Jouke Prop

Spring staging in Brent Geese Branta bernicla: feeding constraints and the impact of diet on the accumulation of body reserves

SummaryThe diet composition of Brent Geese Branta bernicla on a salt-marsh was quantified. Puccinellia maritima was the principal food species, while Plantago maritima and Triglochin maritima were less commonly taken. Festuca rubra only acted as a substitute for Puccinellia when production of the latter species dropped. The metabolizable energy of the food plants ranged from 5 to 11 kJ·g−1. By assessing the ingestion rates of geese feeding on different food species, the net intake rate could be derived. Plantago and Triglochin appeared to be the most profitable plants to eat. The proportion of these species in the diet was restricted by (1) the capacity of the alimentary tract, since high intake rates combined with high water contents of the food plants easily led to overfill; and (2) the limited distribution of these plants, in combination with their rapid depletion by grazing geese. These latter factors led to an unequal allocation among individual geese. Most Plantago and Triglochin was obtained by dominant pairs within the flocks. The high quality of Puccinellia allowed geese to gain mass in spring, but the metabolizable energy of this plant species declined during the staging period, and Plantago and Triglochin increased in importance in supplying the geese with components with which to build their body reserves. The timing of the onset of spring growth of the various food species differed between years, and plant phenology was shown to have a profound effect on the final body reserves of the geese.

Predator protection or similar habitat selection in red-breasted goose nesting associations: extremes along a continuum

Abstract We tested the predator protection and similar habitat hypotheses in relation to red-breasted goose, Branta ruficollis , nesting associations. Geese began laying 1–3 weeks after all associated species. In almost all cases they nested on the mainland only if raptors were also present and always followed raptors when they changed eyries between years. They selected peregrines, Falco peregrinus , and snowy owls, Nyctea scandiaca , as associates in preference to rough-legged buzzards, Buteo lagopus , even though the latter were several times more abundant along river corridors. Nest defence experiments with a surrogate Arctic fox, Alopex lagopus , showed that this could be explained by differences in nest defence intensity, rather than habitat types selected. Similar experiments also suggested that gulls were much less aggressive than owls or falcons. Foxes rarely approached, and were easily repelled from goose colonies associated with owls and peregrines, but gulls were apparently incapable of repelling foxes. Breeding success was much higher for geese nesting with raptors than for those on islands and geese apparently preferred to nest with owls in peak lemming years than to remain on islands. These findings support the hypothesis that red-breasted geese actively choose to nest near raptors; however, they probably associate with gulls primarily because both species select fox-free islands. Compared with other studies, red-breasted geese nesting with raptors and on islands apparently represent two extremes in a continuum of nesting associations generally seen in birds. We discuss why the behaviour might have evolved and argue that this may be the only known bird species whose evolution has been facilitated primarily by the exploitation of the nest defence behaviour of aggressive raptorial hosts. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

The spring range of barnacle geese Branta leucopsis in relation to changes in land management and climate

This study examines recent changes in the distribution of spring staging barnacle geese Branta leucopsis in Norway. From the early 1980s onwards, two major changes have taken place. First, increasing numbers of geese moved from the traditional islands with a semi-natural vegetation towards agricultural fields on larger islands close to the mainland. Secondly, geese expanded their range by exploring more northerly located islands. Data on food exploitation by geese on the traditional islands were collected to interpret these changes. The density of shoots of edible grasses explained the spatial variation in grazing pressure over the traditional islands. This pattern was caused by intensive grazing of available habitat, where apparently a lower threshold of the intake rate set a limit to the amount of food cropped. It is concluded that the carrying capacity of the traditional islands had been reached. On many of the traditional islands, capacity declined, however, as the vegetation type with the highest density of grasses and most preferred by geese disappeared. This negative development seems to have started when people abandoned the islands and summer grazing by sheep stopped. The declining number of geese that can find sufficient food within the traditional range, in conjunction with the rapidly growing size of the total population, are the main causes for the observed expansion of the population. Increasing spring temperatures and a change in agriculture may have allowed the geese to move to the north, and to invade agricultural areas. To lessen conflicts between geese and farmers on the large islands, two lines of management are available: restoration of management on islands within the traditional range, and enhancing the attractiveness to geese of saltmarshes on the larger islands.

ADOPTION OF YOUNG AND INTRASPECIFIC NEST PARASITISM IN BARNACLE GEESE

Prior to use of genetic techniques, extra-pair copulations and intraspecific brood parasitism were rarely observed in long-term monogamous geese. DNA fingerprinting analysis of nine families of Barnacle Geese (Branta leucopsis) revealed one case of intraspecific nest parasitism with the offspring fathered by the attendant male, and one adoption of a foreign gosling. Observations showed that adoptions accounted for 5.8% and 24.6% of goslings hatched, or 13.3% and 24% of families in two successive years. Adoption appears to be common shortly after the young have hatched and has been assumed to result from accidental brood mixing when parent-offspring recognition is not yet fully developed. We found adoptions to occur in goslings as old as 4-12 weeks, when both parents and offspring are capable of recognizing each other, suggesting that accidental mixing alone cannot explain this phenomenon.

