial behavior on feeder, breeders were included and their data pooled with those for retained offspring.
- rank: juvenile, subadult, breeder. First-year birds were classified as juveniles. Individuals older than one year and which were not breeders were classified as subadult. The breeding pair was classified as breeder, and these individuals are normally dominant over other same-sex flock members (Sklepkovych, 1997).
- sex: male, female.
- flock.
I analyzed the time on feeder and vigilance rates with a general linear model type III SS (SAS Proc GLM). Proportions were arcsine or arcsine square-root transformed to attain normality. All data from the same individual were pooled to avoid pseudoreplication. The way of leaving the feeder and the initial behavior are binary response variables, and correlates were therefore tested with logistic regression (SAS Proc GENMOD). "Individual" was included in these analyses to control for their influence on the result and to control for pseudoreplication. I removed factors with p >.3 in a stepwise procedure for the final analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Hawk predation
Hawks, in particular the goshawk Accipiter gentilis, are a main predator of Siberian jays. In addition to the three documented predation events by hawks already reported (Ekman et al., 2001b
), another 17 radiotagged individuals were found killed between 1996 and 2001. They had all been killed by a raptor judging from the feathers (basal shaft part not bitten), and in at least 15 of these cases feces or footprints in the surrounding snow indicated that the predator had been a hawk. Furthermore, hawks were involved in 18 out of the 25 predator encounters that I witnessed between a Siberian jay and a predator between 1998 and 2001 (14 with goshawks and four with sparrowhawks A. nisus).
Parental vigilance rates
Vigilance will have an antipredator function in the Siberian jay because hawks rely on surprise attacks (Kenward, 1978). The joint vigilance in groups will then enhance protection against being taken by surprise (Pulliam, 1973). Often this antipredator effect of flocking is traded in for energy gain by an allocation of time from vigilance to foraging in flocks, although this is not a universal pattern (Elgar, 1989). However, it appears that the response in social groups can be more complex. A pattern with a decrease in vigilance time within group size was only found in feeding associations composed of individuals that were not closely related, whereas parental vigilance was nepotistic and increased in association with a nondispersing offspring (Figure 1). Parents feeding together with a retained offspring may have responded to group size too, but kinship had an overriding effect. Parents increased their vigilance in company of retained offspring, and their vigilance rates were significantly higher than for breeders in groups of corresponding size that did not contain any retained offspring.
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The effect of kinship on breeder/parental vigilance rate was tested with a general linear model including kinship, group size, sex of the parent/adult, and flock as independent variables. This test revealed a significant effect of kinship and the interaction between group size and kinship (Table 1; overall model fit: r2 =.52, F3,36 = 13.32, p >.00001). Sex and flock had no influence on the response variable and were therefore omitted in the final model.
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Retained offspring time use and time constraints of non-related immigrants
Although parental vigilance responded to the presence of a retained offspring, there was no reciprocal response by the offspring. Retained offspring did not respond to the enhanced vigilance of their parents. A test controlling for flock, number of parents on the feeder, and number of other flock members on the feeder showed no difference in vigilance time of retained offspring feeding together with parents or other flock members (general linear model; F2,41 = 0.84, ns, power: 0.19). Hence, according to this test it was not possible to show that retained offspring traded in an enhanced predator protection together with their parents for energy gain by allocating time from vigilance to feeding.
One reason for a lack of response by retained offspring could be that they did not have to compromise feeding time and vigilance. Siberian jay parents concede access to food to their retained offspring. Feeding bouts of the retained offspring was not terminated by adult aggression. This implies that conspecific aggression did not constrain the time when the offspring had access to food. In contrast, about two-thirds of all feeding bouts of nonrelated immigrants were aborted by aggression of the dominant adults of the group (logistic regression testing the way a feeding bout of a retained offspring/immigrant was terminated; kinship: 
2 =, 202.99, p <.0001, N = 568 departures, 33 individuals).
The high frequency of prematurely aborted visits of unrelated immigrants was reflected in the distribution of feeding bout lengths (Figure 2). The median bout length differed significantly between family members and immigrants but not among family members (general linear model: kinship: F1,45 = 19.17, p <.0001). Bout lengths of family members were therefore pooled. The feeding bout length in immigrants followed a negative exponential distribution (r2 =.66), which signifies a "constant failure rate" (Feller, 1968). This distribution implies that a feeding bout could be terminated at any time independently of how long the bird had been sitting on the food, which reflects that the access to food was outside their control. In contrast, the feeding bout length of family members followed a polynomial distribution (r2 =.77) with a pronounced peak at a bout of a length of 21.2 s. Hence, family members had an expectancy for how long they would feed once they had landed, and bout length was more likely under their own control.
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The feeding time constraint for nonrelated immigrants entails that they could spend only a proportion of 0.196 ± 0.017 (mean ± SE) of their total time on the feeder. The lack of a time constraint for retained offspring is reflected in the fact that their corresponding time proportion spent on the food was more than twice as long (0.41 ± 0.026), which is actually even more time spent on food than by parents/adults (0.30 ± 0.014). These differences in time on the food are real when comparing the mean proportion every individual spent on the feeder (general linear model; kinship: F1,45 = 13.45, p <.001; rank: F2,45 = 3.46, p <.05).
