Identification of predators at black-fronted tern Chlidonias albostriatus nests, using mtDNA analysis and digital video recorders

Predators at black-fronted tern (Chlidonias albostriatus) nests on the Wairau braided riverbed in Marlborough, New Zealand, were identified using (1) mtDNA analysis of 438 swabs from shell remains, nest contents, and carcass remains, and (2) digital video surveillance of 85 nests. DNA analysis suggested harriers (Circus approximans) were the main predator of tern eggs (171 of 192 shell samples containing predator DNA). Cats (Felis catus) and stoats (Mustela erminea) were the probable predators of the majority of adult terns killed (9 and 8 respectively, of swabs from 19 carcasses). Video results were broadly, though not entirely, consistent with the DNA results, and showed that harriers were the main predator of eggs (9 of 19 videoed predation events), followed by Southern black-backed gulls (Larus dominicanus dominicanus; 3/19); hedgehogs (Erinaceus europaeus occidentalis; 2/19), ship rats (Rattus rattus; 2/19), pied oystercatchers (Haematopus finschi; 2/19) and stoats (1/19). DNA was analysed from nine of the 19 videoed nests but the only predator DNA obtained was from harriers (four nests). Sixty-four percent of depredated nests (683/1063) contained no eggshell remains at the next monitoring visit after predation. DNA analysis of nest material from 71 of these empty nests yielded only one predator result; video footage was therefore essential to identify the cause of 12 empty nests at 19 videoed nest predations. Terns removed the depredated egg remains from eight nests; black-backed gulls consumed eggs at three nests; and a stoat carried the eggs away from one nest. Hedgehog DNA was not found on shell remains from nests with videoed hedgehog predations. Analysing DNA from eggshell and carcass remains is a valuable new tool in wildlife research and management because it can identify predator species and indicate their relative importance. However, our results show that predator species are not equally detectable using this technique, leading to biases in the DNA results. This ‘detectability bias’ needs to be further quantified, and recognised when interpreting DNA results.