Understanding how birds respond to the activities of people is an important component of conserving wildlife. We measured responses of nesting black skimmers (Rynchops niger) to an approaching boat in Barnegat Bay, New Jersey, USA, by examining distance to first respond, distance to flush, and time to return to the colony. Our objective was to determine if response distances of skimmers changed as a function of year, reproductive stage, direction of approach (direct or tangential), or number of birds nesting in the colony. Generally, reproductive stage had the greatest effect on all responses, followed by direction of approach, number of adults present at the colony, number of nests, and year, which also explained variation in behavioral responses. The distance at which skimmers first flew when a boat approached decreased from the pre-egg-laying period to hatching, and then increased slightly later in the season. Time (x 6 SE) for skimmers to return to the nesting colony varied seasonally, with birds taking longer to return during the pre-egg period (9.5 6 0.6 min) than during hatching (0.7 6 0.1 min). The decision process for determining set-back distances to protect nesting skimmers should involve selecting 1) behavioral response of highest concern, 2) reproductive stage of highest concern, and 3) an appropriate level of response at which to establish the buffer area. We recommend that managers use a set-back distance of L 118 m from the perimeter of the colony for black skimmers, which is the 95% percentile of the distance that skimmers first flew in response to approaching boats. Managers can use these data to set buffer distances for skimmers and other colonial birds.
SUMMARY
Common Terns nested on 34 of 259 saltmarsh islands along 46 miles of Ocean County coastline, New Jersey. They nested on low islands of Spartina with less than 12%, windrow ranging in size from 0.6 to 108 acres. All islands selected by terns faced at least two miles of open water from at least one direction. Selection of islands seemed a response to vegetation, size, distance to the nearest island, distance to the nearest shore and exposure to open water. Of the 225 islands without nesting terns, only three fulfilled appropriate criteria.
Eighty per cent of nests were situated on windrow. Terns laid larger clutches on windrow compated to those nesting in Spartina. Nearest neighbour distance on islands with low predation varied from 85 to 485 cm and was a function of space, vegetation, and the size of the island. Terns nested closer together on windrow compared with Spartina. Considering all colonies, the mean nearest neighbour distance negatively correlated with the number of nests.
Skimmers, Oystercatchers, Laughing Gulls, and Herring Gulls nested in some of the tern colonies. All tern colonies on islands also occupied by nesting Herring Gulls suffered over 10% egg‐loss by predation. Proportionally more nests were preyed upon in windrow than in Spartina. Clutch sizes in nests in windrow on islands with predation were significantly lower than those in windrow on islands without predation, even though clutch sizes in Spartina were similar on islands with and without predation.
Flooding by exceptional high tides destroyed significantly more nests in Spartina than in windrow. The nests that survived in Spartina were built deeper, and their rim heights were higher than nests destroyed by tides.
Choice of colony and nest sites is discussed in terms of the balance of two selection pressures: predation and tidal flooding. Tern nests on windrow are more exposed to predators, but safer from flooding, whereas those nests in Spartina are more susceptible to flooding and less vulnerable to predators. The recent invasion of Herring Gulls into Ocean County has significantly increased the rate of nest predation among affected colonies of Common Terns. It is possible that in future years the terns' behaviour may be modified in response to this new selective pressure.
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