The Willow warblers of Eccup Whin.
In the course of twenty years I have handled over 3000 Willow warblers on Eccup Whin. Annually between 10 and 15 pairs of birds hold territory during the Spring/Summer months. Most of the territories are to found on the edges of the woodland where the trees and grass meet. However some are found away from the edge in places where tall scrub predominates.
Arrival on the Whin is typically during the third week of April but depends on weather conditions on the continent and the South of England. Retrap and Control data suggest that at least some of the Willow warblers have arrived by taking an East - West route along the Ouse and Wharfe valleys. Birds ringed by the Wharfe, in Spring, at Otley and Harewood have been retrapped a few days later on the Whin. Birds ringed along the Humber estuary have also been controlled on the Whin. Perhaps some birds migrate North to the Netherlands and then cross the North sea. At least one Garden warbler is known to have used this path having being ringed in the Netherlands and then controlled on the Whin a month later. The same bird was subsequently retrapped over a six year period. Each year on the Whin and twice in the Netherlands.
During one year, in particular, it seemed to me that the earliest males to arrive were all long winged (wing >68mm).Analysis of historical data of males for the month of April and the first week of May showed that this was not the case; approximately equal numbers arriving across each period of five days.
Males begin singing and setting up territory upon arrival. The older males tend to return to the same area as in previous years. Later arrivals compete song wise with males already holding song posts. During this time there is much antagonistic behaviour amongst males and there are frequent pursuits around the area of the Whin. It is at this time that the males are easiest to catch. Song posts are the near tops of tall scrub, but there appears to be an optimum height for song posts. Few are higher than 5 to 6m. Antagonistic behaviour quietens once song posts are established and the males remain within their territorial areas. When the females arrive antagonistic behaviour resumes and there is much chasing - usually within the males territorial boundary. Females apparently select males on the basis of the territory they hold rather than their singing ability (Lapshin 1978). On the Whin more males arrive than there is territory for. Unpaired males appear to leave the site 14 - 20 days after arrival.
The number of pairs holding territory varies from year to year. In years with mild clement weather more territories exist than in poorer Spring years. The range of occupied territories is from seven (7) to fifteen (15) with an average of ten (10). This gives an average territory size of 0.45/h to 1.0/h. Ten territories give a density of 0.7/h. Nests are built on the ground and usually well concealed. On the Whin grass tussocks, under low bramble or gorse are the favoured site. The nest is domed and is mostly dry grass but also contains twigs, leaves, small roots and sometimes hair. Clutch size 5 or 6 and an incubation period of 12 to 14 days.
An indication of the progress of the breeding cycle
can be gained by examining the 'brood patch' scores of females caught.
The scores are:-
The first young are fledged around YearDay 160 (June 9) but peak trapping numbers do not occur until YearDay 205 (July 23) [Easy to remember, its our wedding anniversary!]. Post-juvenile moult begins about 21 days after fledging. (Norman 1981 - 23 days). Most juveniles have completed their post-juvenile moult by YearDay 220 (Aug7) although some late emerging birds can still be found in moult up to early September.
To be completed
Adult (age 4) Willow warblers, on the Whin, begin their moult about YearDay 170 (June 28) for males and females about 14 days later. Comparing these dates with those for the fledging of juveniles (3J) suggests that males begin their full moult within a few days of the young fledging. Females delay their moult for a further 14 days. This coincides with the onset of Post-juvenile moult. In both cases moult lasts for about 40 days and is complete by mid August.
Whilst the arrival of Willow warblers on the Whin is relatively straightforward to assess it is more difficult to determine departure dates since migration results in a series of new arrivals as well as departures. The new arrivals are 'passage' birds which use the site as a 'stopover' to refuel on their journey South. Juvenile birds begin dispersing as soon as they have completed post-juvenile moult. Adult birds (aged 4F and 4M) do not migrate until they have completed a full body moult. The departure of males and females mirrors their moult strategies with the male's peak departure being about 12 days earlier than for females. There is a steady decline in the number of adult males trapped after YearDay 220 (Aug 8) and also for the number of adult females after YearDay 232 (Aug 20). The latest date for an adult Willow warbler on the Whin was 15/09/1990. Not surprisingly this was a female.