Warblers in mixed passerine
flocks in summer
S. R. D. da Prato
N
umbers of small birds are high in summer, but our knowledge of their
activities 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 observation 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.
T h e 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 overlooked 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. T h e 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.
M a n y 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. T h e 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 offlockswas 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 trochilusfledgein 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 independent. 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 recorded 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 passerineflocksfrom Cousland, Lothian, study area in 1980
Twoflockswere recorded on 18th July
JULY
Species
Dunnock Prunella modularis
Sedge Warbler Acrocephalus schoenobaenus
Whitethroat Sylvia communis
Lesser Whitethroat S. curruca
Garden Warbler S. borin
Blackcap S. atricapilla
Willow Warbler Phylloscopus trochilus
ChiffchafTP. collybita
Goldcrest Regulus regulus
Robin Erithacus nubecula
Long-tailed Tit Aegithalm caudatus
Coal Tit Parus ater
Great Tit P. major
Blue Tit P. caeruleus
Chaffinch Fringilla coelebs
Goldfinch Carduelis carduelis
Greenfinch C. chloris
Linnet C. cannabina
Redpoll C.flammea
Tree Sparrow Passer montams
TOTALS
AUGUST
16 17 18 18 19 21 26 29
1
2
3
1
1
2
2
4
3
1
6 11 16 21
1
1
6 30 10
1
25
1
10
2
4
2
2
8
5
2
3
1
2
1
1
1
2
1
2 2
1 2
9 12 19 10 11 20 11 15 10
1 1 1
1 2
4 6
4
2
2
4
6
9
3
2
8
6
2
2
5
3
6 30
4
4
1
1
1
8
7
2
4 3
2
5
8 10 18 17
3 2
3
8
9
2
3
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 ofwarblers 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, C M . 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