29. Arrival Date of the Swallow
TH Sparks and RG Loxton

Date when the swallow was first seen at coastal observatories in Britain (averaged for Dungeness, Portland, Bardsey and Calf of Man) shown in relation to mean temperatures in February-April in Central England

Specific Indicator
Skilled observers keep daily records of the numbers of about 20 bird species seen at British coastal observatories, linked by the Bird Observatories Council. The specific indicator used is the average date when the swallow is first observed at four coastal observatories (Dungeness, Portland, Bardsey and Calf of Man).
[Source: Centre for Ecology & Hydrology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire PE17 2LS, with thanks to the Bird Observatories Council]

Relevance
Some migrant birds are considered harbingers of spring and are readily recognised by sight or by song. The swallow (Hirundo rustica) is readily observed, is associated with human habitation and is most active when insect production rises to the level that makes aerial feeding possible. In terms of indicators, the first sighting of the swallow may seem on a par with the first hearing of the cuckoo, but the latter is, in fact, less closely related to spring temperatures.

Sensitivity to climatic and other factors
Some bird species migrate several thousands of miles between wintering and breeding grounds. This migration is triggered by changes to day length and is associated with the loss of insect food supplies in the overwintering site - when they set out, they have no way of knowing the weather at their destination. However, climatic conditions along their route affect the duration of the migration and birds will arrive in numbers only when their food supply is plentiful. Thus, their first arrival date is, in fact, partly determined by climatic conditions at their place of arrival.

The swallow is a long distant migrant, overwintering in southern Africa. Fine tuning of its arrival time has been shown to be temperature related, probably because the aerial insects on which it depends must have emerged by the time the swallow arrives. If food supplies en-route are restricted it is likely that the swallow will slow or halt its northwards passage. The arrival dates of swallows, averaged for four observatories, were found to be closely negatively correlated with mean temperatures in Central England in February-April.

Change over time
The time series shows a pattern of late arrival in cold springs, for instance in 1986, and the reverse in years with warm springs. There is evidence of a trend towards earliness in swallow arrival dates since 1970 in response to warmer springs.
The relationship suggests that a 1EC increase in spring temperature is associated with 2-3 day earlier appearance of swallow in the UK. Thus, in the event of climate warming we might expect swallows to be seen earlier and more often in March.