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Higher fitness for philopatric than for immigrant males in a semi-isolated population of great reed warblers.

In virtually all species, dispersal distances range from those of individuals that remain and reproduce in the vicinity of their parents to those of individuals that explore unfamiliar areas (Baker 1978; Swingland and Greenwood 1986). The tactic to stay and breed close to the natal site, that is to be philopatric, has both costs and benefits relative to the alternative tactic of dispersing. Suggested benefits of philopatry include access to parental resources and knowledge of where to find food, nesting sites, and protection against predators (Greenwood 1980; Shields 1982). In the natal area, relatives may be more frequent and this can be beneficial because kin may be more benign neighbors (Waser and Jones 1983). Suggested costs of philopatry are deleterious inbreeding (Bengtsson 1978) and competition with relatives over resources (Johnson and Gaines 1990). Similarly, specific benefits and costs have also been suggested for dispersal. By leaving the natal site, individuals might find less populated areas with abundant resources, but they may also be exposed to transportation costs, and higher risks of encountering predators and unfavorable habitats. Long-distance dispersal may take the individual to a habitat to which it is less adapted (Dias and Blondel 1996) and/or cause it to produce less fit offspring when reproducing with a philopatric individual, due to break down of coadapted gene complexes (Shields 1982; Waser and Price 1994).

The great reed warbler Acrocephalus arundinaceus is a polygynous, long-distance migratory passerine bird that breeds in productive marshes throughout the Palearctic. At our Swedish study site, a relatively high proportion (14.8%) of ringed nestlings return as breeders at their natal site (Bensch 1995) as compared to migratory birds in general (Weatherhead and Forbes 1994). In contrast to most bird species in which natal and breeding philopatry are male biased (Greenwood 1980; Greenwood and Harvey 1982), male and female great reed warblers are similar with respect to emigration rate (Bensch and Hasselquist 1991; Hasselquist 1995). Thus, in this species at our study site, inbreeding is not passively avoided by a sex-biased dispersal pattern (Greenwood 1980).

Several studies of birds have demonstrated that philopatric first-time breeders are at an advantage as compared to immigrants in finding breeding opportunities (Clobert et al. 1988; Part 1994). Here we go one step further in tracing fitness differences by analyzing the lifetime reproductive success of great reed warblers philopatric to our Swedish study site compared to that of immigrants. We examine whether immigrants and philopatric individuals differ in variables that are closely correlated to lifetime fitness, that is, spring arrival date, territory quality, and life span. We compare males and females to elucidate whether a reduction in fitness of immigrants is caused by reduced mating success, breeding success or both. Most studies assume that philopatry confers an advantage relative to dispersal, for example, Greenwood (1980); however, there are few empirical data that support this assumption (Part 1994). Moreover, the relative advantage of philopatry may also vary with population structure (Smith et al. 1996; Negro et al. 1997) and environmental quality (Verhulst et al. 1997). Given that one putative advantage of philopatry is mediated through acquired experience in where to find food, nest sites, and how to avoid predators, we predicted that philopatric males and females should breed in better territories and live longer than immigrants of their gender.

METHODS

The Study Population

The study was carried out at Lake Kvismaren (59 [degrees] 10 [minutes] N, 15 [degrees] 25 [minutes]E), south central Sweden, between 1985 and 1997. Detailed descriptions of the fieldwork can be found elsewhere (Bensch and Hasselquist 1991; Hasselquist et al. 1995; Bensch 1996). Social behavior of the birds was registered at daily visits to all territories throughout the breeding season (May-July). Nests were checked at 1-4 day intervals, with daily visits around the expected hatching date, enabling us to estimate the day of egg-laying, hatching, and fledging. A nest was considered to have fledged young if nestlings survived to their ninth day of life. We assigned parentage by DNA-fingerprinting to 70% of the fledglings (n = 938) raised by the males included in this dataset (Hasselquist et al. 1995; Hasselquist 1998). Less than 3% of fledglings (in 10 broods) were found to be illegitimate, and in all cases these were sired by a territorial neighboring male (Hasselquist et al. 1996). In these ten cases we corrected the observed fledging and recruiting success of the individual cuckolded and cuckolding males, based on their number of sired young. This correction, however, did not change the outcome of the analyses in this paper. There was no consistent difference in extrapair paternity (EPP) between philopatric and immigrant males. Because of the low level of EPP, it is reasonable to assume that counting observed reproductive success as genetic reproductive success in the 30% of the broods not analyzed by DNA-fingerprinting, caused a very small bias in the estimate of fitness of philopatric and immigrant males (see also Hasselquist et al. 1995).

