de Groot R.S. (1983) Origin, status and ecology of the owls in Galapagos. ARDEA 71 (2): 167-182

The Short-eared Owl (Asia flammeus galapagaensis) and the Barn Owl (Tyta alba punctatissima) were studied from November 1979 to May 1980 in the highlands of Santa Cruz Island, Galapagos. Pellets were collected until December 1980. Brief visits were made to the three other inhabited islands San Cristobal, Isabela and Floreana. The study mainly concentrated on feeding and breeding of the owls and their evolution. The Barn Owl occurs only on the five largest islands, the Short-eared Owl is found on almost all (19) major islands (Fig. 3). A rough estimation was made of 9,000 pair of Short-eared Owls and 8,500 pair of Barn Owls in the entire archipelago. The Barn Owl hunts nocturnally and the Short-eared Owl is both diurnal and crepuscular. In the study area on Santa Cruz, 1156 Barn Owl pellets and 31 Short-eared Owl pellets were collected. The Barn Owl ate mainly mammals (85%, weight percentage) and insects (12%), the Short-eared Owl mainly birds (51%) and mammals (47%). Seasonal changes in the diet of the Barn Owl were related to vegetation cover and the reproductive periods of the most important prey species. Another 58 Barn Owl pellets and 184 Short-eared Owl pellets were collected on 8 other islands showing both owl species to be opportunistic feeders: 40 prey species for the Short-eared Owl and 34 for the Barn Owl. It was found that the Short-eared Owl includes seabirds and wading birds in its diet while the Barn Owl does not. Nests of both owl species were found in all months of the year throughout the archipelago although on Santa Cruz there appears to be a concentration in egg laying from November to June, corresponding with the rainy season. Egg measurements, clutch size and fledging success are given. Data on the development of four young Barn Owls and two young Short-eared Owls is presented. Young Short-eared Owls reached their maximum weight around day 30 after hatching, young Barn Owls around day 55. Particularly for the Barn Owl the development of the young is slow compared to subspecies on the continent. Small clutch size and slow development of the young were related to low mortality and lack of predators in Galapagos. Results of a literature study by the author, comparing 101 specimens of the Galapagos owls with four subspecies on the South-American continent confirmed that the Galapagos subspecies are much smaller and darker than the continental subspecies, particularly the Barn Owl. Some evolutionary aspects with respect to sexual dimorphism are discussed and related to diet and colonization times of the owls. An inquiry held among the farmers of the four inhabited islands revealed that people were killing owls in substantial numbers, in the study area alone approximately 350 Short-eared Owls per year (35% of the breeding population). Reasons for killing the owls, other mortality factors and conservation aspects are discussed. Some suggestions for further research are given.