Poor recruitment in marginal areas and gene

Transcription

1 Bird Study (1996) 43, The breeding biology of the Redstart Phoenicurus phoenicurus in a marginal area of Finland S. VEISTOLA*, E. LEHIKOINEN, T. EEVA and L. ISO-IIVARI 1 Laboratory of Ecological Zoology, Department of Biology, University of Turku, FIN Turku, and 1 Kevo Subarctic Research Institute, University of Turku, FIN Turku, Finland The breeding biology of the Redstart was studied in its marginal breeding area in northernmost Finland during the years The mean clutch size was higher than in more southern populations. Losses during the egg and nestling stages were negligible: hatching success (hatchlings per egg) and fledging success (fledglings per hatchling) were 0.91 and 0.89, respectively. Only heavy rainfall decreased nestling survival. Our results show that population size is stable and breeding success is good in this marginal breeding area compared to other breeding grounds. The population size fluctuated in parallel with the vole cycle as expected if small (mammal) predators switch to taking birds when vole numbers are low. INTRODUCTION Poor recruitment in marginal areas and gene flow from more favourable areas may prevent adaptation of a species to its marginal breeding areas. 1 For instance, in many species, clutch size is often smaller 2 4 and breeding success poorer 5 in high northern latitudes than in more favourable breeding grounds. In this paper we present an exception: the Redstart Phoenicurus phoenicurus, a migrant hole-nesting passerine, has large clutches and breeds successfully in its marginal breeding area in Finnish Lapland, compared with other breeding areas. Redstart populations have declined dramatically in both Central Europe and Finland in this century. 6 8 We analyse variation in population size in the Redstart during to find out if the trend is the same in the northernmost population. We also demonstrate that the population size of the Redstart fluctuates cyclically and is coupled with small mammal cycles, strongly suggesting that predator diet switching is involved as Greenwood 9 and Sutherland 10 hypothesized. *Correspondence author. MATERIAL AND METHODS The data were collected in Utsjoki, northernmost Finland (69 N, 27 E) during In 1982, 230 nest-boxes were put up in mixed deciduous and coniferous woodland in the Utsjoki valley ( m above sea level). In 1983 the number of nest-boxes was increased to 700. A detailed description of the study area and set-up of nest-boxes has been presented elsewhere. 11 Laying date of the first egg (later: laying date), final clutch size and breeding success were determined by regular nest inspections (the nests were visited at least four times per season) since During the inspection rate was lower, so data from these years have been omitted from some analyses. Nests abandoned before incubation (11 nests, 7.1%), and four predated nests (2.6%) were omitted when mean hatching or fledging success was calculated. Hatching success and fledging success were calculated only for nests in which clutch size, numbers of hatchlings and fledglings were known. Hatching success was calculated as the number of hatchlings per final clutch size, and fledging success as the number of fledglings per number of hatchlings. Vole abundance was estimated by Lasse 1996 British Trust for Ornithology

2 352 S. Veistola et al. Figure 1. Clutch size ( ), number of hatchlings ( ) and number of fledglings ( ) during Bars denote standard errors. Table 1. Mean clutch size, number of hatchlings, and number of fledglings Mean sd n χ 2 P Clutch size Number of hatchlings Number of fledglings Variation among years tested by Kruskal Wallis test (df = 8). Data include only those nests which could be monitored through the whole breeding period. Iso-Iivari during by standard trapping with 120 snap traps for two nights in the middle of June in the centre of the nest-box area. Weather data were received from the Meteorological Station at Utsjoki Kevo. RESULTS The first Redstart males usually arrived in Utsjoki in the middle of May (mean 10 May, unpubl. data). Laying started on average on 4 June (sd = 6.89, n = 136). Nestlings hatched on average on 24 June (sd = 6.63, n = 124). No second or repeat clutches were found. The mean clutch size and the mean hatchling and fledgling production are given in Table 1. The mean hatching success was 0.91 (sd = 0.17, n = 83). The mean fledging success (fledglings per hatchling) was 0.89 (sd = 0.28, n = 83). The fledgling production was slightly lower in 1987 and 1988, and considerably so in 1992 (Fig. 1). The weather conditions were very bad in the Redstart nestling period in During the main nestling period (from 24 June to 7 July 1992) the precipitation was 71 mm and the mean temperature was 7.9 C. Normally the mean daily temperature lies above 12 C in midsummer and rainfall during a typical nestling period is mm. A logistic regression model showed that nestling losses were connected with heavy rainfall during nestling periods but not with mean temperatures (Table 2). The population density varied greatly from year to year (Fig. 2) but was always low (<2 pairs/km 2 ). The peak in Redstart abundance