Male, Female, and Territory Characteristics

Unbanded males were caught in mist nets 0-2 days after territory establishment and marked with unique combinations of color bands. Females were captured during nest building or when feeding young. All immigrants were captured and individually identified, except for a few males present in our study area for periods shorter than five days (n [less than] 10) and a few females that failed at breeding before capture (n = 7). At capture, birds were sexed by the presence of cloacal protuberance (males) or brood patch (females). However, sex was also confirmed by observations of behavior: singing in males and nest building in females.

One-year-old birds (1 yr) were separated from older birds ([greater than or equal to] 2 yr) using differences in coloration of iris and leg and presence or absence of tongue spots. These criteria are essentially similar to those described in detail for the reed warbler Acrocephalus scirpaceus (Karlsson et al. 1988). A typical one-year great reed warbler has a dull olive grey iris, distal sides of tarsus bluish grey (same color as the feet), and tongue spots clearly visible. A typical three-year bird has reddish-brown iris, pale flesh colored tarsus (in sharp contrast to the feet), and no trace of tongue spots. The typical two-year bird is intermediate in these traits. At capture, each of the three traits was classified as either first-year, intermediate, or three year, giving them scores of 2, 1, or 0. From birds of known age (banded as nestlings), we have found a reliable aging criterion that correctly classifies more than 90% of the birds into two age classes; a grand score of iris, tarsus, and tongue-spots of [greater than or equal to] 4 indicates a one-year-old and a score less than 4 indicates [greater than or equal to] 2 yr. We assumed that the unbanded birds aged to be at least two years old were exactly two years old because (1) the majority of all breeders that were banded as nestlings entered the breeding population during their first or second year (94% of males and 85% of females), and (2) breeding adults showed high fidelity to our study area between years with an average return rate of 55% (Bensch and Hasselquist 1991; Hasselquist 1995). Life span was defined as the age of birds during their last breeding season at our study site.

Philopatric birds were those ringed as nestlings (51 males and 57 females) or captured as independent juveniles during their first summer at our study site (four males and four females), and recaptured in later years. We defined the latter group as philopatric because six of the eight individuals were captured during the first two years of our study (1985 and 1986) when we may not have found all nests in our study area. Immigrants were previously unringed birds (59 males and 72 females) or birds ringed as nestlings at other localities [greater than or equal to] 10 km away (two males and four females).

The accuracy with which we designated birds as philopatric or immigrants relies on how successful we were at finding all nests resulting in fledglings. We believe that over the whole study period we have banded chicks in at least 95% of successful nests, with a somewhat lower figure during the first two years of our study and approaching 100% towards the end of the study period. Our high success rate in finding nests can be explained by (1) the suitable breeding area being well-defined (in two adjacent reed marshes) such that no bird could escape detection by breeding just outside our study area (the nearest breeding site is situated at Segersjo, 10 km towards east) and (2) that we visit all male territories daily during the period of female settlement and that males make an obvious change in singing behavior upon mating that reliably reveals when he has formed a pair-bond (Hasselquist and Bensch 1991). Our assumption that we banded a high proportion of the locally produced great reed warblers is supported by data collected in a bird-banding program administrated by the Kvismare Bird Observatory (KBO), for details see Hall (1996). Each year of our study, this trapping program operated almost daily between late June and September. The netting areas are situated adjacent to prime great reed warbler habitat and, between 1985 and 1996, a total of 196 individual yearling great reed warblers were caught. Of these, 79.6% were previously ringed as nestlings in our study area. The remaining 20% of juveniles captured might either have originated from nests we failed to find or from other localities. That a major part of these unringed birds originated in other localities is supported by data on dispersal of birds ringed as nestlings at Segersjo. During the five years that we banded nestlings at Segerjo (n = 189) one third of the supposedly immigrating juveniles trapped by KBO were previously banded as nestlings at Segersjo.

Of the individuals included in the present analyses, four males and six females were still alive in 1997 (Table 1). Because all of these were already at least four years old (and with an expected mortality of 45% to 1998), lifetime reproductive success of these individuals should be close to their final values. Excluding these birds from the analyses did not change the outcome of the tests to any significant degree. A larger problem in our estimate of lifetime reproductive success might occur if birds first breed elsewhere before being recorded at our study area, or if birds that bred in our study area later dispersed to breed at other sites. This is known to occur; however, it could only involve a small fraction of the birds. Moreover, there is no indication that immigrants and philopatric birds behave differently in this respect. First, the annual return rate of 55% to our study site (Bensch 1995) is [TABULAR DATA FOR TABLE 1 OMITTED] high for a long distance migratory passerine, leaving little room for disappearances not explained by mortality. Second, very few birds (two males and seven females) were present in one year, absent in the next, and present in a third, indicating little movements between years. Third, of the few birds known to have bred first at other sites and later at our study site (or vice versa), some were immigrants (n = 6) and some were philopatric individuals (n = 9).

