Robert W. Furness

Factors That Influence Assimilation Rates and Fractionation of Nitrogen and Carbon Stable Isotopes in Avian Blood and Feathers

By switching great skuas Catharacta skua from one isotopically distinct diet to another, we measured diet‐tissue discrimination factors and tested the assumption that dietary nitrogen and carbon isotope signatures are incorporated into blood and feathers at similar rates. We also examined the effects of metabolic rate and looked for evidence of isotopic routing. We found that blood δ15N and δ13C signatures altered after the diet switch at similar rates (14.4 d and 15.7 d, respectively). Qualitative analyses imply that the same was true with feathers. Mass balance calculations suggest that only a small amount of lipid is likely to be involved in the synthesis of blood and feathers. Differences in diet‐tissue discrimination factors before and after the diet switch may mean that toward the end of the experiment, some of the nutrients for blood synthesis had been coming from stores. Repeated measures mixed models provided evidence that increases in metabolic rate might accelerate fractional turnover rates in blood. There is a need for more laboratory‐based experimental isotope studies in order to address further questions that this study has raised.

Global Seabird Response to Forage Fish Depletion—One-Third for the Birds

One-third of maximum fish biomass must be available for seabirds to sustain high breeding success. Determining the form of key predator-prey relationships is critical for understanding marine ecosystem dynamics. Using a comprehensive global database, we quantified the effect of fluctuations in food abundance on seabird breeding success. We identified a threshold in prey (fish and krill, termed “forage fish”) abundance below which seabirds experience consistently reduced and more variable productivity. This response was common to all seven ecosystems and 14 bird species examined within the Atlantic, Pacific, and Southern Oceans. The threshold approximated one-third of the maximum prey biomass observed in long-term studies. This provides an indicator of the minimal forage fish biomass needed to sustain seabird productivity over the long term.

Using bird feathers to measure mercury in the environment relationships between mercury content and moult

Abstract Feathers can be used to monitor mercury levels in marine, freshwater and terrestrial ecosystems. Previous studies have often failed to take into account the great differences in mercury levels between feathers of individual birds. Feathers replaced early in the moulting sequence have higher levels of mercury than those moulted later. The widely held idea that mercury levels in feathers reflect dietary intake at the time of feather growth is not supported by our data. We suggest that the amount of mercury stored in body tissues is the main factor determining levels in plumage. Although they are most often used, remiges and rectrices may not be the most suitable feathers if mercury levels in birds are to be examined: body feathers provide the most representative sample for estimating whole-bird mercury content.

Changes in fisheries discard rates and seabird communities.

It is clear that discards from commercial fisheries are a key food resource for many seabird species around the world. But predicting the response of seabird communities to changes in discard rates is problematic and requires historical data to elucidate the confounding effects of other, more ‘natural’ ecological processes. In the North Sea, declining stocks, changes in technical measures, changes in population structure and the establishment of a recovery programme for cod (Gadus morhua) will alter the amount of fish discarded. This region also supports internationally important populations of seabirds, some of which feed extensively, but facultatively, on discards, in particular on undersized haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus). Here we use long-term data sets from the northern North Sea to show that there is a direct link between discard availability and discard use by a generalist predator and scavenger—the great skua (Stercorarius skua). Reduced rates of discarding, particularly when coupled with reduced availability of small shoaling pelagic fish such as sandeel (Ammodytes marinus), result in an increase in predation by great skuas on other birds. This switching of prey by a facultative scavenger presents a potentially serious threat to some seabird communities.

Changing priorities: the effect of pre-migratory fattening on the trade-off between foraging and vigilance

SummaryMany birds accumulate fat reserves prior to departure on long distance migration. Since there will be an increased food requirement during the pre-migratory period, it is to be expected that more time will be invested in foraging, at the expense of other activities. The allocation of time to anti-predatory behavior in migratory ruddy turnstones (adults) was found to decrease prior to migration (Fig. 1); non-migratory individuals (juveniles) showed no decrease over the same time period (Fig. 2). This is interpreted as a change in the optimal adult behavior, the cost of a reduced rate of resource accumulation outweighing the additional risk of predation which results from the decrease in vigilance.

Mercury accumulation and excretion in laboratory reared black-headed gullLarus ridibundus chicks

Distribution of mercury between tissues was investigated in black-headed gull (Larus ridibundus) chicks fed doses of methyl mercury. Mercury accumulated differentially in the internal tissues, concentrations in the kidney exceeding those in the liver, which in turn exceeded those in the muscle. All feather types contained higher mercury concentrations than internal tissues. For each dose group, there was a progressive and pronounced reduction in the concentrations of mercury found in the primary feathers as the growth sequence progressed. The amount of mercury given to the birds affected the proportion of mercury deposited in the kidney, carcass, and primaries. The amount of mercury administered had no effect on the proportion of mercury excreted either in the faeces or the feathers. 71% of the doses given was excreted over the fledging period, and 49% of the dose was found in the plumage. These figures allow a more quantitative approach to measuring mercury pollution.

