Amersham o Little. 0. Watford. Chalfont Chorleywood ? Croxley Green. Rickmansworth. O Seer Green ...... Pacific Grove, CA. McNamara, J. M. and Houston, A. I. ...
23: 63-72. Copenhagen1992 ORNISSCANDINAVICA
Variationin the nomadictendencyof a winteringfinch Carduelis spinusand its relationshipwith body condition J. C. Senar, P. J. K. Burton and N. B. Metcalfe
Senar, J. C., Burton, P. J. K. and Metcalfe,N. B. 1992. Variationin the nomadic tendencyof a winteringfinch Carduelisspinusand its relationshipwith body condition. - OrnisScand.23: 63-72. Patternsof residenceandsite-fidelitywere examinedin the nomadicSiskinCarduelis spinus at 14 winteringsites in Britainand Spainwhere excess food was continually available.In spiteof the stabilityof the food source,only 8% of birdscapturedat the study sites became "resident",being caught on over 70% of trappingdays over several months. In contrast,the majorityof birds were classifiedas "transients", staying for short periods and often being caught only once. Both residents and transientscontinuedto arrivethroughoutthe winter.Residentsappearedto become sedentaryas soon as they arrived.Transientswere very mobile (makingmovements of 10-40 km in a single day), whereasresidentsmade only short-rangemovements (usually less than 3 km). There were no sex, age, or size differencesbetween residentsandtransients.An indexof bodycondition(mass/winglength3)varieddaily and seasonally,birdsbeing heaviestin midwinterand in the afternoon.However, residentswere consistentlyheavier than transientsfrom the date of their initial captureonward.The evidencesuggeststhat the observedvariationin site fidelityfits the hypothesisof Gauthreauxwhichrelatesresidenceto dominance. J. C. Senar, Museu de Zoologia, Ap. 593, 08080 Barcelona, Spain. P. J. K. Burton, High Kelton, Doctors Commons Road, Berkhamstead, Herts HP4 3DW, U. K. N. B. Metcalfe, Dept. of Zoology, Univ. of Glasgow, Glasgow G12 8QQ, U. K.
Introduction Many bird species exhibit strong fidelity to either a breeding or wintering area, or both (e.g. McNeil 1982, Rogers et al. 1982, O'Connor 1985, Kricher and Davis 1986). Putative advantages of such residence include better knowledge of the area, improved location of resources, and retention of good quality territories (Hinde 1956, Lundberg 1987). The main exceptions to this pattern are found amongst birds that exploit unpredictable food supplies, such as tree seeds or rodents (Andersson 1980). For instance, cardueline finches feeding on tree seeds not only use different breeding or wintering localities from year to year (Newton 1972, Andersson 1980, Yunick 1983), but also show high mo-
bility within a winter (e.g. Swenson et al. 1988) or even within the same breeding period (Gotmark 1982). Since their food is patchy and ephemeral, it has been suggested that this mobility allows birds to locate alternative food supplies within a large area (Swenson et al. 1988). However, if such movements are made solely as a means of locating food (Andersson 1980, Gauthreaux 1982), birds should remain in an area once a plentiful food source is located. In this paper we examine wintering residence and site-fidelity in the Siskin Carduelis spinus, a typically nomadic species of finch (Newton 1972). Our analyses show that contrary to predictions, only a small fraction of the population becomes resident when given a plentiful and permanent food source; the remainder are tran-
Received 13 May 1991 Revised 17 September1991 Accepted 19 September1991 ? 5
ORNIS SCANDINAVICA
ORNIS SCANDINAVICA 23:1 (1992)
63
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o Markyate @Wheathampstead Aston C inton
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o Bennett's Hemel O Hempstead
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KirteHilltop 0 Kings Langley ? Chesham Prestwood TL o Chesham + Uxbridge Bois * Amersham o Little Watford 0 Chalfont Chorleywood ? Croxley
Fig. 1. Britishstudy area, showingthe different trappingstationsused during the winterof 1986.TL indicatesthe south west corneridentifyingthe TL 100 km squareof the NationalGrid. The map is sub-dividedinto squares spaced 10 km apart.Woody areasare shaded;the remaininghabitatsare farmlandor suburbanareas. Trappingstationslisted in Table3 are indicatedby double circles.
Green
@Terriers Rickmansworth O Seer Green
@Lane End Flackwell @ Heath
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Loudwater ?
. .iii.
0 o Chalfont St. Peter Gold Hill Common
Bourne End
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sients, spending as little as a single day around the food patch before moving on. Having identified both resident and transient individuals utilising the same locality, we then examine (by means of dispersal, demographic and biometrical analyses) whether current hypotheses of the adaptive significance of differential migration and dispersal patterns (e.g. Ketterson and Nolan 1976, Baker 1978, Myers 1981, Gauthreaux 1982) can account for the variation in movement.
I
uous woods and villages. In England, wintering Siskins only utilise garden food sources between January and April (Burton 1986). This short period of use precluded more detailed analyses of site fidelity, but the large number of ringing stations allowed investigation of the movements of individual birds. Both the Spanish and the English wintering regions were at least 250 km from the nearest breeding areas (the Pyrenees and Scotland, respectively), although the majority of Siskins in both areas probably originated from Scandinavia (Senar 1983, Asensio 1985, Burton 1986).
