Pollen on birds

These are Garden Warblers. The closest bird has pollen on its beak. Most warblers fatten up on insects while in Africa then while migrating across stop off in countries along the way and feed on nectar for an energy boost. While feeding they pick up pollen on their faces. This pollen can be analysed to work out which flowers the birds have been feeding on and if the plants are indigenous work out which countries they have been to and map their migration path. When they arrive in the UK they start feeding on insects again to get them in prime condition for breeding.