Birds of prey exhibit a wide range of dispersion patterns, determined in

Transcription

1 Birds of prey exhibit a wide range of dispersion patterns, determined in part by their response to varying prey availability (Newton 1979). The Kestrel Falco tinnunculus has been recorded nesting colonially (Fennel 1954), or solitarily, with pairs defending a restricted area around their nest sites (Tinbergen 1940, Cave 1968); occasionally pairs may defend an exclusive home range during the breeding season, possibly occupying the same range in winter (Shrubb 1970). This paper reports on the year-round territorial behaviour of a small sedentary population of unmarked Kestrels studied in eastern England. Study area Observations were made in a 40-km 2 study site on the southern edge of the continued [Bni. Birds 76: May 1983]

2 Territorial behaviour oj Kestrels 207 East Anglian fens, a flat, open and intensively cultivated area where cereal production predominated, followed by sugar beet and other root crops. Although eight habitat categories were recognised for analysis of foraging behaviour, only the grassy areas need concern us here. These included lode and river banks, ditches and drains, road verges and other grass. Lodes (raised canals carrying run-off water from the chalk hills in the east), together with the ditches and drains which delimit field boundaries, crisscrossed the study area. Other grass was a catch-all category, and included shelterbelts, small copses, meadows, and part of looha of Wicken Fen Nature Reserve in the northwestern corner of the study area. Methods The study was carried out between November 1977 and October On most days until October 1980, a distance of 6km through the middle of two territories was covered either on foot or by bicycle. Observations taking in some part of the remainder of the study area were made on most weekends (no observations were made during mid February to mid May 1980). Between November 1980 and October 1981, more intensive study was possible. Plotting oj territories During the study period, all flight paths, display flights and conflicts between Kestrels were plotted on 1:25,000 Ordnance Survey maps on a 12-monthly basis. During periods of intensive observation of individuals, flight paths were also plotted on l:10,000-scale maps, with particular attention paid to perch sites. Although none of the Kestrels was marked or wing-tagged, which imposes serious limitations on studies of territoriality, by the end of the 1978 breeding season the hunting ranges of each pair had been determined from: (i) Flight paths of different individuals of the same sex seen at a time interval which precluded the possibility of their being of the same individual, (ii) Intensive study of an individual, which, particularly when carried out over a number of days, allowed a pattern of perch usage to emerge; perch changes and other flight paths were used to determine the extent of the range. (This method was also useful in elucidating overlap in hunting areas between adjacent pairs, or when perches were close to territory boundaries.) (iii) The 'turning-back' points of extended flight paths, (iv) Flight paths to and from nesting areas, (v) Display flights and points of conflict between neighbours. Observations in subsequent years reinforced the contention that this was a reliable method of territory delimitation when working with a small population of unmarked birds. Determination oj territory size Territory size was determined by the maximum polygon method: the area enclosed by a line drawn around the outer flight paths of each pair. To investigate annual changes in territory size, the maximum polygon area was worked out for each territory between November and the follow-

3 208 Territorial behaviour oj Kestrels 78. Female Kestrel Falco tinnunculus, Netherlands, March 1975 (Hans Sckouten) ing October. In order to ensure that only those territories which showed no sample size bias were used, the increase in maximum area with the increase in number of flight paths was plotted against number of flight paths observed. It was found that, for those territories for which more than 20 flight paths had been recorded, increase in territory size tailed off at n= 15. Consequently, only territories with 15 or more flight paths were used, Village (1982), using spot observations of wing-tagged Kestrels, found that increase in range size tailed offat n = 20. Since any flight path is equivalent. to at least two spot observations, the maximum area enclosing 15 or more flight paths probably gives an accurate picture of territory size. Extent oj grassy areas in territories To determine the extent of grassy areas within each territory, individual territory size was determined as above for all flight paths recorded during the whole four-year study period. The lengths of ditches, drains, lodes and road verges within the maximum area of each territory over four years were determined from a 1:25,000 Ordnance Survey map. A check was carried out over 18 km to note any differences in the conditions during the study period compared with those at time of map publication. As these were less than 5%, no corrections were made. Widths of these habitats were measured in the field, and the means used to calculate the area of each habitat within the territory boundaries. Results Maintenance oj territories Territories were maintained throughout the year, and defended by both sexes. Territorial conflicts between two Kestrels were seen in all months

