The Social Behaviour of the Common Buzzard Robin J

The social behaviour of the Common Buzzard Robin J. Prytherch ABSTRACT During a long-term study of the Common Buzzard Buteo buteo in Avon, special emphasis was placed on social behaviour.This is shown to be more complex than previously thought, with several new behaviours being described. Furthermore, flight behaviours are shown to be distinctive for the two components of a resident population: territorial pairs and unpaired (non- breeding), mostly one- to three-year-old birds. Buzzards defend their territories from intruding birds by using assertive postures initially. If necessary, further, more demonstrative, behaviours are directed at intruders. These behaviours help the birds to avoid fighting, but this does happen rarely. These and other behaviours serve to advertise the presence of a pair in a territory and are important in pair-bonding. For breeding pairs, activity is normally restricted to the territory, but some males engage in extra-territorial chases, which may involve a form of reciprocal altruism. Some unpaired young birds defend territories, but do not use the full repertoire of behaviours. Some aspects of pair formation and relations within the family group are described, as well as perched postures.The behaviour of juveniles is distinctive especially with regard to play, an activity that can be easily confused with adult behaviours. A few exceptions are described. Suggestions for the origins of the displays are made. crucial aspect of my long-term study seven and 28 visits per month, most visits lasting of the Common Buzzard Buteo buteo some 3–8 hours. Visits were carried out in all A in Avon has been an attempt to unravel months, although less frequently in winter. Early the intricate nature of the species’ social in the year, I check all territories for occupancy behaviour, the results of which I present here. and then check nest-sites during March and Other aspects of the study, which commenced in April. This is when most territorial behaviour is 1980 and still continues, concern population seen, although it can be observed at other times. monitoring, territory size and topography, When entering my study area, observations breeding biology, nest-sites, and individual life often start opportunistically; otherwise, I watch histories, all gathered during over 18,000 hours a single territory or go to a regular observation of fieldwork in my 75-km2 study plot, which lies point (usually at low elevation), from where I just west of Bristol. The area comprises a can monitor several territories. All observations mixture of woodland (mostly deciduous), are noted, timed and attributed to the relevant grassland (meadows for cattle, horses and pairs/territories. I aim to spend a minimum of sheep) and some arable land, and rises from sea 25 minutes in a territory, in order to gain level through wide, low-lying valleys to ridges knowledge about conspicuousness, time and plateaus up to about 180 m. There are many budgets, etc., as well as social and other villages, as well as three large conurbations behaviours. adjacent to the area. At the start of my study, I was heavily To gather information, I spent up to 1,200 dependent on the work already carried out by hours in the field each year, making between Colin Tubbs (1974) and Douglas Weir and Nick © British Birds 102 • May 2009 • 247–273 247 The social behaviour of the Common Buzzard Picozzi (1975), as well as the account given in Structure of the study population Vol. 2 of BWP (Cramp & Simmons 1980), based In my study area, all the breeding adults were largely on the previous two studies. Soon I sedentary, and defended their territories began to realise the value of following individual throughout the year. They comprised the bulk of birds; by knowing their sex and/or age I was the population: 13 pairs in 1982, rising to 97 learning so much more as they interacted with pairs in 2008, with up to 107 juveniles by late others. It was also quickly apparent that the summer, in addition to non-breeders (a group studies mentioned above were far from of unknown size, but probably fewer than 30). complete, as the social signals that the birds use Most of the juveniles dispersed out of the area by to mark their territories are both complex and early autumn, but were replaced to some extent subtle. by juveniles from elsewhere. Many of these Common Buzzards (hereafter referred to juveniles may subsequently have returned to simply as ‘Buzzards’) vary greatly, not only in settle close to their natal area (Davis & Davis their plumages, among individuals and age 1992; Walls & Kenward 1998). Some juveniles classes (see below), but to a lesser extent also in were initially sedentary (remaining in their terms of voice, ‘personality’ and shape. All of parents’ territory for much of their first year and these features helped me to