Effects of Human Disturbance on Reproductive Success in the Black Skimmer

Abstract

-We subjected Black Skimmers (Rynchops niger), nesting in six subcolonies within a tern colony, to either daily or weekly nest checks in order to study the effects of human activity on reproduction. Many prelaying adults left subcolonies that were disturbed daily and settled in relatively undisturbed subcolonies; some pairs in these areas deserted nests early in incubation. Nest density, late nesting, hatching success, and fledging success were inversely correlated with disturbance. In consequence of disturbance, a few chicks ate younger conspecifics. Low fences placed around groups of nests depressed fledging in areas disturbed weekly, but enhanced it in subcolonies disturbed daily. The activities of researchers themselves at waterbird colonies may reduce the birds' reproductive success. Disturbance by biologists has been deemed a major threat to seabirds because their activities have caused desertion and mortality of eggs and young (Nisbet 1978, Schreiber 1979). Determining the effect of observers is important, because the validity of conclusions drawn from scientific investigation is diminished when an observer unintentionally and unknowingly influences the results. Knowledge of the phenomenon can also help to minimize adverse effects on the birds. Natural losses of eggs and chicks cannot be wholly separated from artificial ones because the presence of observers may increase losses. However, the stage at which losses are most likely to occur can be determined (Ollason and Dunnet 1980), and this information can be used to minimize disturbance. Furthermore, the reproductive success of seldom disturbed birds can be compared to that of birds that have been disturbed more often. We designed a study to examine the effect of our field activities on all phases of the breeding season of Black Skimmers (Rynchops niger). We subjected subcolonies of skimmers to different disturbance schedules in order to determine when the birds were most affected by human disturbance. We chose Black Skimmers for this study because they usually nest in clusters or subcolonies on sandy patches within tern colonies (Gochfeld 1978, Erwin 1979), allowing for different treatments of various nesting groups within the same colony. We wanted to quantify the effects of human disturbance, identify stages of the breeding season when skimmers are most sensitive to disturbance, and test the feasibility of enclosing groups of nests in fences to reduce chick mortality. STUDY AREAS AND METHODS We studied Black Skimmers at Cedar Beach on the Jones Beach (Long Island, New York; 400N, 730W) barrier island from early May to early August 1980. The colony contained 2,500 pairs of Common Terns (Sterna hirundo), 200 pairs of skimmers, and 80 pairs of Roseate Terns (S. dougallii). Several pairs of Willets (Catoptrophorus semipalmatus), Killdeer (Charadrius vociferus), and Piping Plover (Charadrius melodus) also nested in the colony. Most of the birds nested between the primary and secondary sand dunes, where the predominant vegetation was beach grass (Ammophila breviligulata) and seaside goldenrod (Solidago sempervirens). Several pairs of skimmers and Common Terns, however, nested on the ocean beach near the primary dunes where the predominant vegetation was sea rocket (Cakile edentula). Within the colony, we selected six subcolonies of skimmers for study, choosing areas that were roughly equivalent in size, substrate, and vegetative cover. To compare the effects of different amounts of disturbance on the birds' reproductive success, we o itored these subcolonies on one of three schedules. Nest checks were made either: (1) daily beginning 22 May, soon after the onset of laying; (2) weekly until hatching and daily thereafter (also referred to as weekly/daily); or (3) weekly throughout the study. In order to check the nests in a subcolony, one or two persons walked through it marking new nests (with numbered tongue depressors), recording numbers of eggs and chicks, and color-banding all new chicks with colors coded