Warblers in mixed passerine flocks in summer S. R. D. da Prato

umbers of small birds are high in summer, but our knowledge of their Nactivities is limited once territorial behaviour ceases and the young fledge. Flocking is well known, but it is not always apparent in woodland species until leaf fall, when mixed species flocks, usually dominated by tits Parus, become obvious. Edwards (1980) recorded an interesting observ­ ation of an August flock of about 80 passerines in southern England, with nearly 50 tits but also 28 warblers (Sylviidae) of five species. A survey into flocking is currently in progress, but is aimed towards the autumn-winter period rather than high summer (Brit. Birds 73: 267). Edwards's Wiltshire observation was published as 'unusual', but flocks of mixed insectivorous passerines are well known in the tropics (e.g. Greig-Smith 1978); warblers do, however, also occur regularly in mixed-species flocks in summer in southeast Scotland. The information shown in table 1 was gathered in the course of an intensive study at Cousland, Lothian, which involved daily visits to a nesting area, checks on bird movements and a considerable amount of colour marking in adjacent woods and hedges. Thus, quantitative data on the timing and composition of the flocks can be given as well as comments on their age-structure and purpose. Data are shown for only one season (1980), but these mixed flocks are a regular feature, although easily over­ looked unless the birds are moving together along a hedge or similar narrow feature. Fortunately, the study area contains many hedges, belts of trees and other linear habitats such as disused railway lines. The 14 records quoted in the table must be a minimum; even so, flocks were seen on approximately every second day between mid July and mid August. The 20 species recorded included seven warblers and four tits; finches and other seed-eaters were relatively uncommon. Many flocks were heard before being seen, since birds constantly 'chattered' when moving through a strip of vegetation. Those at the rear of the flock would fly over the others, land and feed, pecking rapidly at insects on the vegetation. The birds then left at the rear would fly on, setting up a

[Brit. Buds 74: 513-515, December 1981] 513 514 Warblers in mixed passerine flocks constant movement in one general direction. Flocks such as these could be encountered at any time during the day, but were most frequent from one to two hours after dawn until late morning and in late evening. Several flocks located near the main study area were followed on foot or, if possible, by car for up to 2-3 km. This was relatively easy along hedges, but became much harder when the birds reached more extensive cover. They then spread out, still calling, which seemed to attract other birds already in the area, but it was not possible to keep accurate track of the flock once in wood or scrub. Since speed of movement of flocks was not recorded, it is not possible to say whether larger groups moved faster, as found by Deadman (1973) for tit flocks in pine plantations in northeast Scotland. Warblers were not recorded in mixed flocks until mid July. This is linked to fledging dates. In southeast Scotland, the majority of Willow Warblers Phylloscopus trochilus fledgei n late June, but move around in family parties for the first two or three weeks. Other species of warblers are less closely synchronised, with young fledging from late June to late July. Flocking seems to occur once family parties break up and the young become inde­ pendent. By this time, juveniles of many other species also occur in large numbers. The Willow Warbler is the commonest warbler in Scotland, which probably accounts for the frequency with which the species is recor­ ded in the mixed flocks. Sightings of such flocks are, however, biased towards birds in hedges and trees, and Willow Warblers are more arboreal than the two other common species, Whitethroats Sylvia communis and Sedge Warblers Acrocephalus schoenobaenus. The Lesser Whitethroat S. curruca was recorded on four occasions, despite its supposed rarity in Scotland. Fewer flocks containing warblers were seen after mid August. By then, Table 1. Records of mixed-species passerine flocks from Cousland, Lothian, study area in 1980 Two flocks were recorded on 18th July JULY AUGUST Species 16 17 18 18 19 21 26 29 2 3 6 11 16 21 Dunnock Prunella modularis 1 1 Sedge Warbler Acrocephalus schoenobaenus 1 2 2 1 Whitethroat Sylvia communis 4 2 2 1 Lesser Whitethroat S. curruca 3 1 1 1 Garden Warbler S. borin 1 1 2 1 Blackcap S. atricapilla 2 2 1 1 2 1 Willow Warbler Phylloscopus trochilus 6 30 10 5 9 12 19 10 11 20 11 15 10 8 ChiffchafTP. collybita 1 1 1 1 1 2 Goldcrest Regulus regulus 4 4 6 7 Robin Erithacus nubecula 2 2 Long-tailed Tit Aegithalm caudatus 25 1 Coal Tit Parus ater 3 5 3 4 3 Great Tit P. major 2 2 2 2 2 3 2 5 Blue Tit P. caeruleus 10 4 8 3 4 6 9 8 6 30 8 10 18 17 Chaffinch Fringilla coelebs 1 6 3 2 Goldfinch Carduelis carduelis 2 Greenfinch C. chloris 2 4 8 Linnet C. cannabina 4 9 Redpoll C.flammea 2 Tree Sparrow Passer montams 3

