There are several obvious disadvantages in colonial breeding:
1. Competition for food near the colony may be increased; however, this will be moderated as the distances individuals travel to feed increase. Many seabirds travel over 50 km to feed, the exceptions being mainly gulls and terns. In birds generally, particularly those with a short feeding range when breeding, colonial breeding may make it more difficult to exploit food resources in their environment which are evenly or randomly dispersed. Thus, colonial breeding might be expected to be linked only with a clumped food supply and/or the ability to range long distances to feed.
2. Close proximity of birds aids the transfer of microbes and parasites.
3. The re-use of sites year after year may encourage the buildup of large numbers of parasites, such as ticks and mites, which may also act as the vectors for viruses and other microbes.
4. The potential for intraspecific aggression is much greater in colonial species, since many potential recipients are close. One individual can threaten or even attack many individuals in a few minutes.
5. The nature of density-dependent effects is much modified, particularly at the breeding sites where densities are particularly high.
6. The potential adverse effects of predators, particularly mammals, on colonial seabirds can be high. This is well established in the many cases where cats and rats have been introduced to seabird islands. Yet the "swamping" effect may protect all but the edge nesting birds.
7. M0ller and Birkhead (1993) have suggested that cuckoldry is a disadvantage of colonial breeding. While birds are close together in a colony the opportunity for cuckoldry is high, but it still has to be established that cuckoldry is actually higher in colonial birds.
Colonial breeding does not appear to be advantageous to many bird species, since most species breed as solitary pairs and defend large territories. Colonial vs. solitary breeding may be related to the nature of the food supply or the distribution, abundance, and method of feeding of predators, or the presence of parasites.
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