Ian C. T. Nisbet

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.

Older parents are less responsive to a stressor in a long-lived seabird: a mechanism for increased reproductive performance with age?

In many taxa, reproductive performance increases throughout the lifespan and this may occur in part because older adults invest more in reproduction. The mechanisms that facilitate an increase in reproductive performance with age, however, are poorly understood. In response to stressors, vertebrates release glucocorticoids, which enhance survival but concurrently shift investment away from reproduction. Consequently, when the value of current reproduction is high relative to the value of future reproduction and survival, as it is in older adults, life history theory predicts that the stress response should be suppressed. In this study, we tested the hypothesis that older parents would respond less strongly to a stressor in a natural, breeding population of common terns (Sterna hirundo). Common terns are long-lived seabirds and reproductive performance is known to increase throughout the lifespan of this species. As predicted, the maximum level of glucocorticoids released in response to handling stress decreased significantly with age. We suggest that suppression of the stress response may be an important physiological mechanism that facilitates an increase in reproductive performance with age.

Variations in growth of Roseate Tern chicks

We measured growth parameters of Roseate Tem (Sterna dougallii) chicks at two contrasting colonies: Bird Island, Massachusetts, and Falkner Island, Connecticut, from 1987 through 1990. Differences in growth parameters among individual chicks are used to explore differences in parental performance, whereas differences in average growth parameters among colonies and years are used to explore differences in average environmental conditions. Chicks were marked at hatching and weighed at one- or two-day intervals. For each chick that survived to fledging, we calculated two measures of growth: linear growth rate and asymptotic mass. There were no significant differences between growth parameters of first-hatched chicks in broods of one and two. Both measures of growth were significantly lower and more variable for second-hatched chicks than for first-hatched chicks. Both measures were significantly positively related to egg mass and negatively related to hatch date. After controlling for these variables, differences between colonies and among years were relatively small and inconsistent. Mean survival of second-hatched chicks was positively correlated with the mean growth rate of survivors across colonies and years. These findings are consistent with a general hypothesis that growth of chicks reflects individual parental performance and parental quality

Breeding performance of very old Common Terns

Abstract We studied the breeding performance of very old Common Terns (Sterna hirundo) at Bird Island, USA (41°40′N, 70°43′W), in 1999. We studied birds aged ≥18 yr, comprising the oldest 5% of the birds breeding at this site. We located 24 of these old birds and measured their breeding performance, including laying dates, clutch sizes, egg masses, hatching success, chick growth, chick survival, and overall productivity. We compared their performance with that of several reference groups, including a date-matched group of young birds aged 6–9 yr and date-matched birds of all ages. Breeding performance declined with laying date in all groups. The old birds were more successful in raising chicks than any of the reference groups. Within the group of old birds, breeding performance remained high or improved with age, even after controlling for effects of laying date. Males and females were represented equally among the group of old birds; there was no evidence that performance was higher among old males or old females. If the Common Terns at this site experience any senescent decline in breeding performance within their 20–25 yr lifespan, this must be offset either by increased reproductive effort or by selective survival of high quality individuals.

VARIATIONS IN GROWTH OF ROSEATE TERN CHICKS: II. EARLY GROWTH AS AN INDEX OF PARENTAL QUALITY'

We measured growth of Roseate Tern (Sterna dougallii) chicks at a colony in Connecticut in 10 successive years, 1987-1996. Data on body mass during the first 3-4 days of life were fitted to a quadratic regression model, yielding three parameters of early growth for each of 1,551 chicks: mass at hatching, linear growth, and quadratic growth. First chicks in each brood (A-chicks) exceeded second chicks (B-chicks) in each of the three growth parameters; A-chicks from broods of two grew faster during the first 3 days than single chicks. The three parameters of early growth depended upon egg mass, hatch order, hatch date, and year, but not on parental age after controlling for effects of the other vari- ables. The linear and quadratic growth parameters were negatively correlated. Subsequent growth and survival of chicks were predicted by all three parameters of early growth. After controlling for effects of early growth, none of the other variables measured (hatch date, egg mass, parental age, hatching asynchrony, female-female pairing, or trapping) contributed significantly to explaining later growth or survival. Year effects were substantial in only 2 of the 10 years of study. Individual pairs were consistent in performance (as indexed by early growth) in successive years. These results suggest that growth and survival of Roseate Tern chicks are determined primarily by parental quality; much of the information about parental quality is expressed by the time the eggs are laid, and most of it is expressed by the time the chicks are three days old.

