The breeding biology of terns on the Western Isles in relation to mink eradication

American Mink were introduced to Lewis in the 1950s and ‘60s, and their range expanded south to colonise North Uist by the late 1990s. Mink eradication was initialed in 1999 and the islands were almost entirely cleared by 2004. The breeding biology of terns on the Uists prior to colonisation by mink and after eradication was compared with that on Lewis where mink were present during the entire period. The results showed that nest survival was significantly higher on the Uists compared with Lewis in 2005, and this was largely explained by lower mammalian predation rates in the Uists. However, there was no significant additive effect of mink occupation on productivity across years. Productivity was mainly affected by year, with little evidence of differences between archipelagos within years. However, productivity was low in the only two years when good sample sizes were available in both archipelagos, probably due to poor food supply or inclement weather. In these situations, the effects of mink predation would be expected to be compensatory, since they were taking eggs and chicks that would probably have starved subsequently. Improved annual monitoring of colonies on both Uist and Lewis needs to be conducted in order to investigate the interactive effects of mink removal and food availability on tern productivity in the Western Isles.