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Races of Phylloscopus trochilus

The moult strategy of two complete moults each year is unique amongst British Passerines.

This moult strategy may be related to the long distance migratory habit of the Willow warbler, essentially the bird has a new set of feathers before it sets off on each long distance migration.

nominate  trochilus (England)

Adult postN

Ginn & Melville(1983) give start for P1 between early June and late July lasting for 30-40days. No separate data are given for M & F, although they state that females begin a little later than males in Cleveland (Norman?).

Male start extends over 14 days and female21 days. Moult finishing with P10.

My own personal observations at Eccup Whin(1983-1993) suggest Moult in males, beginning around mid-June (earliest June22) and extending over 12 days. Females begin, on average 12 1/2 days later in early July ( earliest July 2) -probably post-fledging of young - and extending over 21 days. Moult lasting for about 40 days and finishing with S6.

Apparently some birds suspend/arrest this main moult (Mead; Baillie; Bensch) although I have never personally observed this in over 3000 birds handled.

P.t.acredula

Williamson (1967) gives this as 1 month later than nominate trochilus (3rd week of July), some males starting during incubation and others not until the fledging of the young. Females probably begin 10-15 days later than males. Some bird begin moult as early as the 2nd week in June but may be non-breeders.

Post juvenile moult

Involves the replacement of Marginal & Median coverts, Greater coverts and in some birds Tertials and Remiges.

Norman (nominate trochilus) suggests all Marginal coverts & some Median coverts in most birds, a few birds moulting 1 or 2 inner greater coverts.

Russian studies ( acredula) suggest only patial renewal of Marginal and Median coverts with no Greater coverts replaced.

In England (nominate trochilus) post-juvenile moult starts about 23 days after hatching (Norman 1981) In Russia (acredula) post-juvenile moult starts 30-33 days after hatching. Duration is about 30 days.

Pre-breeding Moult

Complete moult for both adult and 1st winter birds(*NB). Takes place in the winter quarters mid-December - early April - (Williamson - late November). Duration is estimated at 65 - 80 days for Ugandan birds and 60-70 days for Kenyan birds.

*NB Ginn & Melville found birds in which only body and wing coverts have been moulted. My own personal observations include 7 birds in which the remiges had not been replaced (1987-93).

*NB Ginn & Melville found birds in which only body and wing coverts have been moulted. My own personal observations include 7 birds in which the remiges had not been replaced (1987-93).