Carl Safina

Estimating Annual Survival and Movement Rates of Adults within a Metapopulation of Roseate Terns

Several multistratum capture-recapture models were used to test various hypotheses about possible geographic and temporal variation in survival, movement, and recapture/resighting probabilities of 2399 adult Roseate Terns (Sterna dougallii) color-banded from 1988 to 1992 at the sites of the four largest breeding colonies of this species in the northeastern USA. Linear-logistic ultrastructural models also were developed to investigate possible correlates of geographic variation in movement probabilities. Based on goodness-of-fit tests and comparisons of Akaikes Information Criterion (AIC) values, the fully parameterized model (Model A) with time- and location-specific survival, movement, and capture probabilities, was selected as the most appropriate model for this metapopulation structure. With almost all movement accounted for, on average >90% of the surviving adults from each colony site returned to the same site the following year. Variations in movement probabilities were more closely associated with the identity of the destination colony site than with either the identity of the colony site of origin or the distance between colony sites. The average annual survival estimates (0.74-0.84) of terns from all four sites indicate a high rate of annual mortality relative to that of other species of marine birds.

Effects of Human Disturbance on Reproductive Success in the Black Skimmer

-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

Common tern foraging-seasonal trends in prey fish densities and competition with bluefish

We present a method for making direct quantitative measurements of population dymamics of fish species preyed upon by seabirds. We studied foraging behavior of Common Terns (Sterna hirundo) in relation to density of fish prey of Long Island, New York. We used an echo sounder to record the abundance of small prey fish and large predatory fish near a tern colony. Prey fish abundance was highest when terns were feeding young chicks and declined sharply later in the season, coincident with the arrival of large numbers of predatory bluefish (Pomatomus saltarix). Mean depth of occurrence of prey fish was shallower after the arrival of bluefish. Terns fished in areas with high concentrations of prey, but there was no correlation between numbers of feeding terns and prey density. Number of feeding terns was inversely correlated with prey depth, however. Terns were strongly attracted to predatory fishes feeding near the surface. We suggest that in addition to concentrating prey fish near the surface, predatory fish may have been important as competitors.

Factors affecting vigilance in springbok: importance of vegetative cover, location in herd, and herd size

Abstractu2008u2008Vigilance in vertebrates is often inversely related to group size. We present evidence that distance to bushes and location within the herd are also critical factors in vigilance in springbok (Antidorcas marsupialis) in Etosha National Park, Namibia, where they are the preferred prey of cheetahs (Acinonyx jubatus). Most springbok feed in heterospecific herds, both by grazing on grass and browsing on bushes. We studied 1245 animals; variations in vigilance (time alert) were explained by location within the herd, distance to bushes and roads, number of springbok in each herd, and gender and age. Vigilance time decreased with increasing herd size, with increasing distance to bushes and roads, and with density. Springbok on the edge of herds devoted significantly more time to vigilance than did those in other locations, and vigilance in edge animals decreased with group size. Adults were more vigilant than young, and males were more vigilant than females. Position within the herd, and distance from bushes, were the most important variables influencing vigilance. Location in the herd and gender/age affected both browsing and grazing springbok, although other factors accounted for the differences in vigilance between browsing and grazing springbok: 1) group size was not significant for browsers, but it was for grazers, and 2) distances to bushes and road were not significant for browsers, but they were for grazers. These data relate to the risk from predators and the benefits from other group members. Springbok in bushes cannot see all members of the herd, cannot derive early warning from many group members, and are more at risk from predators because the latter can hide in the bushes.

Evidence for Prey Limitation of Common and Roseate Tern Reproduction

Etude de la dynamique de la reproduction des 2 sternes a plusieurs stades (de la ponte a lenvol) pour savoir quand et comment les oiseaux sont sensibles aux fluctuations de population de leur proie. Deux hypotheses sont testees: 1) la disponibilite de la nourriture influence la date de reproduction; 2) les variations dans labondance et la densite des proies sexpriment dans la «productivite» des oiseaux (taille des couvees, ...)

