Sparrowhawk Accipiter nisus passage through the Calf of Man 1959–1993

Abstract

Daily and ringing logs of the Calf of Man Bird Observatory have been analysed to examine the nature of Sparrowhawk passage recorded on the island. The volume of passage on the Calf during 1959–93 demonstrates a rapid and sustained recovery after 1972, in line with that of the UK population as a whole. The timing of passage shows discrete spring and autumn peaks with virtually no records during June and July and very few records during December to February. Annual bird‐day totals after 1972 fluctuate between 57 and 116 per year and show a significant correlation between low annual totals and diminished autumn passage, while further analysis significantly correlates diminished autumn passage with low hawk numbers in September. Autumn passage is generally trimodal, equal peaks occurring at the end of August and September, with the largest peak occurring in mid‐October. A similar pattern has been noted for Sparrowhawk passage on both Bardsey and Lundy. Late autumn hawk passage is significantly correlated with that of certain prey species, notably the chaffinch. A Manx bird ringed as a pullus has been recaptured on the Calf during its post‐fledging dispersal, while Sparrowhaxvks have been recorded arriving on the island from the west during autumn passage. Birds ringed as pulli in Northern Ireland and Scotland have been recovered on the Isle of Man. The pattern and timing of 14 recaptures suggests that they may refer to birds of Scandinavian origin undertaking migratory movements. Ten birds ringed on the Calf have subsequently been recovered, but only one was found away from the Isle of Man, in ivest Cumbria. Sex‐ratios derived from trapped birds arc heavily biased probably as a consequence of habitat preferences and trapping methods, while ratios derived from observations are more comparable with those calculated from an intensive study. Passage of Sparrowhawks through the Calf in autumn probably includes both Manx‐ and UK‐bred birds undergoing post‐fledging dispersal and Scandinavian birds undertaking migratory movements, while the regular spring passage through the island strongly suggests return movements by Scandinavia migrants.