As a general perspective for migrating birds, the birds that are resident or have been in the area for a few days are the ones with lots of fat on them. Flight birds that have newly dropped in are usually depleted of fat content.
In my bird banding studies at migration stations, we see all types of birds drop in overnight and they are typically in poor condition with no fat. They'll stay for a day or two and build up fat reserves and the move on.
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