4 Territorial behaviour oj Kestrels 209 except December and January, most commonly in the form of mutual soaring along a common boundary (17 of 25 conflicts observed); individuals were seen to fly up to 1.5km to display along their boundary in response to a neighbour already displaying. Some conflicts also involved rapid chasing (seven out of 25), usually when one bird was initially perched. Physical contact was observed twice, the raptors locking talons and tumbling through the air, with much excited calling. Kestrels may continue to display after separating. More than one of the above responses may be seen during a single conflict. Territorial display flights, in the apparent absence of a displaying neighbour, were observed in all months of the year. As with mutual soaring, these were generally at heights above 80m, with the Kestrel frequently displaying along part of its boundary. The soaring may be interspersed with shallow glides, and may end in a rapid, steep stoop to a prominent perch. In 62 of 124 observed display flights where the sex of the participant was identified, males were recorded in 61% of the cases and females in 39%. In actual conflicts, either or both sexes may be involved. Stability oj territories Territorial display flights and conflicts for each year, from November to the next October, are illustrated in figs. 1-4 (note that maximum area boundaries over the four years are used). Three points emerge: (i) Most conflicts occurred along territory boundaries, (ii) Conflicts at the same position were observed at intervals of more than one year, even when a newcomer had replaced the previous occupant which had disappeared 79. Male Kestrel Falco tinnunculus, Lancashire, November 1976 (Dennis Green)

5 210 Territorial behaviour oj Kestrels Figs Territorial display flights and conflicts of Kestrels Falco tinnunculus in East Anglia for each year November-October 1977/78, 1978/79, 1979/80 and 1980/81. Territory boundaries enclose the maximum area over the four-year period, except territory C (November October 1980). Solid lines = display flights of territory owners; broken lines = flights of birds of unknown origin; * = territorial conflict; N = known nest site; (N) = approx. position of used nest; t ) = nest site inspected by Kestrels but not used. Arrow indicates north

6 Territorial behaviour oj Kestrels 211 (e.g. conflicts between males defending territories A and C in 1978/79, and in the same position in 1980/81 involving a different male in territory- C). (iii) Most display flights followed, at least in part, the territory boundaries. Thus, territory boundaries appeared relatively stable from year to year. To check this, the sizes of those territories for which more than 15 flight paths had been plotted for any one year were recorded (table 1). This shows considerable uniformity in individual territory size, even over lour years. An exception is territory G in 1980/81, where habitat disturbance, and a shift in the nest site by 1 km, occurred. Generally, individual territory size did not appear to vary greatly from one year to another, even if one member of a pair was replaced by another (e.g. in territory G in 1978/79 and 1979/80, where the original female disappeared in late August 1979). Table 1. Territory size (maximum polygon area) in hectares of individual territories of Kestrels Falco tirmunculus in East Anglia for each year November-October 1977/ /81 where number of flight paths exceeded 15 Territory 1977/78 YEAR 1978/ / /81 A B C D Seasonal variation in territory size Seasonal variation in territory size has been noted previously for the Kestrel (Village 1982) and also for other raptors (Newton 1979). In the present study, only two territories permitted such analysis, and only in one year. Territories B and Dboth had more than 15 flight paths recorded in 1980/81 for winter (mid October to mid April) as well as for summer (mid April to mid October), and territory size was determined for each season (table 2). There does not appear to be any marked seasonal variation. Table 2. Seasonal variation in size (in ha) of two territories of Kestrels Falco linnunculus in East Anglia in 1980/81 where number of flight paths exceeded 15 in both winter (mid October-mid April) and summer (mid April-mid October) TERRITORY B D Winter Summer Extent oj grassy areas in territories The study Kestrels spent 81% of their foraging time hunting over grassy areas (Pettifor in prep.), and the extents of these habitats enclosed by the maximum area of each territory over four years are compared (table 3). Although the area of any particular grassy habitat varied considerably between territories, the coefficient of variation in total grassy area between territories (- - = 0.151) is less than the variation in size between territories

7 212 Territorial behaviour of Kestrels Table 3. Maximum territory size (polygon area) over four-year period 1977/ / 81 for each territory of Kestrels Falco tinnunculus in East Anglia and extent of grassy habitats (in ha) within each territory \ B TERRITORY C I) E Territory size (ha) GRASSY HABITAT (ha) Lode/river Ditch/drain Road verge Other grass / / ti3 ( ( ( ( TOTAL GRASS %ORASSY HABITAT IN TERRITORY ( /5 ( ( ( x ~ 0.345): i.e., the combined grassy areas are comparatively similar lor all territories, but there is greater variation in territory size. Discussion The Kestrels studied by Tinbergen (1940), Cave (1968) and Village (1982) defended only a restricted area around the nest site during the breeding season. Clave (1968) considered his Dutch Kestrels to be non-territorial during winter, except when voles (Microtinae) were scarce; Village (1982), however, noted that the shared summer hunting ranges of his Scottish Kestrels disappeared in autumn and winter as individuals took up exclusive hunting ranges. In East Anglia, Kestrels maintained and defended a territory throughout the year, with both members of a pair participating in its defence and the range being used lor both hunting and nesting. Seasonal variation in territory size did not appear to be marked, either, although this conclusion is based on a small sample, while the seasonal divisions may be too broad to pick up monthly or bimonthly variations. This contention is, however, supported by the observations that both the area occupied by individual pairs, and the territory boundaries, were stable from year to year. In long-lived species, newcomers replacing a previous owner probably fit into an already stable system; mutual soaring along boundaries reinforces the existing framework, while display flights and conspicuous perching serve to advertise the continued occupancy of a territory. The stability of (and lack of seasonal variation in) territory size over the study period, and the lesser variation in grassy habitat area than in total territory sizes, suggest that these territories represent viable hunting ranges for a pair of Kestrels all year round. Thus, disputes and display flights occurred throughout the year, and most disputes were along the boundaries of territories. Further, these territories were in excess of the size required for the defence of any other resources, such as nest sites. Secondary benefits may occur, however, such as the maintenance of the pair-bond throughout the year, thereby possibly facilitating earlier breeding (courtship-feeding was observed in January, and copulation in early February).

