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Races of Phylloscopus trochilus
Phylloscopus trochilus (Willow warbler)
Movements
All Populations are migratory.
Winter sub-Sahara (South Senegal to Ethiopia in the East and to South Africa in the South)
Very common W.Africa, S.Cameroon; South of 1°N in Kenya.
Widespread Zaire and much of S. Africa
Sporadic winter records in Mediterranean area and even Britain.
Nominate trochilus winters mainly in the West.
P.t. acredula winters in S. Africa but also parts of Zaire and Cameroon. Rwanda and Burundi probably no trochilus.
P.t. yakutensis winters in S. Africa only 2 -5% in E. Africa.
Birds from West of range as far as East Germany and S. Scandanavia (P.t.trochilus) move South to S.West passing through S.W. France.
Birds from N. Scandanavia and Finland (P.t. acredula) move South to South East.
East European populations of P.t.trochilus wintering grounds are not known, may move SE.
Spring movements are on a broader front and probably spread further eastwards. P.t.acredula occurs on East coast of Britain in both Spring & Autumn.
P.t.trochilus
Autumn movements in Britain are short stage - little fat deposition - head South to SSE.
On South coast 'Northern' birds are West of 1°W; 'Southern' birds are further East.
In France landfall well inland; birds change to more SW direction at c48°N. Initial recoveries are from NE Spain, NW Portugal very few in Southern Iberia.
Weight data suggests initial strategy of one night's fuel then dispersal to feed followed by same strategy.
Fuelling for longer staged flights probably occurs in Iberia. In N.Portugal have sufficient body fat to overfly the Sahara( Mead 1966).
Scandanavian birds (acredula & trochilus) probably separate at 60 -63°N. Falsterbo mostly P.t.trochilus ; Ottenby P.t.trochilus & p.t.acredula; Norwegian birds mostly pass through West of Mediterranean; Finnish birds solely in the East Mediterranean - Finnish birds migrate faster.
Some birds have intermittent migration strategy having 'stopover sites' in the 'desert'. Arrive sunrise to noon -inactive -resume at dusk. Lean birds may stop to feed for up to 21 days.
In Britain Southern movement begins late July (juvenile birds disperse locally until then.) Peak movements 11 -20 August, most departed by end of month. Similar timing is shown for adults but arrive 12 days later than juveniles in Iberia. Passage birds moving through Britain mostly complete by late September.
Ottenby passage later July to late September median 27 August. Falsterbo median 19 August.
No evidence that acredula juveniles begin migrating earlier than trochilus.
Studies in NW USSR indicate that juveniles stay within 2km of birthplace for an average of 24 days
Adults probably remain within nesting territory for about 28 days (m) and 30 days (f) - moult period?
Present in winter quarters from mid-September - Ghana; October - Cameroon & Gabon; Sierra Leone not until early November. Mid-october for S. Africa.
Spring
Begins late February to March chiefly March /April most have left by early April although stragglers may remain to early May. Some birds remain in the Niger inudation zone all summer.
Progress through Europe follows the 9°C isotherm, P.t. trochilus moves more slowly than P.t. acredula.
British Birds 73: - 357-358
Late arrival of Willow Warblers in recent years Attention has been drawn to the apparent late arrival of some summer migrants in recent years. Mason (1977) analysed first arrival dates of common summer migrants in Leicestershire during 1942-74 and concluded that, on average, most had been arriving a few days later in 1969-74 than in 1942-68. He suggested that this was correlated with the cooler spring weather of recent years and pointed out that this reversed the general trend of earlier arrivals during the last 30 years compared with previous years' records (Hudson 1973). The collection and analysis of first arrival dates is worthy of attention on a wider scale. These alone, however, do not necessarily reflect trends in the timing of the main migratory movements, and bird-observatory data are probably of little help in this respect. Data from two studies of breeding Willow Warblers Phylloscopus trochilus, however, enable a more detailed analysis for that species. The first was carried out by May (1947, 1949) during 1944-46 at Englefield Green, Surrey; and my own, on colour-ringed individuals, started in 1976 at Witley Common, Surrey. In both studies, records were kept of the arrival dates of warblers comprising the breeding populations. May's (1949) graph of the arrivals of males in 1946 is reconstructed alongside my own for 1977-79 (fig. 1).
The differences are striking: although at the beginning and end of the arrival periods the timings differ by about one week, the dates by which 50% of the populations had arrived differ by nearly a fortnight. Not only would the main arrival appear to be later in recent years, therefore, but also compressed into a shorter period of time. The inconspicuous females tend to arrive some two weeks later than males and are difficult to keep track of; it would seem, however, that differences in their arrival times between May's and my own warblers are similar to those of the males. Fig. 1. Spring arrivals of male Willow Warbler Phylloscopus trochilus in Surrey. Dotted Line: based on 31 individuals at Englefield green in 1946 (after May 1949); solid line: base on 63 records of colour ringed individuals at Witley Common during 1977-79 In view of the late arrivals in recent years, it is interesting to compare the timings of the breeding seasons using nest record data from the same two studies. First-egg dates calculated from May's (1947) data average around 30th April (a date at which my first females are usually only just beginning to arrive), whereas those from my own studies average around 10th May (using first brood data only). There is no indication that the warblers are tending to remain later on the breeding grounds than formerly in order to compensate for the later arrival, but there is a strong suggestion that the frequency of second broods has been considerably reduced since May's studies. The recent later spring arrival, therefore, would seem to result in a shortening of the breeding season. The subject clearly requires investigation on a wider scale, and data on more northerly populations would be of particular interest. M. R. LAWN 20 Croft Road, Godalming, Surrey GU7 IBY REFERENCES HUDSON, R. 1973. Early and Late Dateslor Summer Migrants. BTO Guide No. 15. Tring. MASON, C. F. 1977. Recent later arrivals of summer migrants in Leicestershire. Brit. Birds 70: 342-343. MAY, D.J. 1947. Observations on the territory and breeding behaviour of the Willow Warbler. Brit. Birds 40: 2-Il. -- 1949. Studies on a community of Willow Warblers. Ihis 91: 24-54. We agree with Mr Lawn that the subject requires more investigation; further detailed observations.
most spring recoveries of British ringed birds are in Morocco/S.Spain; NE Spain/central France. Males move ahead of females, probably begin migrating earlier. Earliest records in Britain are from end of March (S.Britain). Early May (Scotland), arriving mainly by South and SW coasts.
Birds used to arrive Otley sewage works before any were found on Eccup Whin (pers obs KAC). Otley sewage works is North of Eccup Whin but lies along the R. Wharfe suggesting a migratory route from the East coast along the river valley.
Central European breeding grounds occupied at end of March and in April. Main arrival in Sweden 21 April to 15 May. N. Sweden 11 - 25 May.