Natal and breeding dispersal in house wrens (Troglodytes aedon)

--We studied the natal and breeding dispersal of yearling and adult House Wrens (Troglodytes aedon) for 7 yr in central Illinois. The forested study areas contained 910 identical nest boxes placed in a grid pattern. On average 38.1% (n = 643) of the adult males and 23.3% (n = 1,468) of the adult females present in one year returned the next; 2.8% (n = 6,299) of the nestlings that survived to leave the nest returned each year. Adult male (median distance = 67 m) and adult female (median = 134 m) breeding dispersal was less than yearling male (median = 607.5 m) and yearling female (median = 674 m) natal dispersal. Females that returned had produced more offspring the previous season than had nonreturning females, and females that successfully produced at least one chick in their last nesting attempt of the previous season moved shorter distances than did unsuccessful females. There were, however, no consistent differences between returning and nonreturning females in two other measures of reproductive success. Females that were unsuccessful in their last breeding attempt of the previous year were more likely to be successful in their next attempt if they moved two or more territories than if they did not move. Reproductive success did not affect the likelihood that a male would return nor the distance that he moved. The success of subsequent nesting attempts by males was also not related to the distance moved. Inbreeding avoidance may explain differences between breeding and natal dispersal, but it does not explain the lack of difference in dispersal of yearling females and males. Differences between adult and yearling dispersal are best explained by advantages accruing to adults that remain near former breeding sites and by the necessity for yearlings to move farther because of their late return from the wintering grounds. The advantages for adults to reoccupy previous breeding sites are counterbalanced, especially in females, by advantages associated with moving after breeding failure. Received 15 June 1987, accepted 4 March 1988. IN many migratory passerines there are genderand age-related differences in the proportion of birds that return to a breeding area and, for those returning, in the distance they settle from their former breeding or natal nest site (see reviews by Greenwood 1980, 1984). The general pattern is for adult males to return in higher proportions and settle closer to their previous nest site than adult females and for adults to settle closer to their previous nest site than yearlings to their natal site. Furthermore, individuals that produce offspring are more likely than unsuccessful birds to return to the area of their last breeding attempt. One explanation for gender-related differences in dispersal is that territorial males gain more from being site faithful than females (Greenwood 1980). Differences in adult breeding dispersal associated with breeding success are usually explained as adaptations for improving breeding performance (e.g. Harvey et al. 1979, Beletsky and Orians 1987; cf. Nolan 1978: 461). Age-related differences in dispersal 480 may be attributable to costs or benefits associated with inbreeding (e.g. Greenwood et al. 1978, Shields 1982). Murray (1967), Moore and All (1984), Liberg and von Schantz (1985), and Waser (1985) suggested that intrasexual competition for nest sites or mates is responsible for some age-related differences in dispersal. We investigated the influence of age, gender, and reproductive success on breeding and natal dispersal in House Wrens (Troglodytes aedon) and examined some of the proposed explanations for differences in avian dispersal patterns. METHODS AND MATERIALS Study areas. --The two study areas, in McLean Co., Illinois (Money Creek Township, T 25N, R 3E), were floodplain and upland forests dominated by oak (Quercus pp.), hickory (Carya spp.), hackberry (Celtis occidentalis), and ash (Fraxinus spp.) (Drilling and Thompson 1984). The forests were surrounded by arable fields. The 108-ha main study area was bisected by the Mackinaw River and contained 585 nest boxes distributed in north-south rows approximately 60 m The Auk 105: 480-491. July 1988 July 1988] House Wren Dispersal 481 apart (Fig. la). Nest boxes within a row were approximately 30 m apart (5.4 boxes/ha). In 1980 there were 327 nest boxes, all north of the river. The study area was expanded south of the river in 1981 by 148 nest boxes and in 1982 when another 110 nest boxes were placed in the remaining forest. The 20-ha East Bay study area (Fig. lb), established in spring 1982, was 1.6 km west of the main study area. The 325 nest boxes there were also arranged in north-south rows, but distances between rows and nest boxes varied, with 183 nest boxes 15 x 15 rn apart (43 boxes/ha), 40 nest boxes 15 x 30 m apart (23 boxes/ha), 67 nest boxes 30 x 30 m apart (10.9 boxes/ha), and 35 nest boxes 30 x 60 m apart (5.4 boxes/ha). Each nest box, made of stained pine, had identical dimensions (floor, 8.4 x 9.0 cm; diameter of entrance hole, 3.2 cm; distance from bottom of entrance to floor, 13.0 cm; distance from entrance to ground, approximately 1.5 m). The entrance hole had a permanently mounted trapdoor. Each nest box was mounted on electrical line conduit, which was greased to reduce predation. Definitions and procedures. --Wrens were categorized as "adults" if they were banded as breeding birds, if their age was unknown (AHY in U.S. Fish and Wildlife terminology), or if they were banded as nestlings on the study area and were 2 or more years old (TY and ATY). "Returning nestlings" and "yearlings" (SY) were banded as nestlings on the study area and returned in a subsequent year to breed. Analyses of breeding and natal dispersal were based on data from 1,219 adult females, 547 adult males, and 6,299 nestlings that survived to leave the nest. Breeding female wrens were identified at 88-95% of the nests in 1980-1987. Females were caught in nest boxes while they incubated their eggs or brooded and fed their nestlings. Breeding males were caught either in nest boxes or in nearby mist nets or visually identified at 10-40% of the nests in 1980-1982 and at 55-85% of the nests in 1983 and 1985-1987. Because males were identified at only 2% of the 1984 nests, data for adult males and yearlings in 1984 were not included in any analyses. Adults and nestlings were banded with a numbered U.S. Fish and Wildlife Service aluminum band and three plastic color bands (2 bands/leg) in 1980, 1981, and the first half of 1982. In the second half of 1982 and in 1983-1987, each adult female and nestling was banded with a numbered aluminum band only. In 1985-1987 males were given 3 color bands and an aluminum band. Nest boxes were checked twice weekly during the May-August breeding season. The day the first egg was laid (i.e. egg-1 day), clutch size, brood size, day the first chick hatched, fate of the nest, and number of chicks surviving to leave the nest were recorded for all nests. Data analysis. --Statistical analyses were performed with Statistical Analysis System (SAS) programs (SAS Inst. 1985). Proportions were compared using a logDiagram of (a) main study area and (b) East Fig. 1. Bay study area. Each dot represents 1 nest box. linear model (LM) contained in the CATMOD procedure in SAS. We used a multivariate rank test with year as a blocking factor (MRANK procedure; SAS Inst. 1986) for most analyses of distances moved. For distance analyses for which sample sizes were inadequate in some years, we used the Mann-Whitney U-test (M-W test) or the Kruskal-Wallis Chi-square approximation (K-W test). The distance between nest boxes was calculated as the straight-line distance. For frequency distributions of distances moved (Fig. 2), a territory was assumed to be 75 m in diameter (Kendeigh 1941). To examine movements with respect to nest-box availability, we used a procedure modeled after that of Rheinwald (1975). This generated a distance distribution of empty nest boxes available at the time of return for each nest box from which a wren moved. We calculated the distances between the nest box from which a wren moved and all unoccupied boxes to which it could have moved. Each unoccupied nest box was classified as being in the former territory, in an adjacent erritory, in a territory 2 territories away, and so forth, based on an average territory diameter of 75 m. We considered a nest box to be unoccupied when no nest was started in the box within 7 days 482 DRILLING AND THOMPSON [Auk, Vol. 105