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I C Nisbet

Publications and source records attributed to I C Nisbet.

12 recordsLinked to original sources
PubMed2001

Detecting and measuring senescence in wild birds: experience with long-lived seabirds.

This paper points out and discusses several practical and methodological problems that arise in attempts to detect and measure senescent declines in survival or breeding performance of wild animals, with specific emphasis on long-lived seabirds. Birds have no anatomical markers of age, so studies of age-related biology require marking individuals at the time of hatching and following them throughout their lives. Seabirds live longer than the working lifespan of biologists, and longer than the turnover times of study techniques or theories of senescence. Seabirds are exposed to changing environmental and demographic conditions and cannot be assumed to be in demographic equilibrium. Sample sizes of the oldest age-classes are always small, requiring either marking very large numbers of birds at hatching or continuing studies of old birds over many years. Incomplete sampling requires the use of mark-recapture models that have only been developed in the last 20years. Mortality selection resulting from demographic heterogeneity (selective survival of high-quality individuals) can offset or confound the effects of senescent changes within individuals. Many of these problems are amenable to solution and will be probably solved within a few years. In the meantime, this paper recommends that reviewers should be cautious about accepting published reports of senescent declines in natural populations.

Aging↗
PubMed2001

Maternal steroids and contaminants in common tern eggs: a mechanism of endocrine disruption?

We looked for evidence for the hypothesis that exposure of female birds to polychlorinated biphenyls (PCBs) results in alteration of blood steroid hormone concentrations and alters subsequent hormone transfer of steroids to eggs. Eggs of three-egg clutches were collected from a PCB-exposed common tern (Sterna hirundo) colony (Ram Island, Buzzards Bay, MA, USA) and from a relatively clean colony (Bodkin Island, Chesapeake Bay, MD, USA), and were analyzed for concentrations of organochlorine contaminants and steroid hormones (17beta-estradiol, 5alpha-dihydrotestosterone, testosterone and androstenedione). There was no relationship between total PCBs and steroid concentrations considering all eggs together, considering eggs of different laying order or considering differences between sequentially laid eggs in a clutch. Similarly, concentrations of di- and tri-chlorinated biphenyls and steroids in eggs were not related. The concentrations of PCBs, mercury and selenium were below estimated thresholds for toxicity to embryos. Maternal steroids, except estradiol, were present in yolk of all eggs, with increasing concentrations in the second and third eggs laid. Our data provided no evidence for a maternal toxicological event that might alter the amount of maternal steroid hormone transferred to eggs.

Androstenedione↗
PubMed1999

Endocrine patterns during aging in the common tern (Sterna hirundo).

Blood samples were taken from breeding common terns (Sterna hirundo) of known age during two successive breeding seasons to determine if plasma concentrations of estradiol, progesterone, androgen, and luteinizing hormone (LH) reflect the process of aging. Males and females were trapped and sampled once between laying of the first and the second eggs in the clutch. The transient capture and sampling did not disrupt incubation or the timing of the subsequent oviposition. Ages ranged from 2 to 21 years, with most in the range 4-11 years. There was a strong inverse correlation (P < 0.0001) between age and laying date in both females and males, with older birds nesting earlier in the season. Plasma LH levels increased (P < 0.005) among mature (>5 year) females. Plasma progesterone and E2 were positively correlated with each other among mature females, but did not change in an age-dependent manner. In males, plasma progesterone levels showed no age-related changes. However, maturing (3-5 year) males showed increasing plasma androgen levels (P < 0.01); mature males showed no change with age. Plasma LH levels were correlated with androgen levels in both maturing and mature males. Plasma progesterone declined with age in maturing males (P < 0.001). These data provide evidence for changing endocrine status with age, but the patterns differ for younger individuals (</=5 year) and mature birds. It appears that in younger individuals, nesting experience and maturity of the pair affect reproductive performance and endocrine status. Individual variability was high for all hormones, possibly masking age-related changes. Only breeding individuals were sampled at all ages, although the proportion of the population that continues to reproduce may decline as the birds age. In summary, common terns continue to breed successfully until at least age 21 years and these individuals maintain relatively stable endocrine status during aging. Although there was some indication of declines in estradiol and androgens among the oldest birds (16-21 years), these declines were not statistically significant and were manifested in only a small fraction (<4%) of the breeding population.

Aging↗
PubMed1994

Heavy metal and selenium levels in feathers of known-aged common terns (Sterna hirundo).

Concentrations of five metals and selenium in the breast feathers of known-aged common terns (Sterna hirundo) were examined at a breeding colony in Massachusetts, USA. Concentrations of selenium, chromium, and manganese increased significantly with age among adults (2-21 years old), whereas concentrations of mercury, cadmium, and lead did not. Concentrations of cadmium, selenium and manganese were lower in fledglings (20-23 days old) than in adults. Concentrations of mercury, however, were higher in fledglings than in adults, and concentrations of chromium were higher in fledglings than in young adults, probably reflecting higher exposure to these metals in the breeding area than in the winter quarters where the adults' feathers were grown. At least for mercury, excretion of metals into the feathers at each molt was an efficient protective mechanism, preventing continued accumulation in the body with increasing age.

Aging↗
PubMed1992

Toxic equivalency factors (TEFs) for polycyclic aromatic hydrocarbons (PAHs).

The polycyclic aromatic hydrocarbons (PAHs; also referred to as the polynuclear aromatic hydrocarbons or PNAs) are commonly encountered at hazardous waste sites and are often the focus of site remediation activities. However, toxicity criteria are not available for all the PAHs. In the past, EPA has assessed risks posed by mixtures of PAHs by assuming that all carcinogenic PAHs are as potent as benzo[a]pyrene (B[a]P), one of the most potent PAHs. The available information on the toxicity of the PAHs suggests that most are considerably less potent than B[a]P and therefore, the EPA approach is likely to overestimate risks. Several approaches have been developed to allow the relative potency of the different PAHs to be considered in a site-specific risk assessment. This paper evaluates these approaches and presents a modified version that we feel more accurately reflects the state of knowledge on the relative potency of these compounds.

Animals↗
PubMed1992

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 bird's tissues reflect the bird's 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.

Animals↗
PubMed1978

Productivity of ospreys in Connecticut--Long Island increases as DDE residues decline.

Nesting success of ospreys (Pandion haliaetus) breeding in the Connecticut--Long Island area has increased since 1973 and is now approaching the levels recorded prior to the 1950's. Simultaneously, DDE and dieldrin residues have declined in unhatched eggs. Levels of polychorinated biphenyls have shown no changes over the period 1969 to 1976. The increase in productivity is attributed primarily to lower levels of DDE contamination. Detrimental effects in the past on ospreys in the Connecticut River estuary are attributed to local contamination with dieldrin.

Animals↗