Birding in Wharfedale:
The Return of Spring Migrants

Preamble

We only have raw data on bird sightings since 1999 - before that we are dependent on the annual report in the Review. In an idle moment I wondered whether I could get any quantitative information from these reports. The obvious parameter to look at is the annual dates when returning migrants are first seen in the spring, since this period is eagerly awaited and perhaps the most reported of the year. In the end it proved a fascinating study since it led me into the complex effects of global warming, which we can glimpse only darkly. I look at the raw results below and will return to the broader effects of climatic change on migration and bird behaviour in future notes.

What does a typical set of results look like?

Important selection criteria are that it has to be a species that has been common throughout the period (and significant parts of the southern WNS area) - and one for which we have a reasonably complete set of records. The weakness of using this parameter, of course, is that it does not reveal anything about the time when the bulk of birds return.

I chose at first to look at sand martins. The annual return date is shown in red in the figure above. The vertical axis is the date relative to April 1st, i.e. + 10 means an arrival date of April 11th. The most obvious feature is its extreme variability from year to year. This is perhaps not surprising. Migration is a very complex phenomenon and arrival will be dependent on a variety of disparate issues including food supply in the wintering country and the weather en-route (but not directly on the climate in Wharfedale). Birds are in a much greater rush to get to their breeding sites than they are in the autumn, but migration involves enormous expenditure of energy and few species make the trip in one long hop. Migration from Western Africa will take the sand martin upwards of a month and most birds will stop off to rest and feed, particularly if the weather is inclement, e.g. rain and adverse winds.

How to make something of this 'spiky' curve. For the moment I chose to look at the 5 year moving average. This is shown in blue and we can immediately see a very distinct trend towards earlier dates, particularly since the late 1980's. The 5-year moving average is used for the rest of this note.

Arrival dates for nine of the commoner migration species

These were chosen since we have relatively complete records:

Martins, Swifts and Swallow

We tend to assume now-a-days that sand martins arrive 2 - 3 weeks before swallows, but that hasn't always been the case. There was a period during the 1950's when the swallows tended to arrive first, and throughout much of the period the gap was quite small. By comparison dates for swifts have not changed much . I will return to the possible cause of the earlier arrival of sand martins (and the general situation for long range migrants) in a later note.

Some common warblers

There were a number of species I would have like to look at but in the end I had the most complete records for willow warbler, chiffchaff and, to a lesser extent, blackcap. The figure above shows that we have another clear case of an increasingly early arrival - the chiffchaff. The blackcap is also arriving earlier, although the situation is less obvious here because the records are very incomplete during the 1990's. The reason for this and for both species earlier arrival reflects very significant changes in their wintering habits, which will become clear in the next note.

Other species

The last figure shows 3 very disparate species - the cuckoo, redstart and common sandpiper. The feature of timings for the cuckoo is the small variation, which is interesting since it winters in the same region of Africa as the sand martin, but has quite different environmental requirements that clearly have not been affected to the same extent as for the sand martin. The feature of the other two species is the variability of the arrival time. The redstart winters from south of the Sahara down through Central Africa almost as far as the Equator. The movements of common sandpipers remain a mystery, since no ringed birds have ever been recovered, but British birds are believed to winter in West Africa.

Summary
The analysis of the arrival dates of a number of species shows that three - the sand martin, chiffchaff and blackcap are now arriving noticeably earlier. The way in which the habits of these last two birds have changed over a remarkably short period, because of climate changes, will be explored in the next note.

Time permitting, I may return to the issue of dates at some time in the future to see if there are more appropriate methods of analysis, and if I can look at the situation for other species

John Flood (25/06/04)

Back