Grant P R, Smith J N M, Grant B R, Abbott I J, Abbott L K
Biology Department, McGill University, Montréal, P. Q.
Zoology Department, University of British Columbia, Vancouver, B. C.
Oecologia. 1975 Sep;19(3):239-257. doi: 10.1007/BF00345308.
Populations of 4 geospizine finches were studied on Isla Daphne Major in April and December 1973. Finches were captured, banded, measured and released in both months, and censused in December. 1640±711 finches were estimated to be present in December, of which probably more than 1000 belonged to Geospiza fortis. G. fortis were 4 times more numerous than G. scandens. The overal density at this time was 27-54/ha. This is the first quantitative estimate of finch population sizes for a whole island in the Galápagos Archipelago. Some G. fuliginosa and magnirostris immigrated between study periods, but overall density remained nearly constant. There was no evidence of immigration of fortis and scandens. Survival rates of the two common species, fortis and scandens, from April to December were estimated, from banded birds, to be 86.9% and 91.3%. Up to 300 finches (fortis and scandens) disappeared between study periods. The amount of predation by owls (Asio flammeus), estimated from an analysis of 49 pellets, could account for more than 50% of the finch losses, but is likely to be less. The estimated loss due to predation was less than 10% of the populations in April.The discovery of the remains of house mice, Mus musculus, and black rats, Rattus rattus, in pellets indicates that owls hunt on one island (e.g. Santa Cruz or Baltra) and regurgitate on another (Daphne). Fruits of a Euphorb, Chamaesyce amplexicaulis (Hook. f.) Burch, that finches eat were found in two pellets that also contained finch remains. This suggests that seeds or fruits in the throat of a victim at the time of a kill may be transported unharmed inside an owl from one island to another. Indirect support to this suggestion is given by the results of flotation experiments with seeds and fruits of 22 species of plants. Most did not float, and are too heavy to be transported by normal winds. Many do not have hooks or sticky surfaces, and internal transport by birds (including owls) is therefore strongly suggested.
1973年4月和12月，在达芙妮主岛对4种食籽雀种群进行了研究。在这两个月里，对食籽雀进行了捕捉、环志、测量并放生，并在12月进行了普查。据估计，12月时有1640±711只食籽雀，其中可能有1000多只是勇地雀。勇地雀的数量是仙人掌地雀的4倍。此时的总体密度为每公顷27 - 54只。这是对加拉帕戈斯群岛一个全岛食籽雀种群规模的首次定量估计。在研究期间，一些纯暗色雀和大嘴地雀迁入，但总体密度几乎保持不变。没有勇地雀和仙人掌地雀迁入的证据。根据环志鸟类估计，4月至12月期间，两种常见物种勇地雀和仙人掌地雀的存活率分别为86.9%和91.3%。在研究期间，多达300只食籽雀（勇地雀和仙人掌地雀）消失了。通过对49个食丸的分析估计，仓鸮（短耳鸮）的捕食量可能占食籽雀损失量的50%以上，但实际可能更少。估计的捕食损失量不到4月种群数量的10%。在食丸中发现了家鼠和黑家鼠的残骸，这表明仓鸮在一个岛屿（如圣克鲁斯岛或巴尔特拉岛）捕食，然后在另一个岛屿（达芙妮岛）反刍。在两个含有食籽雀残骸的食丸中发现了食籽雀食用的大戟科植物——抱茎大戟的果实。这表明，在捕杀时受害者喉咙中的种子或果实可能会在仓鸮体内完好无损地从一个岛屿被运到另一个岛屿。对22种植物的种子和果实进行的漂浮实验结果为这一推测提供了间接支持。大多数种子和果实不会漂浮，而且太重，无法被正常风力运输。许多种子和果实没有倒钩或粘性表面，因此强烈表明是通过鸟类（包括仓鸮）进行内部运输的。
