Читать книгу Migration of Birds (1979) онлайн

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It has been suggested that small birds migrate by night to avoid their enemies. To a certain extent this may be true because the group includes not only weak fliers, such as the rails, but also the small song and insectivorous birds, such as wrens, small woodland flycatchers, and other species that habitually live more or less in concealment. These birds are probably much safer making their flights under the protecting cloak of darkness. Nevertheless, it must be remembered that night migrants include also the snipe, sandpipers, and plovers. Most shorebirds are usually found in the open and are among the more powerful fliers, as some of them make annual migratory flights over 2,000 miles nonstop across the ocean.

Night travel is probably best for the majority of birds chiefly from the standpoint of feeding. Digestion is very rapid in birds and yet the stomach of one killed during the day almost always contains food. To replace the energy required for long flight, it is essential that either food be obtained at comparatively short intervals or stores of fat be laid on prior to migration. If the smaller migrants were to make protracted flights by day they would arrive at their destination at nightfall almost exhausted, but since they are entirely daylight feeders, they would be unable to obtain food until the following morning. Unless reserve energy was carried in the form of fat, the inability to feed would delay further flights and result in great exhaustion or possibly even death should their evening arrival coincide with cold or stormy weather. By traveling at night, they can pause at daybreak and devote the entire period of daylight to alternate feeding and resting. This schedule permits complete recuperation and resumption of the journey on a subsequent evening after sufficient energy has been restored.