Occurrence of rats and their impacts on colonial waterbirds in a Danish fjord

Human development around the globe has led to great expansion of the Brown Rat (Rattus norvegicus), which has implications for local wildlife and especially ground-breeding birds. In this study, we analyse the colonisation and persistence of rats on small islets important to breeding waterbirds in a Danish fjord, and investigate the effect of rat presence on the number of breeding pairs of eight waterbird species. The islets had an annual rat colonisation probability of 6% and an annual rat population persistence rate of 65% (equalling an annual population survival rate of 62% when adjusting for re-colonisations upon extinctions). Contrary to our hypotheses, rat colonisation and persistence was uncorrelated with islet size, distance from the mainland and the presence of shrub cover. Rat presence had a significant negative effect on the number of breeding pairs of four waterbird species, including Avocet (Recurvirostra avosetta, reduced to 30% compared to years without rats), Black-headed Gull (Chroicocephalus ridibundus, reduced to 45%) and Common Tern (Sterna hirundo, reduced to 52%). The smaller bird species in particular seemed to be affected by the presence of rats, and control efforts aimed at reducing rat presence on important breeding bird islets may consequently have a positive effect on the occurrence and breeding success of these species. We found no evidence of birds acting on a memory of where rats had been present in the previous year, and further research is needed to investigate the precise mechanisms behind the recorded negative effects in the contemporary year, i.e. how do prospecting as well as settled breeders detect and behaviourally respond to the presence of rats.