Pilot Survey of Migrant Dates

Background

This project stems from the Devon Birds article last April which presented analyses of the spring arrival dates of our breeding summer migrants over the past 80 years.. The results showed not only the expected great variations from year-to-year but they also suggested that some species have been arriving earlier from Africa in recent decades whereas others have not changed their migration timing. Similar apparent trends in other counties have widely been claimed as responses to changing climates. Or, they might be artefacts of our traditional recording methods which leave much to be desired as they were not designed to gather information for such ‘modern’ uses. It would be useful to know what, if anything, is happening!

Recent interpretations have relied upon dates when just the first bird of a species was seen each year in the county and do not distinguish between local breeders and passage migrants. Our personal experiences are that most Devon summer visitors often do not return to local nesting areas until an appreciable time after the first (Swallow, Cuckoo or whatever) has been reported, usually on the coast. Many firsts may, in fact, be birds heading further north. A better way to understand any true migration changes would be to obtain many more records for birds returning to their actual nesting places across Devon each spring. These would allow us to see patterns of arrival, and how they vary between years, and to measure more confidently any trends based on average dates of (say) the first 10 sightings each spring rather than those of just one bird of each species.

Aims

The current SMADD project was hatched early last summer, soon after the DB article appeared, to try and assess how many members would like to get involved in such a new approach to gather more information. Its primary aim was to concentrate upon spring returns. [Coastal passage migrants, of unknown origins/destinations, are already well recorded by DBWPS.] By then, of course, the 2009 migrants had allarrived, so the trial aimed instead at requesting dates when birds departed from known nesting sites in 2009. Such knowledge is also important, for conservation purposes, as it can tell us the length of breeding seasons in Devon (poorly known at present) and whether some might change in response to environmental changes in the Devon countryside. We also cast our net wider by adding requests for late autumn passage records at the coast and for the first autumn arrivals of some familiar winter visitors. From the numbers of respondents we hoped to gauge the level of interest in migration recording by the general membership; rather than by just the most active coastal birders. Records for just one common species (Swift, Willow Warbler, say) nesting near an observer’s home would be just as valuable as those for scarcer species nesting in a few special habitats  (Pied Flycatcher, Wood Warbler).

Special recording forms and instructions were distributed with the Harrier newsletter, and members were requested to return them to us, either on-line or by post, before the end of December last.

Results

The overall response was disappointing with just 17 contributors. Nevertheless, some interesting observations were sent in, including the arrival dates for Swallows at one place going back to 1990 (see below). This shows the potential of such an inquiry if more members become involved. A summary of observations, with a list of contributors, is attached.

(i) Departure dates from nest sites:
As expected, these were easier to record for conspicuous aerial feeders nesting close to human abodes than for unobtrusive species inhabiting woods and other dense cover. The latter fall silent once breeding has finished and then slip away unnoticed. The spread of dates for Swift, Swallow, House Martin and Spotted Flycatcher presumably reflects individual nesting success and some pairs rearing second or late broods.

(ii) Coastal passage last dates in autumn:
Many more species could readily be found along the coast during autumn, again as expected, although many are unlikely to have been from Devon breeding populations. Many more such records will be submitted by other members to the county recorder. Nonetheless, even our very small autumn samples can usefully be compared with breeding site departure dates. For three of the above four species there are wide intervals between their two sets of dates. For these reasons, autumn coastal observations are unlikely to be reliable indicators of any changes in the length of breeding seasons (as had been mooted in some conservation newsletters).

(iii) Autumn arrival dates of winter visitors:
The reporting rate was quite high for the two winter thrushes from Scandinavia. Both were generally later than usual, perhaps owing to the prevailing winds (from south-west to westerly) being unfavourable for their North Sea crossings into Britain. The very early Fieldfare near Dartmoor was unexpected. Bramblings were scarce this autumn.

(iv)  Spring arrival dates:

These are intended to be the focus of an annual project starting in 2010. An excellent example of what we are looking for is shown on the chart, derived from the diary records of Swallows returning to nest at Hilary Marshall’s home in north Devon during the past 20 years. In this case, the well-known annual variability is evident and there is no significant trend of birds returning earlier over this period (the very slight trend is due to the very early 2009 bird). How typical these Swallows are is not known. So, further information of this kind is of considerable historical importance.

BBAD 2010 (Pilot Year 2)

The survey has now been re-organised and new record forms have been produced. These are also now available from the website at BBAD 2010 in this Fieldwork section. Following support from the DBWPS council the survey has also been renamed simply as Breeding Bird Arrival & Departures or BBAD. Meanwhile, your continuing support this year would be much appreciated. We are therefore hoping that you will have received a copy of a modified version of the recording form with suitably amended instructions, and look forward to hearing from you!

Peter Dare & Peter Reay

February 2010