Summary. Natal philopatry and recruitment were measured in two populations of willow ptarmigan; one near Chur- chill, Manitoba and the other in northwestern ...
Oecologia
Oecologia (Berlin) (1987) 71:518-524
9 Springer-Verlag1987
Natal philopatry and recruitment of willow ptarmigan in north central and northwestern Canada K. Martin 1 and S.J. Hannon 2 1 Boreal Institute, University of Alberta, Edmonton, Alberta, Canada, T6G 2E9 2 Department of Zoology, University of Alberta, Edmonton, Alberta, Canada T6G 2E9
Summary. Natal philopatry and recruitment were measured in two populations of willow ptarmigan; one near Churchill, Manitoba and the other in northwestern British Columbia. We examined the return of tagged offspring in subsequent years with respect to geographical area, annual variation, their age when tagged, their sex, their body weight, age and number of their parents, and time of hatch (first nest or renest). Most chicks were tagged before they fledged, but chicks tagged after that had the highest rate of return. We also observed a strong positive relationship between fledging success of broods and offspring return in following years. Patterns of offspring return were similar in both populations except that male offspring in Manitoba settled closer to their natal sites than those in British Columbia and more yearlings captured in Manitoba had been tagged as chicks. Return of offspring did not vary with year, their body weights shortly after hatch, or with the age or number of parents raising them. However, a significantly higher proportion of offspring hatched from first nests (first-initiated clutches) returned compared to those hatched from renests (replacement clutches). The low return of chicks hatched from renests may due to low survival, low philopatry, or both. We observed no differences in the mating status (recruitment) of returning offspring with respect to the time they hatched or the number of parents that raised them. Key words: Philopatry Hatch date
Recruitment - Willow ptarmigan
Recruitment of offspring into the breeding population is the best available estimate of annual reproductive success for individuals, but often this is difficult to measure directly or completely (Howard 1979). Generally, we know little about the factors, both intrinsic and extrinsic, that affect offspring survival and return to their natal area. Is probability of return related to characteristics of the parents or of the offspring themselves, to ecological factors, or to other variables? Because most dispersing young are not found again in the study population, the degree of natal philopatry usually influences estimates of recruitment, and thus measures of fitness.
Offprint requests to. K. Martin
Here, we present information from two geographical areas on characteristics of willow ptarmigan (Lagopus lagopus) offspring that survived and returned in a subsequent year. We examine return of offspring in relation to geographical area, year, their age when tagged, their sex and body weight, age and number of their parents, and time of hatch (first nest or renest). We define natal phitopatry as the return of tagged juveniles to the study area in a year subsequent to hatching. Returning offspring were considered recruits when they established a territory and acquired a mate. The willow ptarmigan is a territorial, usually monogamous grouse in which males contribute substantial time and energy to nest and brood defence (Wittenberger 1978). Both sexes can breed as yearlings, thus we could begin to measure recruitment in the second year of study. Ptarmigan breed in arctic or subalpine tundra and migrate variable distances to wintering areas, with females migrating further than males (Weeden 1964). Both sexes show territorial behaviour (Hannon 1983). Study sites
Populations of willow ptarmigan were studied at two field sites. One study was conducted by KM during 1981-1984 on 10 sq km of subarctic tundra near La Perouse Bay (58~ 94~ 40 km east of Churchill, Manitoba, Canada. Martin (1984a, 1985) gave a detailed account of the study area and field methods, as well as general biology of the resident willow ptarmigan population, and Jefferies et al. (1979) described the vegetation. The second study was conducted by SJH during 1979 1982 and 1984-1985 on a 2.5 sq km area (enlarged to 4.5 sq km in 1985) located in subalpine tundra at Chilkat Pass, northwestern British Columbia, Canada (59~ 136~ Descriptions of the climate and vegetation of the area are found in Weeden (1960) and Hannon (1983, 1984). The Manitoba site was heterogeneous with many large and small lagoons and with the vegetation (willow (Salix spp.) and birch (Betula glandulosa)) approximately 0.3-0.8 m high with occasional taller strips (1.5 2.0 m). In contrast, British Columbia was drier and the vegetation a more uniform height of about 1.5 2.0 m. Territories were substantially larger in Manitoba than in British Columbia (e.g., in 1981, mean territory size was 4.1ha+0.31SE (N=13) in Manitoba, and 2.2ha _+0.22SE (N=18) in British Columbia). Field seasons in
