Wintering Blackcaps in Worcestershire 1998/9 - A Simple Survey

This survey arose from my interest in Blackcaps (Sylvia atricapilla) and what seemed to be an increasing number present during recent winters and an example of micro-evolutionary change amongst some individuals of the north-western population, the majority of whom continue to migrate to the traditional wintering grounds in the Mediterranean.

The survey followed the same methodology employed by Iain Leach in his national survey for the British Trust for Ornithology (BTO) during the winter of 1978/9. Worcestershire's contribution to a national total of 1,714 birds found during that survey was 39. I was interested in finding out what had happened since.

Radio and press appeals were made for sightings of Blackcaps between the 1st October 1998 and 31st March 1999, and asking that people ring Worcestershire Wildlife Trust to report their sightings.

Information requested included place, sex, number, food items, aggressive episodes, and whether the callers participated in the BTO Garden Birdwatch Survey, and were members of the Trust.

To date I have received 249 survey forms, detailing the sightings of 245 male and 115 female Blackcaps.

While there are issues of duplicated sightings particularly in centres such as Malvern and Worcester, the winter population has clearly increased since 1978/9, to a figure in the low hundreds at least. Interestingly BTO data suggests the winter population is declining nationally.

Aggression between species largely involved Robins (Erithacus rubecula) and Great Tit (Parus major) and may point at similar feeding niches. Aggression between Blackcaps was almost exclusively male to female.

Food and feeding behaviour proved one of the surveys most interesting aspects, both in the range of foods taken and the variety of presentations they are willing to tackle, making them as adaptable and adroit as any of our usual garden feeding species.

To my mind it is these abilities, given milder winters, that is the most influential factor in their wintering success.

So how well have the objectives of the survey been met ?

We know that we have wintering Blackcaps of both sexes, distributed throughout Worcestershire. We know what they eat, and their ability to survive cold spells. We know that some show a site fidelity, both through the winter, and from year to year. The preference for built up areas, rich in gardens and bird feeding stations is clear. Competition is evident, with Robins and Great Tits prominent rivals. Blackcaps have learnt to exploit a variety of foods, and most significantly in a variety of presentations.

As for the size of the winter population educated guesses are required due to the fact that the survey did not ask for nil returns, i.e. absence of Blackcaps, (although it is interesting that I did not receive any records from the Trust reserves) so that densities cannot be assessed, but I think we can make some broad assumptions.

The population of wintering Blackcaps has clearly increased since 1978/79, from a figure of 39 individuals, we now have a population at least in the low hundreds if we assume that the population density in more rural areas is similar, and that many Blackcaps have not been found.

The populations in places like Malvern, Worcester, Redditch are more problematic as the same birds are surely being seen by many observers, and a garden ringing and retrapping programme would be useful in providing a more conclusive estimate.

The use of Mahonia japonica has proved particularly interesting, and while feeding on nectar in known in Mediterranean wintering populations, it is less well known elsewhere. The same observations have also been made in this survey on Blue Tits (Parus caeruleus) and is probably worth a little research in its own right.

Many observers saw both male and female Blackcaps, and 43 respondents had both sexes present together, and where aggression between Blackcaps was noted, it was predominantly male to female.

One respondent, Mrs F.M.Baldock, was able to keep records of two males with distinctive features (injured leg and missing tail feathers), one of which continued to visit her garden for the months of December, January and February.

Mark and Christine Turner were able to provide extensive records of Blackcaps in their garden back to 1996.

During March I began to receive records of, and heard for myself, male Blackcaps in full song, and in one instance display flight - what is the origin of these birds? Are they very early summer migrants or overwintering British birds ? It is hard to believe they are continental wintering individuals - surely they would not waste precious resource and time (and advantage) in delaying their return migration ?

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