Effect of investigator disturbance on nest attendance and egg predation inEurasian oystercatchers

Eurasian Oystercatchers (Haematopus ostralegus) breeding on the salt marsh of Schiermonnikoog (Dutch Wadden Sea) lose many eggs to predators, mainly H erring (Larus argentatus) and Mew gulls (L. canus). We estimated that the p robability for an egg to survive from laying until hatching was 69%. Daily egg mortality was higher during the laying period than during the incubatio n period. When researchers were present in the study area, oystercatchers s pent more time at greater distances from the nest. We investigated whether human disturbance resulted in more eggs being lost to predators. Two experi mental areas were in turn visited at high and at low frequency. From a prel iminary analysis, we estimated higher daily egg mortality rates when nests were checked three times per day instead of once every other day. However, high-frequency nest checks provided more information on newly laid and lost eggs, especially during the laying period. After correcting for that extra information (by simply deleting it), the egg mortality rates were no longe r different. We conclude that human disturbance did not increase egg loss, rather egg mortality rates were underestimated when nests were checked only once per two days.