1. Factors controlling the distribution of the bird fleas Ceratophyllus styx, the sand-martin flea, C. gallinae, the hen flea, and Dasypsylla gallinulae, the blackbird flea, were investigated in the field.

2. Sand-martin fleas (C. styx) pass the winter as adults within cocoons in the old nests of their host. Observations indicate that the fleas are stimulated to emerge from the cocoons by the rise in temperature in the spring, and some of the adult fleas emigrate in the spring and invade new nest burrows. Observations and experiments showed that sand-martin fleas disperse from old burrows both laterally and vertically and that emigrating fleas could reach areas as far as 33·8 m. from the old nests. The detection of the new burrow entrances is not due to vision, or to the recognition of difference of humidity between the burrow and the cliff face, or to the reaction to air current differences between the burrow and the cliff face, but to recognition of the horizontal floor of the burrows and a tendency to congregate upon horizontal surfaces. How the fleas distinguish the horizontal burrow surface from the cliff top is still unknown. The colonization of new burrows by the fleas does not occur at night. It is suggested that, in addition to finding their hosts in the spring by invading new burrows, sand-martin fleas may jump on to the birds when they are hovering near the cliff face. Many adult fleas leave the burrows within 3 days of the fledging time of the young sand-martins but a small number remain within the nest. The fate of these specimens is not known.

3. C. gallinae, the hen flea, and D. gallinulae, the blackbird flea, pass the winter principally as adults enclosed in the cocoons in the old nest material. In the spring they emerge from the cocoons, emigrate away from the old nest and are free-living on the ground. Probably the adult C. gallinae and D. gallinulae jump on to birds when they are feeding on the ground in the spring.* Therefore the absence of hen fleas in nests situated on the ground or in open nests built in low vegetation, is not due to lack of opportunity of contact between the adult flea and the birds which construct such nests.

4. It is concluded that in host-seeking, adults of C. gallinae and D. gallinulae emigrate away from old nests in the spring and come in contact with the host birds on the ground in the birds' feeding area, whereas C. styx adults emigrate into the hosts' breeding area and come in contact with the birds in the new nesting burrows.