Sunday, 4 April 2010

Early Arrivals

Sometimes birding raises more questions than answers. Anecdotal records this year appear to suggest that Swallows and to a lesser extent House Martins may be arriving earlier albeit in small-ish numbers. Start thinking about this and you quickly realise that they can't possibly know the weather conditions a couple of thousand miles further north so either some change in their wintering grounds is triggering an early return or they are leaving at the same time and encountering less adverse weather conditions or an increased food supply as they move north encouraging them to continue at a pace faster than they used to.
I headed to The Euring Swallow Project to look for answers but most of research quoted is looking at swallows heading back post-breeding, it is interesting though so worth a glance.
 There is a paper printed in Ecology Letters in 2004 entitled 'Ecological conditions during winter predict arrival date at the breeding quarters in a trans-Saharan migratory bird' this study found that the arrival date of old but not young individuals captured during consecutive breeding seasons was earlier after winters with favourable conditions. Change in arrival date in relation to change in NDVI (normalised difference vegetation index) was similar in the two sexes. Change in arrival date significantly and positively predicted change in breeding date. As a result of increased frequency of second broods determined by earlier arrival, the number of fledged offspring per season was larger after African winters with good in comparison to poor ecological conditions for barn swallows.
So perhaps the answer to our early arrivals this year lies in the weather conditions during Dec-Feb in the wintering areas and maybe we're in for a bumper breeding season, fingers crossed.

Reference
Ecological conditions during winter predict arrival date at the breeding quarters in a trans-Saharan migratory bird

Nicola Saino, Tibor Szep, Maria Romano, Diego Rubolini, Fernando Spina, Anders Pape Moller
Ecology Letters (2004)
Volume: 7, Issue: 1, Pages: 21-25

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