Damage by rats Rattus norvegicus to breeding birds on Danish islands

Occurrence of rats Rattus norvegicus was recorded in Danish islands during the period 1970–1979. Rats apparently became common during the 1960s and 1970s. Their frequency fluctuated, indicating a possible 4-year cycle. Most rat colonisations lasted a single year. Geographical differences in their frequency may be caused by regional differences in distribution of larid colonies. Rat distribution did not follow the pattern suggested by the theory of island biogeography; presence of larids may be of major importance. When rat populations in hedges, ponds and streams are at maximum population density in autumn, pregnant females emigrate to, and so colonise, islands. Flooding and the severity of the winter may cause extinction. Food supply and predation may be of importance in limiting rat numbers. Breeding birds suffer, especially those placing their nests near rat runs; rat predation on adults may have serious consequences for a long-lived species, whereas that on eggs and young is insignificant. Some larid species renest in the immediate vicinity when predated by rats; others desert their dense colonies. Rat control is successful in small islands, whereas large island populations are only exterminated with difficulty. They should be controlled to prevent predation and to keep breeding grounds usable. This should be done in late winter when population density is low and food-supply insufficient.