attribute particular rarely much longer) but most of them, and behaviours to individual birds and therefore to almost all other non-breeders, were wanderers interpret their purpose (I have identified as or ‘floaters’ – settling for a period (from a few individuals at least 60 birds during the study days to several months) in locations free of period, by a mixture of plumage and other breeding birds, but often adjacent to an characters). In this paper, I describe first the occupied territory. Many of these birds, like various behaviours of both breeding (territorial) some breeders, became very inconspicuous in and non-breeding birds, and then describe their midsummer. function and how the behaviours relate to each Before leaving the natal territory, juveniles other. The social behaviour of territorial, can be identified quite easily: they are tolerated breeding pairs is distinct from that of other by their parents, and also call distinctively when Buzzards and is crucial in interpreting the in the territory. The call is usually a double or activities of a group of two or more interacting triple ‘ki-ki’ or ‘pi-ya – pi-ya – pi-ya’ (but very birds. Indeed, breeding and non-breeding birds variable), rather higher pitched and shriller can be described as forming two components of than the better-known adult calls (which I the population. usually note as ‘ca-au’, but these are also highly variable). The calls of juveniles are commonly thought to denote hunger, but they also act to appease their parents; once fledged, juveniles normally call only when a parent is in view, and do so even when the caller has a full crop. Later, when making their first flights beyond the territory, they often call on their return as they cross Robin Prytherch the boundary, as if 132. Part of the study area in Avon: a view from Clevedon looking east-northeast to repress any attack along the Gordano Valley (which forms the western third of the study area); from a parent (see autumn 1994. It is an area of mixed woodland (mostly deciduous), damp also below). Having meadows separated by ditches, and a small amount of arable land. Much of the flat area is within the Gordano Valley National Nature Reserve. abandoned the natal 248 British Birds 102 • May 2009 • 247–273 The social behaviour of the Common Buzzard territory, a juvenile will normally cease to call, Age classes and ‘basic’ flight behaviour unless attacked (by another Buzzard), when it Adult Buzzards are distinguishable from juvenile might squeal shrilly, until it occupies a territory and some 2Y birds in terms of plumage of its own two to three or more years later. characters (see Appendix 1). Nonetheless, Once independent, juveniles did not plumage variation is such that some juveniles appear to defend the areas in which they (especially dark ones) can look very similar to settled and so these were not obvious older birds. Furthermore, in silhouette, which is territories. In this they differed from many a typical view of a flying Buzzard, plumage 2nd-year (2Y) or older birds, which settled in features are of limited value. There are, however, a territory of their own. (Note that I use ‘bird subtle structural differences and, more years’ rather than calendar-years to importantly, postural differences that help, and distinguish age classes, since the major moult which lead neatly into social behaviour. takes place from May/June in juveniles and Almost all the territorial interactions are not from July/August in most adults, and lasts for between adjacent pairs, as might be expected, at least four months.) Third-year and older but between a pair and non-breeding (mostly birds could be referred to as adults, and were juvenile), intruding birds. There are some rarely unpaired. Intruders were chased out of exceptions, which I will describe later. A pair in these single-bird territories, but the full its territory dominates all others that enter it repertoire of behaviour (see below) was not (including its own young, some of which may used. Single-bird territories were usually held remain in the territory for variable periods after from a few months to a year or more, until a fledging). The residents signal their status to mate arrived or the bird moved on. Territorial intruders by a distinctive assertive body pairs usually comprised two adults but, rarely, language. The intruders, regardless of their age, one of the pair was a juvenile or 2Y bird. adopt passive postures. These are most often These paired youngsters exhibited most of the demonstrated in flight but equivalent signals are behaviours of older birds, but often in a less given by perched birds. Flapping flight, circle- demonstrative way (particularly juveniles). and slope-soaring, sailing and gliding all have Four of my territories have been occupied by their distinctive forms (fig.

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