A relaxed time constraint of retained offspring is also reflected in their initial behavior on landing on the feeding site. Retained offspring and parents/adults practically always started feeding bouts with a scanning behavior (N = 601 arrivals with scanning, N = 2 arrivals with feeding), whereas nonrelated immigrants (especially females) took the risk to start feeding immediately on 38% of feeding bouts without securing that there was no risk of an imminent attack (N = 82 arrivals with scanning, N = 50 arrivals with feeding; logistic regression; kinship: 2 = 59.05, p <.0001; individual: 2 = 16.79, p <.0001; rank: 2 = 12.44, p <.0005; sex: 2 = 11.82, p <.001; N = 735 arrivals, 45 individuals).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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This study demonstrates that Siberian jay offspring that delay dispersal benefit in several ways by associating with their parents. Parents are nepotistic in their vigilance behavior by protecting their offspring from surprise attacks by hawks (Table 1, Figure 1). To be successful, attacking hawks depend on surprising their target (Kenward, 1978
). Therefore, increased vigilance of parents will reduce predation risk for their offspring. Furthermore, access to food is not constrained for retained offspring by conspecific aggression (Figure 2), and they are therefore not forced to trade in safer feeding conditions for energy gain. Unrelated immigrants, in contrast, do not have this benefit of more safe feeding conditions, and they have a more constrained access to food (Figures 1 and 2). My results link parental nepotism with vigilance behavior and provide behavioral mechanisms that may explain the difference in first winter survival between retained offspring and unrelated immigrants (Ekman et al., 2000).
Studies investigating vigilance behavior have mainly focused on the effect of group size on vigilance rates. Numerous studies have reported a negative correlation between group size and vigilance effort (Elgar, 1989) according to the "many eyes" principle (Pulliam, 1973). However, this relationship could be confounded by effects of food density, competition, or variation in the quality of individuals (Elgar, 1989). Accordingly, several studies have failed to detect a group-size effect on vigilance (see Elgar, 1989; Treves, 2000, for examples). Only a few studies have focused on the social context of vigilance behavior beyond group size, but most of these studies were performed in captivity. Field studies showed that subdominant individuals are less vigilant than dominant individuals (Krams, 1998) or are forced to forage in more dangerous microhabitat (Ekman, 1987). A study by Heinsohn (1987) suggested that vigilance behavior could contain a nepotistic component. He showed that adult white-winged choughs (Corcorax melanorhamphos) have higher vigilance burden than subadults when foraging in family groups. A similar effect was found in wintering barnacle geese (Branta leucopsis; Black and Owen, 1989). Parents wintering together with offspring were more aggressive toward neighbors and more vigilant than parents wintering alone. My study shows that vigilance behavior in Siberian jay parents is nepotistic, lowering the risk of surprise attacks by predators for retained offspring. This is a novel approach to vigilance behavior and could explain why individuals organize in kin societies where mutual aid provides an evolutionary payoff.
Kin societies form when grown offspring forego independent reproduction and remain on the parental territory for prolonged periods. However, offspring can delay reproduction without remaining on the parental territory (Brown, 1987). The question of why offspring wait on their parents' territory rather than elsewhere is therefore crucial to the understanding of the evolution of kin societies. The parental territory differs from other territories in at least two ways. First, offspring associate with their parents when delaying dispersal. Second, offspring remain in a familiar area and, unlike individuals with prompt dispersal, are not faced with the struggles of an unfamiliar environment.
Parents should behave nepotistically only when their own prospects to survive are good (Ekman and Rosander, 1992; McNamara et al., 1994). Conditions under which delayed dispersal evolves may therefore be restricted and promoted by a long life span (Arnold and Owens, 1998). Given a role for parental nepotism, avian kin societies may require cooperation in that the offspring will not delay dispersal unless parents offer them preferential treatment that is denied nonrelatives (Kokko and Ekman, 2002). Preferential treatment of retained offspring by parents has been found in several species involving food sharing (Barkan et al., 1986; Ekman et al., 1994; Pravosudova, 1999) and predator protection through nepotistic alarm calling (Hoogland, 1983; Sherman, 1977, 1981). Conceivably, it is a consequence of such behaviors that offspring with delayed dispersal have a higher survival than individuals with prompt dispersal (Blumstein and Arnold, 1998; Ekman et al., 1999; Kraaijeveld and Dickinson, 2001; Moses and Millar, 1994). As further benefits, Siberian jay sons with delayed dispersal acquire better territories (Ekman et al., 2001b) and have a higher lifetime reproductive success (Ekman et al., 1999).
The hypothesis that parental nepotism facilitates delayed dispersal is rather unexplored. Studies on kin mutualism focused mainly on cooperative breeding (Brown, 1987). However, if cooperative breeding is a selected trait, selection will have no opportunity to operate on any inclusive fitness gains from cooperative breeding unless there are already coherent families. Cooperative breeding is not the only form of kin mutualism, and offspring delay dispersal even in species without cooperative breeding (e.g., Ekman et al., 1994; Gayou, 1986; Green and Cockburn, 2001; Robinson, 2000). The benefits reported here should promote delayed dispersal together with other benefits already reported (Ekman et al., 1994, 1999, 2000, 2001b). This study therefore demonstrates a potential role of parental nepotism for the dispersal decision, and it may help explain why offspring in the Siberian jay fight to stay at home (Ekman et al., 2002).
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ACKNOWLEDGEMENTS |
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I am grateful to Jan Ekman, Vittorio Baglione, and Sönke Eggers for help in the field and for many useful suggestions on the manuscript. Many thanks to Lisa Shorey for correcting my English and to Jon Stone for calculating the r2 values. Gunnar and Ingrid Person provided a perfect base at Lappugglan. This study was supported by Zoologiska Stiftelsen and the Swedish Natural Science Research Council (to Jan Ekman).
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