1965/05/01 by D. W. Snow · 2 citations
Environmental Science · #Avian ecology and behavior #Ornithology #Snow #Tying #Geography #Beech #Bird migration #Archaeology #Genealogy #History #Ecology #Southern Hemisphere #Biology #Meteorology #Forestry #Computer science
paper · doi:10.2307/1365398
openalex publication_date 1965/05/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
In the course of field work in the Galapagos Islands from February, 1963, to April, 1964, a limited study was made of the breeding of the Red-billed Tropic Bird (Ph&hon aetltereus).Because visits to the breeding colonies could be made only at fortnightly or monthly intervals, there were many points on which accurate information could not be obtained; but since the visits were at least regular, it was possible to determine the pattern of breeding activity throughout the year.Two colonies were compared: that of Daphne Island, off the north coast of Santa Cruz Island, which was visited monthly, and the colony on South Plaza Island, an islet off the east coast of Santa Cruz, which being nearer to the Charles Darwin Research Station on the south coast of the same island could be visited every two weeks.Beebe (1924) has vividly described Daphne Island, a well-known sea-bird breeding station, which members of most of the expeditions to the Galapagos Islands have visited.The island is a crater of pelagonite (tufa) some 500 feet high and about 400 yards in diameter, sparsely vegetated on the outer slopes, where most of the tropic birds nest.Like most such pelagonite cones in the Galapagos Islands, it is stratified like a dome-shaped cake which has been covered with successive layers of thin icing, and subsequent weathering of the steep sides has produced numerous small caves and recesses under the down-sloping strata.The tropic birds nest in these recesses, many of which are quite shallow although a few are so deep that it is difficult or impossible to extract the nesting birds.South Plaza Island is quite different.It is a low-lying islet of basaltic lava, 700 yards long and about 100 yards wide.On one side, a broken cliff up to 60 feet high runs along most of the length of the island, whereas at the other side the land slopes gently down to the sea.The tropic birds nest in cracks, niches, and shallow recesses in the cliff, a certain number of the nest sites being inaccessible without a rope.Daphne Island is so large that it was not possible to check the whole island in the few hours available at each visit.Instead, we worked the lower part of the southern slopes systematically, attempting to find all the nests in this limited area.On South Plaza Island we covered the whole length of the cliffs at each visit, but some parts of it were inaccessible.On both islands we made a point of checking all the known sites at each visit. COMPARISONOF THE TWO COLONIES Murphy (1936) and Lack (1950) both concluded that in the Galapagos Islands the Red-billed Tropic Bird must nest continuously, as eggs and young had been found in many months of the year.On Daphne Island, whence much of their information was derived, this is certainly so.Table 1 shows the months of laying of 148 eggs in the 116 nest sites that were found.No pattern is apparent, the generally higher numbers in the later months being simply due to the fact that we kept on finding additional nest sites in the course of the year.As we checked all the known sites at each visit, and as old sites are regularly reused, the potential number in use increased slightly month by month.