At our study site, territories differ in their order of settlement supposedly indicating quality (Bensch and Hasselquist 1991). We obtained a measure of a territory's quality in a given year ([year.sub.t]) by calculating the mean of the ranked dates on which it was established in the preceding ([year.sub.t-1]) and the following year ([year.sub.t+1]), see Bensch and Hasselquist (1991) and Bensch (1996) for details. The average ranks of the males' settlement dates in the two flanking years were standardized for each year and marsh by means of Z-transformation (Wilkinson 1992). Even though the between-year return rate to our study area is high (55%), our estimate of territory quality is only marginally affected by the same male occupying the same territory in different years; only 6% of returning males settle in the same territory in successive years (Bensch and Hasselquist 1991). Our measure of territory quality is significantly correlated with both male and female reproductive success (Table 2).

Spring arrival at the study area was defined for males as the first date on which a male was recorded singing and for females as the first date she was observed in the study area, which usually is the date when she settles in a male's territory. To standardize for differences in phenology among years, the date of arrival of the first warbler (males and females separately) was set to day = 1. For individuals breeding in two or more years (58 males and 66 females), we computed their average values of spring arrival day and territory quality. Fitness was estimated as lifetime number of social mates (for the males), fledglings, and recruited offspring (i.e., fledglings recorded on breeding grounds in any of subsequent years).
TABLE 2. Pearson correlation coefficients between three male or
female characteristics and estimates of fitness. Dependent variables
were square root transformed.

                                   Lifetime number of

Male/female variable        Mates       Fledglings    Recruits

Males (n = 116)

Mean arrival date        -0.431(***)   -0.472(***)   -0.480(***)
Mean territory quality    0.309(**)     0.341(**)     0.303(**)
Life span                 0.756(***)    0.711(***)    0.625(***)

Females (n = 137)

Mean arrival date                      -0.332(***)   -0.315(***)
Mean territory quality                  0.245(**)     0.228(**)
Life span                               0.685(***)    0.575(***)

** P [less than] 0.01, *** P [less than] 0.001.


Before performing parametric tests, the total number of females, fledglings, and recruits were square root transformed to improve normality of residuals. The analyses were performed using SYSTAT 5.0 (Wilkinson 1992).

RESULT

Males

Of the 116 males aged as having hatched between 1985 and 1993 and later recorded as adults at our study site, 47.4% were philopatric (Table 1). The proportion of philopatric to immigrant males did not differ among cohorts ([[Chi].sup.2] = 3.33, df = 8, ns). Birds of the nine cohorts differed significantly in lifetime number of fledglings ([F.sub.8,107] = 2.04, P = 0.048) and recruits ([F.sub.8,107] = 2.08, P = 0.044), but not in lifetime number of mated females ([F.sub.8,107] = 1.31, P = 0.244).

Philopatric males attracted more females over their lifetime than immigrant males (P = 0.007), and they also produced more fledglings (P = 0.013) and recruits (P = 0.006) [ILLUSTRATION FOR FIGURE 1 OMITTED]. This pattern remains when we restrict the analysis to the males for which data on lifetime reproductive success is most [TABULAR DATA FOR TABLE 3 OMITTED] likely to be complete, that is, the males that entered the breeding population when exactly one year old (P [less than] 0.01 for all three fitness measures, philopatric n = 38, immigrants n = 24). Also, the annual production of fledglings remained consistently lower for immigrants as they aged [ILLUSTRATION FOR FIGURE 2 OMITTED]. Below we examine whether the observed pattern is confounded by differences in lifetime fitness between cohorts or the three other variables also known to affect reproductive success, viz. territory quality, spring arrival date, and life span.

Philopatric males and immigrant males did not differ significantly in arrival day, territory quality, or life span (Table 3). In these analyses we used the average values of arrival day and territory quality over the males' life spans. We tested whether a difference in these two variables might exist during the birds first breeding attempt at our study site. However, neither arrival day, nor territory quality differed between philopatric and immigrant males during their first breeding season (P [greater than] 0.3).

