Individual study: Control taste aversion using methiocarb-treated eggs is likely to reduce predation of roseate tern Sterna dougallii eggs by yellow-legged gulls Larus michahellis but not starlings Sturnus vulgaris, Vila Islet, Azores archipelago, Portugal
Neves V.C., Panagiotakopoulos S. & Furness R.W. (2006) A control taste aversion experiment on predators of roseate tern (Sterna dougallii) eggs. European Journal of Wildlife Research, 52, 259-264
In the Azores archipelago, European starlings Sturnus vulgaris are a main egg predator at Vila Islet, which supports a colony of the endangered roseate tern Sterna dougallii. An earlier experiment showed that control taste aversion was effective in controlling raven Corvus corax predation in a colony of California least tern Sterna antillarum browni. In the study summarised here, at Vila Islet a control taste aversion trial was undertaken on yellow-legged gulls Larus michahellis and starlings eating eggs of terns in the mixed common Sterna hirundo and roseate tern colony.
Study site: The experiment was conducted on Vila Islet (a 10 ha rocky island) off Santa Maria island (36°55′N, 25°10′W). There are no mammalian predators on the island.
Experimental design: The trial was conducted from 17 April to 26 May 2003. Domestic quail Coturnix coturnix eggs (approximately of the same size and general patterning as the tern eggs) were treated with methiocarb (11.25 mg/egg) and placed in artificial nests (shallow scrapes) within the tern colony. This was undertaken at two stages in the nesting cycle; before and after they had started egg-laying.
It was not possible to have a control group, therefore untreated quail eggs were placed in the artificial nests for the first 3 days (24 untreated quail eggs on 19 April; replaced if predated or broken), followed by treated eggs. The difference in predation rates on untreated and treated eggs was used as a measure of modification of gull and starling egg-predation behaviour. Artificial nests had one egg per nest (a typical roseate tern clutch is 1-2 eggs). All nests were checked daily and observations made from a hide to identify predators and record behaviour and abundance.
On 14 May when terns had commenced incubation, 18 treated quail eggs were placed in artificial nests; again nests were checked regularly and observations of egg predators made.
Pre-laying period: Quail eggs started to disappear from day 1, only yellow-legged gulls were seen taking eggs. They took 32 untreated eggs during days 1–3, and 15 methiocarb-treated eggs during days 4–9. Rate of egg predation thus decreased from 10.6 to 2.5 eggs/day when methiocarb was used.
Egg-laying period: Gulls were not observed taking eggs. Starling predation rates on treated eggs decreased, but not of tern eggs. The number of roseate tern nests and eggs changed daily; therefore, percentage of available eggs predated was calculated to compare between days. From days 1 to 6, the average daily predation was 13.0% for tern eggs and 9.3% for treated eggs; when the methiocarb concentration was increased to 22.50 mg/egg, average daily predation on treated eggs dropped to 5.6%, but predation on tern eggs remained high 12.3%.
The authors conclude that control taste aversion using methiocarb-treated eggs may reduce predation by gulls but not starlings; starlings appeared attracted by the movement of incubating adults, not the presence of eggs alone, unlike the gulls.
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