Materialand methods The data presented here are based on captures of wintering Siskins at permanent ringing stations in suburban areas. We have used two data sets. The Barcelona data set was obtained from a ringing station in an area of orchards, small pine woods (Pinus halepensis) and gardens in Barcelona, Spain. In this region, Siskins are winter visitors with an irregular annual appearance (Senar 1983), although in those years when they appear the study area is utilised throughout the winter (October to April) (Senar 1989). This extended utilization of the area allowed detailed analysis of the extent to which individual birds used a plentiful food source over a prolonged period. However, the low density of other bird ringers in Spain and the difficulties of establishing other local ringing stations restricted the amount of information that could be obtained on movements. More complete information was provided by a second data set. The English data set comprised birds captured over a network of suburban gardens in southern England, in a mixed habitat of farmland, small decid64
Methods for Barcelona sample Siskins were mainly caught at one site (Sarria), although two secondary sites (at a distance of 1 and 3 km) were used in 1988-89 (see Senar 1989 for more details). Three trapping methods were used: a Yunick Platform Trap (Yunick 1971), a clap net (Davis 1981, McClure 1984), and a mist net (see Senar 1988, 1989 for an evaluation of the different trapping methods). The platform trap and the clap net were normally associated with permanently baited feeders and were operated from a hide. The food supplied was a mixture of rape Brassica napus and niger (thistle) Guizotia abyssinica seeds, and was continually provided in excess, thus mimicking an abundant localised food patch such as a tree with a good crop. Data collected in two winters are analysed here. In 1985-86 we caught 200 individual Siskins and made 200 recaptures in 318 trapping hours (over 78 days, from the second half of October to the first half of April). In ORNIS SCANDINAVICA 23:1 (1992)
Table 1. Mean distancee (km) moved by English wintering were recorded: sex, age (i.e. first year or adult), wingSiskinsbetween ringingstationsin each month of the study. length (maximum chord, to the nearest 0.5 mm), and Only movementswithin each calendarmonth are included. mass (to the nearest 0.5 g). Eight northerlymovementsof more than 250 km have been excludedfrom the April*category(see text). WinterMonths February March 12 8.2 2.4
N Mean SE
April
April*
33 147.0 45
25 10.5 1.4
59 15.1 3.4
Results Movement patterns of wintering Siskins
1988-89 we caught 2015 individuals and recorded 10452 recaptures in 433 trapping hours over 139 days. The analyses of the Barcelona data presented here refer to the (larger) 1988-89 sample unless stated otherwise.
Methods for English sample Birds were caught using mist nets associated with permanently baited feeders of peanuts. The sample analysed here was obtained in 1986, when trapping in Hertfordshire and Berkshire was carried out on 118 days (246 trapping sessions) at a total of 37 ringing stations over an area of about 900 km2 (Fig. 1). The minimum distance between stations was 1 km, the maximum 44 km. The time spent trapping at each station varied between two and 75 hours. For further details see Senar (1989). We caught 1958 siskins, and made 810 recaptures at the original site of capture and 232 at different stations. A small additional data set was obtained from a ringing station in Essex (S.E. England), where 134 birds were caught over 31 days in 1986. All birds in both samples were given numbered metal rings upon first capture, and the following variables 9
The English sample contained 232 movements between ringing stations within the same winter, with rates of movement of up to 40 km in one day (Table 1). Siskins moved farther in April than in February and March (Table 1) (one-way ANOVA: F= 9.25, d.f. = 2,101, p3 weeks in 1985-86, >8 weeks in 1988-89) was divided into those that used the area intensively (residents) and those that only used it intermittently (visitants). This was achieved by calculating an index of site use, defined as the number of days on which each resident or visitant bird was caught as a percentage of the total number of trapping days between first and last capture. The cumulative frequency distribution of this index indicated a binomial distribution (Fig. 3; Harding 1949). The point of inflection in this plot was then used to separate residents (birds ORNIS SCANDINAVICA 23:1 (1992)
o,
but two stations (Table 3). Residents and visitants could not be distinguished here as with the Barcelona data due to the smaller sample sizes. No clear bimodal distribution was apparent when the pooled data from these English stations were plotted on probability paper (cf. Fig. 3), possibly because of variations in trapping effort between sites. All birds staying more than expected at British stations were therefore defined as residents.
0
100
99 95 90 80 70 60 50 40 30 20
Variation in population composition between samples
10 5 2 1
A comparison of the two Barcelona samples and the 11 English samples from Table 3 with defined resident and \ transient birds showed that the percentage of residents using an area during a winter was quite stable, being between 6% and 14% of the total number of birds . ...2. .... caught (mean = 8.4%, SD = 2.3, N = 13). This percent0_ ....... . . . 100 90 so 70 60 50 40 30 20 100 age was not correlated with either the total number of birds ringed (Spearman r = -0.31, ns, N = 13) or the PERCENTAGE USE OF THE STUDY AREA trapping effort (number of trapping days) (Spearman r = -0.19, ns, N = 13). Fi. 3. The cumulativefrecuencvdistributionof the index of site use, plottedon probabilitypaper,for Siskinspresentin the The percentage of birds that were transients Barcelonastudyareafor at least9 weeksin 1988-89.The index SD=16.1, N=13, (mean=72.6%, range: 24.9is calculatedas the percentageof availabledayson whicha bird was caught. The point of inflection (arrowed) separates 89.2%) was also uncorrelated with either the total num"resident"birds (caughton >70% of days) from "visitants" ber of ringed birds (Spearman r = -0.30, ns) or the number of trapping days (Spearman r = -0.39, ns). (