The demands of incubation and avian clutch size

We reviewed information on the demands of incubation to examine whether these could influence the optimal clutch size of birds. The results indicate that appreciable metabolic costs of incubation commonly exist, and that the incubation of enlarged clutches can impose penalties on birds. In 23 studies on 19 species, incubation metabolic rate (IMR) was not elevated above the metabolic rate of resting non‐incubating birds (RMR), but contrary to the physiological predictions of King and others, IMR was greater than RMR in 15 studies on 15 species. Across species, IMR was substantially above basal metabolic rate (BMR), averaging 1.606 × BMR. Of six studies on three species performed under thermo‐neutral conditions, none found IMR to be in excess of RMR. IMRs measured exclusively within the thermo‐neutral zone averaged only 1.08 × BMR contrasting with the significantly higher figure of 1.72 × BMR under wider conditions. 16 of 17 studies on procellariiforms found IMR below RMR, indicating a significant difference between this and other orders. We could find no other taxonomic, or ecological factors which had clear effects on IMR. Where clutch size was adjusted experimentally during incubation, larger clutches were associated with: significantly lower percentage hatching success in 11 of 19 studies; longer incubation periods in eight of ten studies; greater loss of adult body condition in two of five studies; and higher adult energy expenditure in eight of nine studies. Given that incubation does involve metabolic costs and given that the demands of incubation increase sufficiently with clutch size to affect breeding performance, we propose that the optimal clutch size of birds may in part by shaped by the number of eggs the parents can afford to incubate.

Mercury accumulation in great shuas Catharacta skua of known age and sex, and its effects upon breeding and survival

(1) Great skuas Catharacta skua Brunnich have among the highest tissue mercury concentrations of British seabirds, and many of the birds breeding on Foula, Shetland in 1988 and 1989 had concentrations well in excess of those known to affect reproduction adversely in some terrestrial bird species. Comparison of mercury concentrations in feathers of great skuas collected before 1900 and in 1988-89 indicates a significant increase during this century, which is assumed to be due to anthropogenic activities (Thompson 1989). (2) Mercury in muscle tissues of great skuas was entirely methyl mercury, whereas approximately half the mercury in liver and kidney tissue was inorganic. Feather mercury concentrations correlated with total mercury concentrations in soft tissues of birds culled during incubation. (3) Mercury concentrations in feathers of adult great skuas on Foula, Shetland were found to be higher than those in feathers of chicks, but were independent of adult age and sex. (4) There was no evidence of accumulation of inorganic mercury in soft tissues with age. This suggests that dietary variation and specialization are more important than age as determinants of mercury concentrations in this species. (5) There was no relationship between diet during the breeding season and feather mercury concentration, either comparing mercury concentrations in different years covering a period of major change in diet at the colony, or comparing different birds with markedly different diets in the same year. Great skuas disperse widely during the winter, and differences in exposure to mercury during this period may be more important than differences in diet during the breeding season. (6) Despite evidence of increased mercury pollution (Thompson 1989), there was no evidence of a relationship between the mercury concentrations of individuals and their breeding performance or survival.

A review of the use and the effects of marks and devices on birds

This paper reviews the use and the effects of marks and devices on birds. Although most papers reporting research on birds make use of marks or devices, very few studies test for harmful effects or data biases caused by these. Many research projects have used marks and devices without encountering any harmful effects of these on the birds being studied. However, where assessments have been made it is clear that all of the methods of marking can have adverse effects, while most devices attached to birds do alter their behaviour to some extent. We conclude that more attention should be given to these effects before the normal assumption of no influence on the biological parameters being studied can be made with confidence. There is a need for careful tests of the effects of marking methods to be undertaken. We hope that this review will assist people studying birds to plan their use of marks and devices in such a way as to minimize problems caused by these research tools.

Mercury levels in eggs, tissues, and feathers of herring gulls Larus argentatus from the German Wadden Sea Coast.

The relationships between mercury levels in eggs, tissues, and feathers of male and female herring gulls caught at their nests at a colony on the German Wadden Sea Coast were investigated, and an assessment of mercury intake and excretion of these birds was made. Samples of the liver, ovary, pectoral muscle, and body feathers, as well as the primary feather (in some cases), and eggs were taken from 37 adult herring gulls. Analysis of total mercury in all samples showed that body-feather and tissue levels were independent of sex. There was, however, a significant difference between levels of mercury in the primary feathers of male and female birds. Egg levels were not correlated to pectoral muscle, ovary, or feather levels although they were positively correlated with mercury levels in the liver. Liver levels were in turn correlated with mercury levels in the ovary. The ratio of mean feather to mean egg concentration ranged from 3.7 to 5.5 according to which feather was used. It was estimated that herring gulls from the Wadden coast ingested between 825 and 1337 microg of mercury in the year prior to analysis. It was also estimated that female birds may excrete over 20% more mercury via their eggs than could be excreted by male birds.