Migrants Arrive to Breed in the Arctic With a Full Lunch Basket

Abstract from Ibis
2nd September 1996

Studies of small migrant birds arriving to breed in the arctic spring have sometimes shown that they are quite fat when they arrive. This would seem to be counter-productive as they will have to have carried the extra weight for many hundreds of kilometres. Now Roland Sandberg has carefully measured this extra load in nine species over six years in Swedish Lapland.

His results, published in the latest Ibis (138, 514-524), clearly show that this is a deliberate strategy related to the shortness of the breeding season and the need for these little birds to start it with significant reserves. Indeed for the two species which come a very long way, reserves. The long-distance migrants arrived with the equivalent extra fat load which would allow them to go between 242 and 500 kilometres further.

Indeed for the two species which come a very long way, and where the sexes arrive at the same time, the females actually have a bigger fat store, on average, than the males. The female Pied Flycatchers and Bluethroats are immediately going to have to build a nest and produce the eggs. For the Willow Warblers the females arrived later but carried about the same fat loads. However the males have evolved a longer wing for more efficient flight - possibly to allow them to arrive back very early and claim the best territories. The females then just have to chose the best males - and bring their fat reserves! This may seem a lot of bother but the arctic summers, though short for the breeding birds to fit everything in, are very productive. The insects which are the bane of the Human interloper are the proverbial free lunch.