Arrival date, territory quality, and life span were all significantly correlated to the three measures of fitness (Table 2). Because of the slight numerical difference between philopatric and immigrant males in these three correlates to fitness (Table 3), we controlled for the effect of these three variables and the variable cohort (df = 8) in a multiple partial correlation analysis. Lifetime number of females still differed significantly between philopatric and immigrant males (partial r = 0.271, df = 103, P = 0.005). This was also true for lifetime number of fledged young (partial r = 0.278, df = 103, P = 0.004) and recruits (partial r = 0.265, df = 103, P = 0.006).

Males living longer will on average attract more lifetime number of females (Table 2). To test whether annual mating success differed between philopatric and immigrant males we analyzed the mean number of females per recorded breeding season and found a significantly higher mating success for philopatric than for immigrant males ([F.sub.1,114] = 5.60, P = 0.02). On average, 16% of the philopatric males remained unmated, 38% were monogamous and 46% polygynous. The corresponding proportions for immigrant males were 32%, 31%, and 37%, respectively. Hence, twice as many immigrant as philopatric males failed to attract females. When the number of females was statistically held constant, philopatric and immigrant males did not differ in lifetime reproductive success (P [greater than] 0.5). Thus, the higher lifetime fitness of philopatric males seemed to be a result of these attracting more females than did immigrant males.

Females

Of the 137 females that were hatched during the period 1985 to 1993 and later recorded as breeders at our study site, 44.5% were philopatric (Table 1). The proportion of philopatric to immigrant females did not differ between cohorts ([[Chi].sup.2] = 10.5, df = 8, ns). The nine cohorts differed in lifetime numbers of fledglings ([F.sub.8,128] = 2.48, P = 0.015) but not in recruits ([F.sub.8,128] = 1.53, P = 0.153).

Philopatric and immigrant females did not differ in numbers of fledglings produced in their lifetime (7.26 [+ or -] 7.13 vs 5.83 [+ or -] 5.30, Mann-Whitney U-test, P = 0.37) or in numbers of recruits (1.06 [+ or -] 1.62 vs 0.75 [+ or -] 1.17, Mann-Whitney U-test, P = 0.41). As in males, philopatric and immigrant females did not differ in arrival date, territory quality, or life span (Table 3). Arrival date, territory quality, and life span were all significantly correlated with the two measures of fitness (Table 2). When the effect of these three variables and the variable cohort (df = 8) were controlled for in a multiple partial correlation analysis, there was no difference between philopatric and immigrant females (fledglings, partial r = 0.031, df = 123, P = 0.71; recruits, partial r = 0.017, df = 123, P = 0.84).

DISCUSSION

The lifetime fitness of great reed warblers was lower for immigrant than for philopatric males. The fitness difference remained after we controlled for life span implying that the higher lifetime reproductive success of philopatric males was not a result of them living longer. Also, the pattern still held true when we restricted the analysis to the males for which lifetime data were complete (individuals that started to breed as one-year-olds), indicating that the result was not an effect of immigrants breeding at other sites before they were recorded at our study site. We did not detect any difference in lifetime reproductive success between immigrant and philopatric females. This contrasts to recently reported findings in a population of great tits Parus major in which immigrant females produced fewer lifetime recruits than philopatric females (Verhulst and van Eck 1996).

If the habit of dispersal is partly genetically determined (Greenwood et al. 1979), immigrants are expected to produce a lower proportion of local recruits because a greater proportion of their fledglings would be expected to disperse. This would result in an apparent fitness difference between philopatric and immigrant individuals. However, in our great reed warbler study population, the proportion of local recruits per fledgling is very similar between philopatric and immigrant males (13.5% vs. 14.5%) suggesting no difference in dispersal (Bensch et al., unpubl.). In most species, much of the observed variation in dispersal seems to be explained by ecological factors such as population density (Negro et al. 1997) or the degree of population isolation (Weatherhead and Forbes 1994).

In yearling collared flycatchers Ficedula albicollis, male mating success is higher for philopatric males than for immigrant males (Part 1994). In this species, philopatric males established themselves at higher quality nest sites, which indicates that prior local experience facilitates nest site selection. In great reed warbler males, lower mating success reduced the immigrants' lifetime fitness even though immigrants did not occupy lower quality territories than philopatric males. This was also true for males breeding for the first time. Also, a fitness difference between philopatric and immigrant males was still detectable after several years of breeding experience [ILLUSTRATION FOR FIGURE 2 OMITTED]. Because philopatric and immigrant females experienced similar lifetime fitness and life span, and philopatric and immigrant males did not differ in life span, it is unlikely that the immigrants suffer from difficulties in finding food, nesting sites and protection against predators (Greenwood 1980).