TOTALS 47 40 20 11 16 21 40 37 34 59 34 35 37 65 Warblers in mixed passerine flocks 515 most of the local breeding population and their progeny have left the study area and, by the end of the month, only stragglers of unknown origin are encountered (da Prato & da Prato 1977). Although early departure from the breeding area could account for the drop in warbler numbers, other more sedentary species also seem to become scarcer. Perrins (1979) stated that there is considerable mortality among juvenile tits in late summer; this may also occur with young warblers, especially in cold, wet summers when insect populations and warbler feeding efficiency are adversely affected. Only with Long-tailed Tits Aegithalos caudatus trapped in July has there been any evidence of family parties being involved in the flocks, since adults and juveniles were netted together from a mixed-species flock. Although it might seem obvious to attribute the break-up of family parties in other species to second brooding, this is most unlikely since the species concerned are, at least in Scotland, normally single-brooded. Perhaps moult is a factor, since the Parus tits and many of the warblers have different moult strategies between adults and young. Priority for adults by mid summer must be either to replace their plumage completely (e.g. Willow Warbler) or to leave breeding sites early on migration (e.g. Sedge Warbler). From knowledge gained by watching marked warblers of known age, these flocks are made up mainly, if not entirely, of juveniles. Juvenile warblers have a partial moult in their first summer (Spencer & Mead 1979, and own data), which, in the case of Lothian warblers, seems to take about 35-40 days. They do not migrate in this condition, but certainly wander throughout the study area, with distances of up to 4km recorded, and they probably go farther. This summer wandering may serve to acquaint young warblers with breeding sites for next year, since a number of juveniles colour-marked as wanderers in the main study site were discovered holding territory there or in surrounding areas the next summer. Adults would not need to prospect for territories, since there is good evidence from this and other studies that breeding birds are site faithful. Other explanations for flocking in birds, such as food location (Ward & Zahavi 1973), are less convincing for warblers, since their invertebrate food is fairly widely distributed. Although young warblers could wander individually, flocking probably improves predator detection without cutting feeding time, as described by Bertram (1978) for a variety of other vertebrates. References BERTRAM, B. C. R. 1978. Living in groups: predators and prey. In KREBS, J. R., & DAVIES, N. B. Behavioural Ecology. Oxford. DA PRATO, S. R. D., & DA PRATO, E.S. 1977. Feeding ecology of warblers in a Midlothian scrub valley. Edinburgh Ringing Group Report 5: 31-39. DEADMAN, A.J. 1973. A population study of the Coal Tit and the Crested Tit in a Scottish pine plantation. PhD thesis, Aberdeen University. EDWARDS, S. B. 1980. Unusual roving flock of tits and other birds. Brit. Birds 73: 265. GREFG-SMITH, P. W. 1978. Formation, structure and function of mixed species insectivorous bird flocks in African savanna woodland. Ibis 120: 284-295. PERRINS, CM. 1979. British Tits. London. SPENCER, R., & MEAD, C.J. 1979. Hints on ageing and sexing, Part 3. Ringers'Bulletin 5: 63-72. WARD, P., & ZAHAVI A. 1973. The importance of certain assemblages of birds as information centres for food finding. Ibis 115: 517-534. S. R. D. da Prato, 38 Caerlaverock Grove, Tranent, East Lothian