Climate change and the increasing impact of polar bears on bird populations

The Arctic is becoming warmer at a high rate, and contractions in the extent of sea ice are currently changing the habitats of marine top-predators dependent on ice. Polar bears (Ursus maritimus) depend on sea ice for hunting seals. For these top-predators, longer ice-free seasons are hypothesized to force the bears to hunt for alternative terrestrial food, such as eggs from colonial breeding birds. We analyzed time-series of polar bear observations at four locations on Spitsbergen (Svalbard) and one in east Greenland. Summer occurrence of polar bears, measured as the probability of encountering bears and the number of days with bear presence, has increased significantly from the 1970/80s to the present. The shifts in polar bear occurrence coincided with trends for shorter sea ice seasons and less sea ice during the spring in the study area. This resulted in a strong inverse relationship between the probability of bear encounters on land and the length of the sea ice season. Within, 10 years after their first appearance on land, polar bears had advanced their arrival dates by almost 30 days. Direct observations of nest predation showed that polar bears may severely affect reproductive success of the barnacle goose (Branta leucopsis), common eider (Somateria mollissima) and glaucous gull (Larus hyperboreus). Nest predation was strongest in years when the polar bears arrived well before hatch, with more than 90% of all nests being predated. The results are similar to findings from Canada, and large-scale processes, such as climate and subsequent habitat changes, are pinpointed as the most likely drivers in various parts of the Arctic. We suggest that the increasing, earlier appearance of bears on land in summer reflects behavioral adaptations by a small segment of the population to cope with a reduced hunting range on sea ice. This exemplifies how behavioral adaptations may contribute to the cascading effects of climate change.

Latitudinal variability in the seroprevalence of antibodies against Toxoplasma gondii in non-migrant and Arctic migratory geese

Toxoplasma gondii is an intracellular coccidian parasite found worldwide and is known to infect virtually all warm-blooded animals. It requires a cat (family Felidae) to complete its full life cycle. Despite the absence of wild felids on the Arctic archipelago of Svalbard, T. gondii has been found in resident predators such as the arctic fox and polar bear. It has therefore been suggested that T. gondii may enter this ecosystem via migratory birds. The objective of this study was to identify locations where goose populations may become infected with T. gondii, and to investigate the dynamics of T. gondii specific antibodies. Single blood samples of both adults and juveniles were collected from selected goose species (Anser anser, A. brachyrhynchus, Branta canadensis, B. leucopsis) at Arctic brood-rearing areas in Russia and on Svalbard, and temperate wintering grounds in the Netherlands and Denmark (migratory populations) as well as temperate brood-rearing grounds (the Netherlands, non-migratory populations). A modified agglutination test was used on serum, for detection of antibodies against T. gondii. Occasional repeated annual sampling of individual adults was performed to determine the antibody dynamics. Adults were found seropositive at all locations (Arctic and temperate, brood-rearing and wintering grounds) with low seroprevalence in brood-rearing birds on temperate grounds. As no juvenile geese were found seropositive at any brood-rearing location, but nine month old geese were found seropositive during spring migration we conclude that geese, irrespective of species and migration, encounter T. gondii infection in wintering areas. In re-sampled birds on Svalbard significant seroreversion was observed, with 42% of seropositive adults showing no detectable antibodies after 12 months, while the proportion of seroconversion was only 3%. Modelled variation of seroprevalence with field data on antibody longevity and parasite transmission suggests seroprevalence of a population within a range of 5.2-19.9%, in line with measured values. The high occurrence of seroreversion compared to the low occurrence of seroconversion hampers analysis of species- or site-specific patterns, but explains the absence of an increase in seroprevalence with age and the observed variation in antibody titre. These findings imply that even though infection rate is low, adults introduce T. gondii to the high Arctic ecosystem following infection in temperate regions.

Baseline Immune Activity Is Associated with Date Rather than with Moult Stage in the Arctic-Breeding Barnacle Goose (Branta leucopsis)

Variation in immune defence in birds is often explained either by external factors such as food availability and disease pressure or by internal factors such as moult and reproductive effort. We explored these factors together in one sampling design by measuring immune activity over the time frame of the moulting period of Arctic-breeding barnacle geese (Branta leucopsis). We assessed baseline innate immunity by measuring levels of complement-mediated lysis and natural antibody-mediated agglutination together with total and differential leukocyte counts. Variation in immune activity during moult was strongly associated with calendar date and to a smaller degree with the growth stage of wing feathers. We suggest that the association with calendar date reflected temporal changes in the external environment. This environmental factor was further explored by comparing the immune activity of geese in the Arctic population with conspecifics in the temperate climate zone at comparable moult stages. In the Arctic environment, which has a lower expected disease load, geese exhibited significantly lower values of complement-mediated lysis, their blood contained fewer leukocytes, and levels of phagocytic cells and reactive leukocytes were relatively low. This suggests that lower baseline immune activity could be associated with lower disease pressure. We conclude that in our study species, external factors such as food availability and disease pressure have a greater effect on temporal variation of baseline immune activity than internal factors such as moult stage.