Long-distance migration is ordinarily explained as the pursuit of food. A species breeding at high latitude exploits a brief abundance of insects and departs when the abundance fails; the movement is represented as a continuous tracking of resources around the annual cycle. The explanation accounts for the general phenomenon and encounters difficulty with the particular cases.
Consider a bird that crosses several thousand kilometres of ocean without feeding. Reading this as resource-tracking is not possible, since the whole journey is made where there is nothing to be had, and the bird arrives with its reserves far down. Take a shorebird that weighs no more than a small apple. It feeds for weeks on a mudflat until it is half again as heavy as it was, then lifts off and flies south over open water for eight days and nights without once coming down. There is nothing to eat on the way and nowhere to rest. It lands on the far shore light and worn, and it will feed for weeks before it is fit to breed. On any reading of the flight as a search for food the flight makes no sense at all: the bird leaves a place where it is eating well, crosses a stretch where there is nothing, and comes down where it must eat hard to get back what it spent. Such flights are not exceptional. A considerable proportion of long-distance migrants overfly ground where suitable food is plentiful, and do so in weather that makes the crossing costly.
The resolution proposed is that the destination rather than the journey is what selection has operated on. What a high-latitude summer offers is not merely abundance but duration: daylight persists for eighteen or twenty hours, and a species that provisions its young during that interval can raise a larger brood than any competitor confined to a twelve-hour day. The arithmetic is easy to see. A pair working an eighteen hour day can make half again as many trips to the nest as a pair working twelve, and the young that get the extra food grow faster and leave the nest sooner. The advantage is available nowhere else. Against an advantage of that size, an expensive journey is not an anomaly requiring explanation but the price of admission.
The reformulation generates a testable expectation. If duration rather than abundance is the operative variable, migration distance should correlate with photoperiod at the destination rather than with the quantity of food available there, and where the two diverge the correlation should follow the daylight. Where this has been examined the daylight has predicted the pattern and the food supply has not.
None of this displaces food from the account. A destination without provisions is useless whatever its daylight, and abundance remains a necessary condition. What the reformulation displaces is the assumption that a necessary condition is the thing being sought, which is a confusion the original explanation invited and the anomalous flights exposed.