519 both areas were from mid April to mid August, except during 1982 in British Columbia (27 April to 11 May only). Parts of both populations were manipulated in spring by removal of either males or females (see Harmon 1983, ]984; Martin 1984a, 1985; Martin and Cooke 1987, for details). Methods Methods of capture and census in both studies were similar, with most birds being captured before onset of incubation with a noose (Zwickel & Bendell 1967) or net, and weighed and marked individually with one metal and three coloured plastic leg bands. More than 90 percent of the resident population, including nonbreeders, were located, captured and marked. Ptarmigan could be classified as yearlings (hatched the previous season) or adults (2 + years) by comparing pigmentation on the eighth and ninth primaries (Bergerud et al. 1963). Sexes were distinguished by differences in plumage, voice, and wing length (Bergerud et al. 1963). Birds of both sexes were classified as either territorial and mated, territorial and unmated, nonterritorial (lacking both territory and mate during the breeding season), or nonresident (seen only occasionally during migration or off the study area). Nests were found by searching, sometimes with pointing dogs, around roost sites of mated males. After a nest was located, clutch size was recorded and the nest was visited several times weekly until hatch. Breeding chronology within both populations was asynchronous because of differences in timing of nest failure and renesting intervals. Eggs from "first nests" (first-incubated clutches) hatched in late June or early July, whereas subsequent attempts ("late nests" or "renests") hatched from mid July through early August. Renests were more frequent in Manitoba than in British Columbia. At hatch, or when first encountered thereafter, chicks were captured, weighed and marked with metal tags attached to the patagium. Subsequent recaptures of chicks revealed that tag loss was rare in both sites, and was unlikely to be biased in relation to any of the variables measured in these analyses. Unknown hatch dates were estimated by determining ages of chicks and backdating. Onset of incubation was determined by estimating an incubation period of 22 days, and date of clutch initiation was determined by assuming a laying rate of one egg per day (Martin I984b). Upon location of a brood, the number of flying chicks was recorded; juveniles began to fly (fledge) at 11 to 12 days of age. As most broods were observed several times, fledging success of pairs for a season was considered as the highest number of chicks recorded from broods 15-25 days old. Single parents (both sexes) were created by removing their mates either during laying, at onset of incubation, or at hatch. SingIe parents also occurred naturally when secondary hens hatched broods or when mates were killed. Because of differences in habitat heterogeneity, breeding density, and territory size on the two sites, we measured degree of natal philopatry in both areas by determining the number of territories away from the natal territory that returning offspring settled. Shields (1982) argued that dispersal is a discrete process involving quantal leaps by an individual from territory to territory before it settles successfully, and thus measures of dispersal using linear distances are not always comparable between species or populations. Although the Manitoba site was larger than that
in British Columbia, we searched roughly the same number of territories (65-70) each year, except in 1985 when the study site in British Columbia was enlarged. Statistical analyses Sample sizes differed for the variables measured because complete details of reproductive histories were unavailable for all individuals. Data from various years and first and late nests were pooled when homogeneous. Frequency data involving three variables were analysed with Multidimensional Contingency Analyses (MDCA; Bishop et al. 1975; Colgan and Smith 1978). Log-likelihood ratios were calculated with a Yates' correction for continuity, Two-way contingency analyses were performed with G-tests using a Williams' correction, for cells with small sample sizes (Sokal and Rohlf 1981). Means are given with one standard error (SE). Statistical probabilities greater than 0.05 are considered nonsignificant. All tests are two-tailed. Results
Return of tagged juveniles In Manitoba, during the 1981-1983 breeding seasons, 789 chicks (sexes unknown) were tagged from 125 broods of resident pairs, and 46 of these (35 males, 11 females) were observed on the study area in subsequent years. Five other male offspring returned from another 85 chicks tagged from 33 broods of nonresident pairs. Forty-nine offspring returned as yearlings and the remaining two as twoyear olds. Four of the tagged yearlings that returned were "grand-offspring" (offspring of tagged offspring). In British Columbia, during 1979-1981 and t984, 746 chicks were tagged from 193 broods, and 31 of these (25 males, 6 females) returned to the study area; 29 as yearlings and two as two-year olds.
Annual and geographical variation Rate of offspring return was similar for each year in Manitoba (Z2= 0.02, df= 2, P = 0.99) and also in British Columbia (Z2= 4.97, d f = 3, P = 0.17), despite the fact that median date of clutch initiation varied among years by as much as 13 days (Table 1). When years were pooled, no difference was observed between study areas in proportion of offspring returning (G = 1.48, d f = 1, P-- 0.22).
Age of tagging and offspring return Age of chicks at time of tagging was divided into three categories: hatch (1-3 days old), preflight (4-12 days old) and fledged (>12 days). No overall difference in return of chicks relative to age when tagged was observed between sites (2,2=1.13, df=2, P=0.57). Return rates of chicks tagged at hatch (4.9%, N=1,164) and during preflight (3.5%, N=290) were similar (G=1.17, d f = l , P=0.28). Chicks tagged after fledging returned at a significantly higher rate (11.]%, N--81) than chicks tagged earlier (4.6%, N=1,454; G=5.06, d f = l , P=0.02). Chicks captured at all ages from each site were pooled in the subsequent analyses, because 94.7% (N= 1,535) of chicks were tagged before they fledged, and there were similar proportions of chicks tagged in each age category between areas.
