Szukana fraza: [Abstract = "It is suggested that, when the colony size falls below a certain level, the birds cease to breed because of the lack of social stimulation and the colony collapses \(the \"Allee Effect\"\). This stage has been reached in the centres of some large towns, where lack of nesting sites has been the critical factor, and in the \"lafy suburbs\", where the separation of the buildings on which the birds prefer to nest leads more rapidly to lack of social stimulation than in the case of the other residential areas where the houses are much closer together. Any decrease in these latter areas has occurred though an increased spacing of the colonies rather than by a decrease in colony size."]

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