VEEN J (1980) Why do birds breed in colonies?. LIMOSA 53 (2): 37-48.

For a long period studies on the function of colony breeding behaviour in birds have emphasized the ways by which colonial nesting may reduce predation. More recently, attention has been given to other possible functions, such as food-finding and in a few cases, thermo-regulation.
      In a number of studies colony nesting and solitary nesting individuals of a particular 'species have been compared for breeding success (figure I), showing that colony nesting individuals were more successful. In Sterna sandvicensis a number of characteristics of a colony e.g., colony size, nest position, nest density and synchronization were related to success allowing the conclusion that the species-specific nest distribution pattern is adaptive (figure 2).
      A number of species incubate their eggs and raise their young at very low temperatures. Aptenodytes forsteri which incubate during winter huddle together in dense groups during a period of extreme cold. Close body contact enables these birds to reduce heat loss. The same applies for A. patagonica chicks which remain in the colony through their first Antarctic winter.
      Colonial nesting has been shown to reduce predation in a number of ways: (I) A colony may act as an early warning system and thus increase the likelihood that a predator is detected at a long distance. Until now, the warning function of a colony has been poorly studied and the results obtained are not very convincing (figure 3). (2) If the number of predators within a particula area is limited, concentration of broods increases the number of prey per predator and may thus decrease predation pressure. Examples are given for Larus ridibundus (figure 4) and L. argentatus preying on eggs and chicks of Sterna sandvicensis. (3) Colony nesting enables the breeding birds to defend the nesting area communally. In a number of cases such combined attacks have shown to be very effective in reducing predation (figure 7,8).
      There are some marked relationships between group life e.g. colony nesting and group foraging, and the distribution of prey. Sturnus vulgaris, for instance, feed solitarily when the location of prey is predictable, and group feeding occurs when the location of prey is unpredictable (figure 9). Colony size of seabird species breeding in the Caribbean is correlated with feeding distance (figure 10). In European terns colony size is related to both feeding group size and feeding distance (figure 11). It is suggested that colony nesting is to a large extent determined by prey distribution and a number of possible ways in which colony nesting might enhance prey location are discussed:
(I) Colony nesting allows Pelecanus onocrotalus to synchronize foraging activities. This is important since feeding activities in this species require a high degree of co-operation.
(2) Sterna sandvicensis of a particular colony usually forage within the same area. They strongly react to each other's success and a school of fish which is usually detected by a single individual is exploited by many colony members (figure 12).
(3) For a number of species it has been suggested that unsuccesful individuals follow succesful ones from colony to feeding grounds (figure 13). In Riparia riparia the incapability of parents of late hatching young to follow colony members to the feeding grounds is correlated with severe mortality (figure 14).
(4) Observations on Euphagus cyanocephalus suggest that breeding birds measure success of colony members bringing food to the nest. Unsuccessful birds subsequently follow successful ones (figure 15).
      The complex relationship between colony nesting on the one hand and environmental factors on the other is shown for Sterna sandvicensis in figure 16.

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