3 Breeding biology of Redstart 353 Figure 2. The proportion of occupied nest-boxes (solid line), and the relative abundance of voles (individuals caught in standard trapping, dotted line) during Total number of nests = 138. occurred in the same two-year periods when voles were most abundant (Fig. 2). The years of low Redstart abundance occurred two years after vole peaks. Serial correlation analysis of Redstart and vole numbers, carried out with increasing lag (Fig. 3), suggests that Redstart and vole numbers are related. DISCUSSION The clutch size of the Redstart increases from Central Europe to Lapland. 6 As far as we know, the mean clutch size in our study area is the largest reported. 3,6,12 The increase in clutch size with latitude is caused partly by different breeding strategies of populations: many females lay two broods in Central Europe, 6,12 but in the north the Redstart is single-brooded. In Lapland, the average clutch size decreases with altitude. 3,13 Thus, in the Redstart, geographical variation in clutch size is similar to that of many other passerines: the mean clutch size increases with latitude, 14,15 but towards very marginal breeding grounds it decreases. 2,3 Large broods and a high breeding success suggest that conditions are not very harsh for the Redstart in our study area. The breeding Table 2. The relationship between nestling mortality and weather variables (mean temperature and rainfall during nestling period) Parameter estimate Intercept Mean temperature Rainfall A logistic regression model, in which a binary independent variable had values 0 (no dead nestlings) or 1 (one or more dead nestlings). Data include all nests where nestling mortality could be monitored, n = 92. success is lower in Central Europe than in our study area. Breeding success (fledglings per egg) ranges between 0.46 and 0.77 in Central Europe, 12 but in our study it was Which factors explain the success of the Redstart in Utsjoki? We suppose that the light nights, broad foraging niche and, in most years, low predation risk allow for large broods. Lack 16 suggested that the increase in clutch size with latitude was partly explained by the increase in day length. Because here the Redstart can feed the brood at midnight, 17 longer daily feeding time may explain larger P

4 354 S. Veistola et al. Figure 3. Variation in the Spearman correlation coefficient of Redstart density and vole abundance with lags of 0 7 years. Dotted lines denote significance level at P = broods in northern Redstart populations. Heavy predation is believed to select for smaller clutches. 18 However, in the north, where the predation rate is normally low in hole-nesting passerines, 19 such selection pressure must be weak. In many summers a bottle-neck for successful breeding in the north is caused by bad weather. 20 Many passerines, e.g. the Pied Flycatcher Ficedula hypoleuca (unpubl. data) and the Great Tit Parus major, 4 suffered heavy losses during bad weather in 1987, 1988 and 1992 in our study area. The fledging success of the Redstart was poor only in 1992 (Fig. 1), when weather conditions were very bad during the nestling period. Our data show that the Redstart is well adapted to northern conditions, and only extremely bad conditions cause losses. 5,20 We suggest that the broad foraging niche of the Redstart 6,21 is important during bad weather spells. Whereas the Pied Flycatcher is dependent on mobile insects, the Redstart also picks immobile food items from ground and trunks (own obs.). Thus, the Redstart can satisfy the minimum energy demand of the young even during bad spells; the nestling mortality is remarkable only during very long and severe periods. Although the Redstart population has declined in Finland, it breeds abundantly, although sparsely, in its marginal breeding grounds of northern Lapland. 22 In our study area, the population has not declined during the study. The peak abundance of Redstarts and small rodents occurs normally in the same years in high mountain areas or in high latitudes. 23,19 Greenwood 9 and Sutherland 10 proposed that a high predation rate after a peak vole year causes a synchronous population fluctuation in small rodents and birds. In our study, the lowest abundance of Redstarts lagged two years behind the vole peak. Other studies have also found that the population size of the Redstart 19 and other small passerines 23 was usually lowest two years after the vole peak. So, the results from passerine studies in high latitudes in Fennoscandia fit very well with Greenwood s and Sutherland s hypothesis. REFERENCES 1. Perrins, C.M. (1990) Concluding remarks: dispersal and gene flow. In Population Biology of Passerine Birds (eds J. Blondel, A. Gosler, J.D. Lebreton & R. McCleery) pp NATO ASI Series, Vol.

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