Effects of Human Disturbance on Colonial Species, Particularly Gulls
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0042567 Effects of Human Disturbance on Colonial Species, ParticularlyGulls JOANNABURGER Department of Biology and Center for Coastal and Environmental Studies, Rutgers University, New Brunswick, New Jersey 08903 USA Colonial birds are particularly vulner- presence of vegetation and rocky or uneven able to human disturbance because of the terrain. large concentrations of birds nesting in Reproductive success can be lowered close proximity. Vulnerability varies de- either directly by causing desertion of nests, pending on species, nest location, and the eggs or young, by adults, or indirectly by type of disturbance (see Manuwal 1978). causing thermal stress, predation, and Nearness to human activities is one key cannibalism. In this paper I discuss the factor, as the potential for disturbance is direct and indirect ways human activities much greater regardless of a species' in- affect colonial birds, particularly Larids, and trinsic vulnerability. Mere distance is not I report on some of the sub-lethal effects sufficient to assess the potential for dis- of human disturbance including behaviors turbance because the surrounding habitat contributing to the lowered reproductive affects human (or predator) access. Birds success. I present results on the behavioral nesting a half mile from a town on an responses of gulls, terns, and skimmers to island surrounded by deep, rough waters human disturbance including decreased egg are surely less vulnerable than those and chick attendance, shifts in the mate in- surrounded by solid ground. Similarly, nest- cubating, movements and entanglements of ing habitat is crucial: nests on cliffs are less chicks, higher brood sizes, and greater accessible than are nests on flat ground, aggressive interactions. nests in trees are less accessible than are ground nests, and burrows are less accessible GENERAL METHODS than are surface-areas. Ground-nesting The results in this species such as gulls, terns, and some herons, presented paper were in several tern and egrets, ibises, and cormorants are thus par- gathered gull, skimmer colonies from 1970 to 1980 in- ticularly vulnerable to human disturbance Carvel, and Clam Islands because they are accessible and often nest cluding Islajo, Cedar Beach and in close proximity to human activities. (New Jersey), Jamaica Isles of Shoals The types of human activity that affect Bay Refuge (New York), National Wildlife colonial bird populations are varied, (Maine), Agassiz Refuge and at ranging from overall destruction to direct (Minnesota), Murphy (Argentina). The methods to observe intervention such as entering colonies, specific employed the effects of human disturbance varied and collecting eggs, and killing adults, eggs, or will be described in the chicks. The response of colonial birds to appropriate sections. human activities varies markedly, but no species can withstand the direct effects of killing adults, eggs, or chicks for sustained RESULTS periods of time. Assessing the effects of Nest Attendance human disturbance on nesting colonial birds is a difficult task because the observer can When disturbed frequently, gulls can never completely remove himself. The respond either by habituating (i.e. remain- problem is further complicated by the de- ing at the nest) or by decreasing nest at- creased accuracy of success measurements tendance. While studying Herring Gulls with fewer egg or chick censuses. Observa- (Larus argentatus) on Clam Island, New tion of reproductive success from afar is Jersey in 1977 (see Burger 1979 for de- usually impossible in the egg stage, and is scription of Clam Island) I compared adult complicated in the chick phase by the behavior in one section where my assistants 28 Colonial Waterbirds 4: 28-36 1981 0042568 Vol. 4, 1981] HUMAN DISTURBANCE OF GULLS 29 walked through the colony for 30 min. every I found that males usually resumed incu- other day (= disturbed) with a second bating after a disturbance (x2=8.0, df=-1, section in front of my hide where no one P < .005, based on 36 exchanges). Pre- walked among the birds. These data were sumably, energy demands are similar for taken in an area with bushes, where gulls birds incubating and for their mates stand- could not see the intruder until he ap- ing nearby. However, during the brooding proached, but they could see other gulls phase, the non-brooding parent not only flying above the head of the intruder. In rests nearby, but chases intruders that land the disturbed section the gulls flew when my near the nest, and periodically flies off to assistants were 10-12 m from the plot, obtain food for the young. Thus, females whereas in the undisturbed section the gulls displaced from brooding might be expend- did not fly when the assistants approached ing more energy foraging and defending (but walked by) as close as 5 m. The gulls nests than they