1. Homing experiments were carried out with 338 Manx Shearwaters from Skokholm Bird Observatory, Pembrokeshire. The especial advantages of this species for such work are described.
2. With sunny conditions, strong homeward orientation was shown by birds released at points unknown to them. Early in the season these releases gave very fast, complete returns, more than half back on the first 2 nights. Those on the first night were unequivocal evidence of long, direct flights over unknown country.
3. One shearwater homed at least 3050 miles across the Atlantic in 120½ days.
4. There was a decline in the proportion of returns later in the season, probably due to a loss of stamina.
5. With conditions of heavy cloud the homeward orientation broke down and returns were poorer.
6. The stage of the individual incubation shift has some effect on the homing performance.
7. For the first time we have a demonstration of complete evidence for navigational ability in a wild bird. The suggestion of a form of sun navigation is in line with evidence obtained with pigeons and gulls. The failure of workers with other species to produce such evidence is not considered to weigh against the possibility of navigational ability of this kind in all proven homers.
8. The sun-navigation hypothesis is investigated. Although shearwaters fly to the burrows after dark they are unable to navigate after sundown. Determination of the latitude from the sun's altitude can only be tested in this species by transequatorial experiments.
9. Determination of longitude by time differences was tested by attempting to desynchronize the necessary internal ‘chronometers’. The first attempt was in conclusive owing to cloudy conditions at release. A second attempt was also partially thwarted by the weather, but gave suggestive results favouring the hypethesis. More experiments of this nature are required.
10. An analysis of the range of shearwaters from the colony, based on banding results, is given.
11. The Wilkinson flight-duration recorder was used on this species, but did not give satisfactory results.