TEMPORAL PATTERNS IN REPRODUCTIVE SUCCESS IN THE ENDANGERED ROSEATE TERN (STERNA DOUGALLII) NESTING ON LONG ISLAND, NEW YORK, AND BIRD ISLAND, MASSACHUSETTS

Roseate Terns (Sterna dougallii) nest in few colonies in the northeastern United States, and the population is listed as endangered. We compare reproductive success from 1987 through 1990 at Cedar Beach, Long Island, and in 1980 and 1987 through 1990 at Bird Island, Massachusetts, to examine yearly and seasonal differences. Productivity was highest for terns breeding in the first six days of the egg-laying period and decreased thereafter. Clutch size, hatching success, and productivity declined significantly during the season, with some variations in pattern among years and between colony sites. Pairs initiating nests after 22 June fledged almost no young. Patterns of reproductive success were more irregular at the smaller colony (Cedar Beach) than the larger colony. Reproductive success was related to age of adults; young birds (two to three years old) had lower clutch sizes, had lower reproductive success, and laid later than older birds. The effective reproductive population of the colony included primarily birds that bred in the early and peak periods; thus, moni- toring reproductive success only from early or peak nests overestimates overall reproductive success. Selection against even earlier breeding in this species may be due to lower food resources early in the season and higher predation rates on early nests. Received 20 April 1994, accepted 2 July 1994.

Bluefish Mediation of Foraging Competition between Roseate and Common Terns

The presence and foraging activities of predatory bluefish (Pomatomus saltatrix) differentially affect the foraging success of Roseate (Sterna dougallii) and Common Terns (S. hirundo), and depress the foraging activities of Roseate Terns. In the absence of bluefish, Roseate Terns had higher mean fishing success frequencies than Common Terns. This advantage was lost in the presence of bluefish, and Roseate Terns foraging success was inversely correlated with bluefish feeding intensity. Roseate Terns left foraging flocks more frequently as predatory fish activity increased and flock density increased. Conversely, the rate at which Common Terns left flocks was inversely correlated with predatory fish feeding and was unrelated to flock density. Bluefish may be considered keystone competitors in this system. See full-text article at JSTOR

Hibernacula and Summer Den Sites of Pine Snakes (Pituophis melanoleucus) in the New Jersey Pine Barrens

We examined eight summer dens (used only in summer) and seven hibernacula (occupied both in winter and summer) of the snake Pituophis melanoleucus in the New Jersey Pine Barrens, comparing above ground characteristics of hibernacula and summer dens with characteristics at nearby random points. Temperatures at the soil surface and at 10 cm depth were significantly warmer, and there was less leaf cover around the random points compared to the entrances of the hibernacula and summer dens. Hiber- nacula had significantly more vegetation cover within 5 m, more leaf cover over the burrow entrance, and were closer to trees than were summer dens. Most hibernacula and summer dens were beside old fallen logs (73%), the entrance tunnels following decaying roots into the soil. Excavation of the hibernacula and summer dens indicated that most hibernacula appeared to be dug by the snakes and had an average of eight side chambers and 642 cm of tunnels, compared to less than one side chamber and 122 cm of tunnels for summer dens. Except for hatchlings, most snakes in hibernacula were located in individual chambers off the main tunnel; all snakes were at depths of 50-111 cm (X = 79 cm). Pine snakes may select optimum

Population interactions among free-living bluefish and prey fish in an ocean environment

SummaryWe used sonar to measure relative abundance, location, and depth of prey fish schools (primarily Anchoa and Ammodytes) in the ocean near Fire Island Inlet, New York from May to August for 4 years to examine predatorprey interactions. Prey fish numbers built through May, peaked in June, and thereafter declined coincident with the arrival of predatory bluefish. Bluefish abundance and feeding behavior correlated inversely with prey fish abundance and depth. Bluefish may drive seasonal patterns of prey abundance and distribution in this area through direct predation and by causing prey to flee.

Colony differences in response to trapping in roseate terns

Both members of seabird pairs are normally required to fledge young. Seabirds that nest in sites accessible to predators usually have one parent in attendance during the egg/chick phase (Burger and Gochfeld 199 1 a). Time devoted to foraging can vary with individual skill and age, prey availability and abundance (Searcy 1978), and distance to foraging grounds (Safina 1990). Although average skill of similar-aged individuals should not vary from colony to colony (Ryder 1980), prey availability and abundance, and spatial distribution of foraging grounds may vary. Thus, the percent of time both members of a pair are present at the nest site may vary in different colonies. In this paper, we examine parental behavior in response to trapping in Roseate Terns (Sterna dougallii) nesting in three of the six major colonies in the northeastern United States: Cedar Beach, New York; Falkner Island, Connecticut; and Bird Island, Massachusetts. Roseate Terns were listed on the United States Endangered Species List in 1987. We were interested in differences among colonies in how often both parents were present, how soon a mate returned to the nest if one parent was temporarily removed, how soon a trapped bird returned to the nest after release, and the time during which the nest was left unguarded. We feel it is important to recognize and make management decisions based on colony differences where they exist. Our study follows directly from earlier work at Cedar Beach on trapping vulnerability of Common Terns (Sterna hirundo) and Roseate Terns (Burger and Gochfeld 1991 b). Nisbet (1981 la) reported that Roseate Terns at Bird Island required about three hours to return to the nest after trapping.