520 Table 1. Date of clutch initiation and return of willow ptarmigan chicks in Manitoba and British Columbia Year
Median date of clutch initiation S
Total chicks tagged
Chicks returning in next season
% return
Manitoba: 1981 June 1 1982 May 30 1983 June 12
214 240 335
11 15 20
5.1 6.3 6.0
British Columbia: 1979 May 31 1980 June 5 1981 June 1 1984 May 31 1984 (expanded plot)
193 179 214 160 160
9 6
4.7 3.4 1.4 5.0 8.1
14
iit x-
9
6
C~
3b
8 13
a Only first completed clutches of the season were included b Capture effort in 1982 was lower than other years due to shorter field season
Fledging success as an indicator of offspring return Fledging success has often been used to measure annual reproductive success (Howard 1979), but usually data are unavailable to test if counts after fledging are accurate assessments of offspring survival to breeding age. To examine this relationship we compared brood size after fledging (measured in current season) to the probability of offspring returning from that brood in a subsequent season. D a t a were available for 7 6 b r o o d s in Manitoba (1981 11 broods; 1982 - 29 broods; 1983 - 36 broods). One or more tagged chicks returned in a subsequent season from 23 of 48 (48%) broods that had > 50% of the chicks in a brood fledge compared with only 6 of 28 (21%) broods in which < 50% of the juveniles fledged. D a t a were available for 29 broods (1979 - 8 broods; 1980 - 3 broods; 1981 - 10 broods; 1984 - 8 broods) from British Columbia. Here, 7 of 19 (37%) broods that had at least five chicks fledge had returns in the subsequent season compared to 2 o f 10 (20%) broods with fewer chicks fledging. Contingency analyses revealed that data from the two sites were homogenous (Z2 = 0.19, d f = 1, P = 0.66), and that significantly more offspring returned from broods that experienced high fledging success in the previous year compared to broods with poorer survival to fledging 0f2=5.77, d f = l , P : 0 . 0 2 ) . Hence for willow ptarmigan, fledging success appears to be an accurate assessment of annual reproductive success.
Sex of offspring and natal philopatry I f one assumes an equal sex ratio at hatch (chicks cannot be sexed until well after fledging), then male chicks had a higher probability of returning to the natal area than females in both studies. In Manitoba, 35 of 395 (8.9%) tagged male chicks (half of 789 chicks) and 11 of 395 (2.8%) tagged female chicks were observed in subsequent seasons ( G = 13.79, d r = 1, P=0.0002). In British Columbia, 25 of 373 (6.7%) tagged males (half of 746 chicks) returned compared to 6 of 373 (1.6%) tagged females ( G = 12.81, d f = 1, P=0.0003). When return o f each sex was compared be-
z
o-
I,,l,, 0
1
2
3
4
5
6 9
>10
Number of territories Fig. 1. Number of territories moved from the natal territory by willow ptarmigan male (closed) and female (open) offspring in the first year of their return in Manitoba and British Columbia
tween areas, rate of offspring return was not significantly different for either male offspring ( G = 1.24, d f = 1, P = 0.27), or female offspring (G = 1.21, df--- 1, P = 0.27). To compare degree of natal philopatry between sites, we defined four movement categories based on the distance returning offspring settled from their birth sites: those settled on their natal or one to two territories away, three to five territories from natal, six to nine territories away, and ten or more territories. Number o f territories moved was heterogeneous between Manitoba and British Columbia (Z2= 7.85, d f = 2, P = 0.02), thus sites were analyzed separately. In Manitoba, male offspring returned significantly closer to their natal territories than females (Z2 = 32.61, d f = 3, P < 0.0001 ; Fig. 1). Three returning males were captured on their natal territories. In each case, their fathers did not return in that year. In two cases, their mothers had returned, but these hens moved to a n adjacent territory and to a territory about one km away, respectively. In British Columbia, no offspring settled on their natal territories, and in general offspring were found further from their natal territories than in Manitoba (Fig. 1). In British Columbia, males did not settle closer to their natal areas than females (Z2=3.62, d f = 3 , P = 0 . 3 1 ) . However, the sample size of females was small, and there was a tendency for males to settle closer to their natal territories than females. The proportion of yearlings captured that had been tagged in a previous year did not vary annually, except for males at La Perouse Bay (Table 2). More tagged male yearlings were recaptured than females in both Manitoba (G=27.40, d f = l , P