8 Territorial behaviour oj Kestrels 213 Davies (1978) has suggested that, when territories are linked to foraging, territorial behaviour is in part related to food availability; and Newton (1979) has argued the same for various raptor species. No index of preydensity is available for the study areas, but agricultural operations probably maintain rodent populations at a relatively low level and dampen any oscillations in numbers. Consequently, the advantages of either a nomadicstrategy as adopted by other open-country vole-hunters, such as skuas (Stercorariidae), some owls (Strigiformes) and kites Elanus and Milvus (Pitelka et at. 1955; Lockie 1955; VV. R. Tarboton verbally; Newton 1979). or the shared hunting ranges of the Kestrel when voles are abundant (Cave 1968), are not feasible options when vole densities are low over wide areas. 80. Female Kestrel Falco tinnunculus with nestlings, Dyfed, June 1974 (Graham F. Date) Since the Kestrels in my study spent much of their foraging time hunting from a perch, particularly in winter, it would be beneficial to them to be familiar with the better hunting sites, especially if their prey were sparse or patchy. Hence, an exclusive range for hunting would be to their advantage. Southern (1970) suggested a similar reason for the territorial behaviour of Tawny Owls Strix aluco. I noted that the Kestrels appeared to be intimately familiar with their territories, and confirmed this when I made observations on perch usage by an individual Kestrel (recognisable by plumage characteristics) along a 200-m lode bank for a four-month period: it used 13 of the 34 trees available to it, and spent over 50% of its hunting time perched on just two of these 13. Still-hunting is energetically less expensive than hovering, and any Kestrel able to obtain sufficient prey by this method is at an advantage over one procuring prey mainly by hovering.

9 214 Territorial behaviour oj Kestrels I consider, therefore, that an exclusive feeding territory provides the most viable spacing system for Kestrel pairs in areas where the nature of the habitat results in locally patchy hunting areas. Further, where prey availability is influenced by the patchiness of a habitat, intimate knowledge of such sites and of hunting perches is beneficial to a Kestrel, since these sites are often constant over time. In order to obtain this familiarity with foraging sites, and to prevent interference or competition, an exclusive range is necessary. Acknowledgments Warwick Tarbolon"discussed this project with me in its earlier stages, while Dr N. B. Davies and Dr I. Newton made many helpful comments and suggested improvements on an earlier draft. Summary The territorial behaviour of a small sedentary population of Kestrels Falco tinnunculus (five pairs) was studied over a four-year period, from November 1977 to October 1981, in the intensively cultivated East Anglian fens. Pairs defended territories throughout the year. These territories were stable over the study period, even when replacements occurred. Seasonal variation in territory size was not thought to be marked. Territory size varied between 288ha and 750ha, while total grassy area (important hunting sites) in each territory did not differ so markedly (63-95ha). The territorial behaviour was thought to be linked to the nature of the habitat influencing foraging opportunities. References CAVE, A. J The breeding of the Kestrel. Falco tinnunculus L., in the reclaimed area, Oostelijk Flevoland. Neth.j. Zool. 18: DAVIES, N. B Ecological questions about territorial behaviour. In KREBS, J. R., & DAVIES. N. B, (eds.) Behavioural Ecology an Evolutionary Approach. London. FENNEL, C. M Notes on the nesting of the Kestrel in Japan. Condor 56: LOCKIE, J The breeding habits and food of Short-eared Owls after a vole plague. Bird Study 2: NEWTON, I Population Ecology oj Raptors. Berkhamsted. PETTIFOR, R Bird Study, in press. PITELKA, F. A., TOMICH, P. G., & TREICHEL, G. VV Ecological relations of jaegers and owls near Barrow, Alaska. Ecot. Monogr. 25: SHRUBB, M The present status of the Kestrel in Sussex. BirdStudy 17: SOUTHERN, H.N The natural control of a population of Tawnv Owls (Slrix aluco).j. Zool. 162: TINBEROEN, L Beobachtungen liber die Arbeitsteilung des Turmfalken (Falco tinnunculus) wiihrend die Fortpflanzungszeit. Ardea 29: VILLAGE, A The home range and density of Kestrels in relation to vole abundance. J. Amm.Ecol. 51: Richard A. Pettijor, Slade Farm, Swajjham Prior Fen, Swajjham Prior, Cambridge CB50LQ