Metal levels in regrown feathers: Assessment of contamination on the wintering and breeding grounds in the same individuals

Birds are useful indicators of environmental contamination because they are relatively large, conspicuous, top predators in food chains. However, concentrations of contaminants in a birds tissues reflect the birds exposure over wide temporal and spatial scales. Birds are most useful as monitors of exposure when these scales are known. In this paper we report concentrations of lead, cadmium, mercury, and selenium in breast feathers of common terns (Sterna hirundo) and roseate terns (S. dougallii) trapped during incubation at breeding colonies in New York and Massachusetts. Terns arrived on the breeding grounds with breast feathers grown on their wintering grounds, and regrew certain feathers that were plucked for analysis. The regrown feathers were themselves plucked, and both sets of feathers were analyzed. For roseate terns at Cedar Beach and common terns at both sites there was a significant increase in mercury levels in the feathers grown on the breeding grounds compared to those grown on the wintering ground. The differences in mercury were far greater at Bird Island than at Cedar Beach. Selenium levels at Cedar Beach were higher for the regrown feathers than the initial feathers for roseate terns, but not for common terns. Lead and cadmium levels were not significantly different at either site for either species. These results suggest that terns are exposed to significantly higher levels of mercury in the northeastern United States than they are in the wintering grounds in South America.

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.

Age-related changes in efficiency among breeding Common Terns Sterna hirundo: measurement of energy expenditure using doubly-labelled water

We used doubly-labelled water to measure field metabolic rates of breeding Common Terns Sterna hirundo, and hence to investigate differences in reproductive efficiency among birds of different ages. Field metabolic rates, activity patterns, changes in body mass, chick provisioning rates, and chick growth rates were measured for 14 males of known age (6-24 yr) during a 48-h period immediately after hatching of the first chick, when males provide most of the food. Males expended energy at high rates (3.3-6.3 x estimated basal metabolic rates). We found no significant relationship between male age and energy expenditure. Some aspects of male performance (feeding rates and food provisioning rates) were age-related due to changes in efficiency. Efficiency in the use of time and energy was low in the youngest breeding males, increased thereafter to peak at about 12 years of age, but appeared to decline again amongst the oldest breeding males. Changes in body mass and measures of activity were not correlated with energy expenditure and hence were not appropriate measures of reproductive effort.

Contribution of research to management and recovery of the roseate tern: review of a twelve-year project

-The Northwest Atlantic population of the Roseate Tern (Sterna dougallii) is largely confined to a small breeding area along the northeast coast of the USA between 400 and 420N. This population was listed as endangered in the USA in 1987 because it was dangerously concentrated into a few breeding sites (85% on two islands in the 1980s). The nesting population in the area from Long Island, New York to Cape Cod, Massachusetts has been studied intensively since 1987, in conjunction with a program of management of the breeding colonies. This paper summarizes the results of the research program and discusses the extent to which it has contributed to effective management. The regional population now numbers about 4,000 breeding pairs and has been increasing slowly since 1987, except between 1991 and 1992 when it declined by about 17%. Noteworthy features of the demographic data collected since 1987 are: skewed adult sex-ratio (about 127 F to 100 M), high average productivity (1.0-1.2 fledglings per pair), low annual adult survival (0.83), and probably low survival from fledging to first breeding (about 0.2). This species is a specialized forager and may be limited within this region by the distribution of suitable feeding sites. When this regional population was listed as endangered in 1987, managers postulated that predation and displacement by gulls were important factors limiting numbers and productivity. Research since 1987 has suggested that the primary effect of gulls is to limit the number of secure sites available for nesting, and that high postfledging mortality and skewed sex-ratios are probably more important as limiting factors on population size. Research activities have contributed both directly and indirectly to management of the regional population, but it has taken longer than expected to obtain needed information on demographic parameters, causes of mortality, and other limiting factors. Received 5 February 1999, accepted 18 April 1999.

Characteristics and Performance of Common Terns in Old and Newly-established Colonies

Abstract In 1999, we compared demographic and biological parameters in Common Terns (Sterna hirundo) among three colonies located 10-26 km apart in Buzzards Bay, Massachusetts, USA. The three colony sites differed primarily in size, number of pairs, and length of occupancy by terns: Penikese Island (8 ha, 132 pairs, 3 yr), Ram Island (1.1 ha, 1,900 pairs, 8 yr), Bird Island (0.6 ha, 1,800 pairs, >65 yr). This study compared parental ages and origins, nesting phenology, clutch-size, chick growth rates, productivity, chick diets, feeding rates and foraging trip times among the three colonies. Common Terns breeding at the new colony on Penikese Island were significantly younger (mean age 6.8 yr) and laid later (mean laying date 25 May), had higher productivity (2.2 fledged chicks/pair) and higher chick growth rates than those at the old colony on Bird Island (means 11.7 yr, 20 May, 0.8 chicks/pair, respectively). Feeding rates were highest and foraging trip times for all types of prey were shortest at Penikese Island. Birds at Ram Island were intermediate in all these respects. Although productivity of Common Terns at Bird Island was relatively low during the period in which the other two colonies were established (1992-1999), both breeding adults at Bird Island and chicks raised there have been very slow to move to the other colonies, which were colonized mainly by immigrants from outside the region.