"Rassing"

Since 1st March my time has been almost exclusively occupied by “rassing”. This means participating in a programme of ornithological research with the acronym “R.A.S.”, namely Retrapping Adults for Survival. The scheme is directed by the British Trust for Ornithology (BTO) and a full description of the scheme is found here.

It is to do with measuring survival of a nominated species, of which mine is the Marsh Tit Parus palustris. In this context, survival means the proportion of a population which survives from one year to the next. In a stable population, the number of new births in a year is balanced by the number of deaths. If the survival rate should decline, a population might spiral into extinction; on the other hand, if it increases we could end up with the eponymous proverbial “knee deep in Marsh Tits”. My study area is the Roudsea Woods and Mosses National Reserve which I have previously described here .

The Marsh Tit - recognised by its glossy black head and small neat black bib

The fieldwork is conducted during the “RAS Season”, nominally the breeding season which for Marsh Tits is designated as March to August. However, it is during March that the birds are most readily captured as they still come to feeders, and that explains the intensity of field observation during that month.

Survival rates usually differ at different stages of the life cycle. This is called age-specific survival rate and it is known that young birds have lower survival rates than adults. So “RAS” specifically investigates the survival rates of adult birds. Once we can measure survival, we are in a position to examine factors which affect it, for example habitat management practices.

In order to determine the number of birds which survive from one year to the next, individuals have to be caught and ringed with uniquely numbered metal rings issued under licence by the BTO. However, it may be difficult to catch every bird in the population every year, and so the task is facilitated by fitting each bird with a unique combination of coloured rings (up to three, in addition to the metal ring). The colour combinations can be seen in the field with binoculars, especially as they come to feeding stations placed around the habitat. This enables individual birds to be recognised without the need to retrap them.

A ringed Marsh Tit. Each individual has its own unique colour combination

The research is ongoing, and should persist for at least five years. This enables the accumulation of a sufficient sample size to minimise the margin of error in the survival estimate. The objective is to ring, in any one year, enough birds to generate at least 25 retraps (or re-sightings) the following year. So far this season we have accounted for 60 individuals of which 28 are retraps from previous years, so we are well on target.

This year, 2007, is the fifth year of my study period, during which time I have caught and ringed (to date) 155 adult Marsh Tits. Preliminary analysis of the results from the first four years indicates that adult Marsh Tits have a comparatively high annual survival rate of about 70%, compared to about 60% for other tit species.

These results are fully analysed by advanced statistical computer packages operated by the BTO and the outcome may give an insight into the natural history and conservation of this attractive little bird.