Our results indicate that the difference in lifetime reproductive success between philopatric and immigrant males is explained by the latter being less successful in obtaining females. According to the theory of optimal inbreeding, there is an optimal genetic distance between partners that maximizes fitness (Shields 1982). We have previously shown that nonincestuous inbreeding in our study population of great reed warblers results in reduced hatching success (Bensch et al. 1994). Nonincestuous inbreeding would be reduced if philopatric birds preferentially mated with immigrants, but in fact the opposite seems to occur. A too large, genetic distance between partners may result in breakdown of coadapted gene complexes. This could make females reluctant to mate with immigrant males and contribute to lowered male mating success and fitness. Our data cannot be used to test whether males avoid mating with immigrant females because we cannot register females that fail to find a male. In polygynous species, however, males are expected to accept all females as partners (Searcy and Yasukawa 1989), irrespective of origin.

In many bird species song varies geographically, in some species over remarkably short distances (Catchpole and Slater 1995), and song dialects may act as dispersal barriers (Baker and Mewaldt 1978). Much of this geographical variation seems to be maintained by immigrants learning the local dialect (Payne and Payne 1977; Payne et al. 1987). However, even if they are able to learn the local song dialect, this may take time. Hence, before immigrant males have learned to sing the local dialect, song is a possible cue by which resident females could discriminate between philopatric and immigrant males. In the great reed warbler, Fisher et al. (1996) found that song differed between populations when these were located at least 40 km apart. Though male song repertoire size in great reed warblers appears to change over a male's life (Hasselquist 1994), it may contain elements that are diagnostic for its natal area. If so, females might be able to discriminate between philopatric and immigrant males by listening to their song.

For a subsample of the males analyzed in the present paper (26 philopatric and 28 immigrants) we found (unpublished) that philopatric males on average had larger song repertoires (standardized for age) than immigrants (37.2 versus 34.4 syllables, t = 2.64, P = 0.011; see Hasselquist et al. (1996) for methods). We have previously shown that males with large song repertoires are more successful in obtaining extrapair fertilizations and that their offspring have higher postfledging survival (Hasselquist et al. 1996). Thus, the low song repertoire size of immigrants may suggest that they are of lower phenotypic quality than philopatric males. This explanation, however, was not confirmed by our data as we did not find any differences between philopatric and immigrant males in the other quality measures, that is, spring arrival date, territory quality, and life span.

Another explanation is that immigrants do not use their complete repertoire size at our study site. This might be adaptive if females avoid males with unfamiliar songs (e.g., Hegelbach 1986; Baker et al. 1987) and would be consistent with the honest convergence hypothesis (Rothstein and Fleischer 1987) which proposes that only males experienced with the local area are capable of singing the local dialect. If this is the case, one would expect any fitness difference to disappear once immigrants had learned the local dialect. In contrast to this hypothesis, we found that the fitness difference remained as the birds aged [ILLUSTRATION FOR FIGURE 2 OMITTED]. Another explanation for why immigrants exhibit a lower repertoire size could be through suppression of certain song themes, for example, in relation to social dominance as in cowbirds Molothrus ater (West et al. 1981). Similarly, immigrant males could partly conceal their alien origin by adjusting their song repertoire to include only syllables shared with the other males in the new area. If this is the case in the great reed warbler, it may result in a lower song repertoire size, and thus lowered mating success (Catchpole et al. 1986; Hasselquist et al. 1996). However, to express a reduced song repertoire may still be more advantageous than if the male were to advertise his immigrant origin. Still, however, we do not know in what way females may benefit from avoiding mating with immigrant males.

ACKNOWLEDGMENTS

Numerous people have assisted in the field and especially U. Ottosson, P. Frodin, M. Haraldsson, F. Haas, A. Kvist, H. Westerdahl and O. Ostman have helped with fieldwork for long periods. We are indebted to E. Ketterson, T Madsen, T Price, T von Schantz and two anonymous referees for comments on earlier drafts of this manuscript. The study was financially supported by Elis Wide's Foundation, Royal Swedish Academy of Science (Ahlstrands, Hierta-Retzius), Olle and Signhild Engkvist's foundation, Magn. Bergvails foundation, Lunds Djurskyddsfond, Crafoord Foundation, Swedish Natural Science Research Council (NFR) and Swedish Forest and Agricultural Research Council (SJFR). This is report no. 98 from the Kvismare Bird Observatory.

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Author:Bensch, Staffan; Hasselquist, Dennis; Nielsen, Bo; Hansson, Bengt
Publication:Evolution
Date:Jun 1, 1998
Words:5304
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