Many small insectivorous birds eat spiders as well as insects. This they do almost with impunity in temperate latitudes, where only occasionally do spiders make webs strong enough to trap a bird. But in the tropics, where there are more large spiders, their webs must be a greater hazard to birds. That the hazard exists in both climes, however, is shown by a goldfinch reported caught in a spider's web in Cape Cod, Massachusetts, and a dusky flycatcher caught in a spider's web in Cameroons, West Africa.
SURVIVAL OF THE UNFIT [Ref]
To care for the weak, the unfit, and the cripple is usually considered an extremely highly developed altruism in our society. As our society progresses, more and more provisions are made for the unfit.
In nature the unfit usually is soon weeded out. If an animal is unable to feed itself it is doomed; or if it is less successful than its fellows it has less chance of leaving progeny. That is natural selection.
Hence on both counts it comes as a surprise to find two well-authenticated cases of crippled birds, unable to search for food for themselves, surviving for long periods.
The first is T. R. Peale's record in 1848 of a brown booby on Enderby Island in the Pacific. An adult bird whose plumage indicated it was several years old was found on the island, and it had only one wing, the other having been lost by some accident and the wound completely healed. The bird was unable to go to sea and get its own food, and was being fed by its fellows.
The second was a frigate bird, found on the Revillagigedo Islands, reported by A. W. Anthony in 1898. This bird, too, was fat, and had one wing withered and useless, evidently from hatching. It had never flown. Frigate birds are masters of the air that snatch their food on the wing from the surface of the water, and a flightless frigate bird would be as badly off as a flightless swallow. The cripple had been fed all its life by its neighbors.
At first the uncritical might think, What altruism, what charity, for the healthy to feed these two cripples. But an explanation involving less advanced principles, principles more in keeping with what we know about bird behavior, is possible. Remember that young birds that are unable to begin feeding themselves at the proper time may continue to beg for food, and be dependent for a long time, as I have shown with young shrikes under the chapter, "Conditioning in Birds." Remember that a young bird begging for food may be fed by adults, not its parents, and even by other young birds (shown in "Bird Helpers at Nesting Time"); and we have the clue.
The cripples, hungry, begged for food; the healthy birds responded by feeding, as they might do to other begging young, and owing to the unusual circumstances both were continued.