"Our data can be used to assess habitat quality and the importance of specific sites for migratory birds, and this can contribute to decisions about whether migratory sites are protected and which sites are prioritised for protection", explains Williams.
Two techniques were used to study Western Sandpipers on their spring journey along the Pacific 'Flyway' from Mexico to Alaska: 80 birds were fitted with radio-telemetry tracking devices and a further 400 had blood samples taken to give measurements of fattening rate. Williams found that birds fatten more rapidly as they move further north as they get closer to the breeding grounds - and that the longer the birds spend hanging around at San Francisco Bay (one of the more southerly refuelling sites), the lower their fattening rates.
Such differences in fattening rates cannot simply be explained by differences in the availability of food. "Our current thinking is that the difference in fattening could be caused by differences in behaviour (birds simply feed more intensively in the north) or changes in physiology that the birds experience as they move further north", says Williams. "We know there are major differences in gut structure and digestive enzymes between non-migrating and migrating birds, so there might also be similar alterations in migrating bird's physiology further north, which allow more efficient digestion."