would be doing in an un- in the undisturbed section had apparently disturbed colony. habituated to the nearby presence of humans (who never came into their area), Aggressive Behavior while the birds in the disturbed section re- to an human. Human disturbance in gull colonies also sponded quickly approaching increases the rate of behavior Although the birds in the undisturbed aggressive toward as well as towards section did not actually see the intruders, conspecifics human intruders. I examined the did see the cloud of mobbing gulls changes they in rates of Gulls toward above the intruder's head, and apparently aggression Herring a human intruder on Clam Island, New habituated to this mobbing behavior. Jersey in 1978. I made observations each week from early April to June, from hides Disrupted Incubation in two habitats; grass (Spartina patens), In most gulls the sexes spend equal and bushes (Iva sp.) As an intruder walked amounts of time incubating eggs and brood- steadily through the area, I recorded the number of the mean ing chicks (see Burger 1980a). Repeated diving gulls, height disturbance may disrupt this equal sharing of the dives above the intruder's head, the of incubation and brooding activities if the number of dives, and the number of attacks sexes behave differently after a disturbance. on conspecifics in 1 min. samples (10 per In 1977 I watched 12 pairs of Herring Gulls habitat). For both habitats the number of and attacks in- from a blind (8-12 hr/day) during the in- divers, dives, conspecific cubating period. All birds were color creased, while the mean height above the marked for individual identification. These intruder's head decreased toward hatching birds, in a disturbed section, were visited (Fig. 1 and 2). For all measures, the re- every other day for 30 min. during the incu- bation period. I saw 11 disturbances, which provided 132 opportunities for the disturbed birds to resume incubation after the dis- turbance. Males incubated after a dis- turbance more often than expected by chance (X2 = 13.2, df = 1, P < .01). That is, males incubating before a disturbance usually resettled on nests, whereas when females were disturbed while incubating, their mates (when present) resumed incuba- tion after the disturbance. Males did not allow females to incubate the eggs for some time afterwards. APRIL MAY JUNE In Black Skimmers (Rynchops niger), Fig. 1. Top: Mean height of Herring Gulls both sexes are usually the diving at a human intruder in the bush habitat. present during Bottom: Number of While 11 nests Herring Gulls diving, the day (Erwin 1977). studying number of dives, and the number of conspecific of skimmers at Cedar Beach, Long Island, attacks in bushes as a function of season. 0042569 30 BURGER [Colonial Waterbirds JKAO5 APRIL MAY JUNE Fig. 2. Top: Mean height of Herring Gulls diving at a human intruder in the grass. Bottom: Number EGGS CHICKS EGGS CHICKS of Herring Gulls diving, the number of dives, and BBG HG the number of conspecific attacks when a human intruder walks through the grass habitat. Fig. 3. Mean aggression rate (number of en- counters/bird x hour) when humans walked through sponses were more intense in the bushes the area (hatched bar) and when the birds were compared to the open grass. This difference undisturbed (solid bar). BBG = Great Black- was partially due to differences in nesting backed Gull, HG = Herring Gull, Eggs = birds were = density. In the bush habitat several gulls incubating, chicks parents had chicks over one day old. nested within 5 m of each other, whereas internest distances in the grass habitat aggression rates were lower in the egg phase rarely were as low as 8 m. Gulls nesting in (May) compared to the chick phase (early the bushes usually (at hatching) flew to June); and lower in the undisturbed period within 2 m of the intruder's head and fre- compared to the disturbed period. When quently struck his head; whereas gulls nest- two people walked through the plot aggres- ing in the grass never touched the intruder sion rate for Herring Gulls were about five and rarely came closer than 5 m. Thus, ag- times greater, and those for Black-backed gression stimulated by the presence of a Gulls were four to 18 times greater than human varied temporally with stage of they were in periods when the birds were nesting and as a function of nest density not being disturbed. I then examined the and habitat. aggressive behaviors involved (Fig. 4). I further examined disturbance-induced When undisturbed by humans both species changes in aggressive behavior in a mixed tended to use low intensity behaviors toward species colony of Herring Gulls and Great an avian intruder, such as walking toward Black-backed Gulls (L. marinus) located on the intruder, long-calling, grass-pulling, or Appledore Island (Isles of Shoals, Maine, flying at the intruder (ground chase). Such see McGill 1977), in late May and early encounters rarely evolved into ground fights June 1980.