Decline of Canada geese (Branta canadensis) and common goldeneye (Bucephala clangula) associated with a collapse of eelgrass (Zostera marina) in a Nova Scotia estuary

Mean numbers of migrant Canada geese (Branta canadensis) in Antigonish Harbour in the southern Gulf of St. Lawrence (Canada) during October to December were similar (approx. 450–500 birds) for the period 1998–2000. Similarly, during this period, geese used two foraging sites. However, in 2001, the average number of birds decreased by half and the primary foraging sites were used only rarely. This coincided with a decline of about 95% in the biomass of roots and rhizomes of eelgrass (Zostera marina) that occurred between October 2000 and 2001. Eelgrass is the principal food of geese in this estuary. In addition, there was a reduction of around 50% in the numbers of common goldeneye (Bucephala clangula), which feed on invertebrates associated with eelgrass. Lower than usual weekly abundances of geese and goldeneye are probably the result of an unusually short residence time in the estuary, rather than a decline in the total number of visiting migrants. We attribute these changes in the distribution and abundance of geese and goldeneyes to the dramatic decline in eelgrass. Electronic Publication     

中国鸟类新纪录——加拿大雁

<正>2014年12月21日,在天津市宁河区永定新河水域(39°15′24″N,117°26′2″E,海拔7 m)观察到4只加拿大雁(Branta canadensis)在此栖息觅食,同域亦有少量骨顶鸡(Fulica atra)、绿翅鸭(Anas crecca)、罗纹鸭(A.falcata)及赤膀鸭(A.strepera)在此活动。次日有观鸟者前往观察,未能再发现该群加拿大雁。本次观察到的加     

Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis

1. Individual variability in prey preferences can have marked effects on many demographic parameters from individual survival and fecundity to the vital rates of entire populations. A population level response is ultimately determined by individual prey choices; however, the effect of individual dietary choice is often overlooked. 2. We determined prey choice by individual consumers, light-bellied Brent geese Branta bernicla, during the overwintering period. Two hundred and eighty-one individuals were sampled at distinct temporal points over two winters. Stable isotopic ratios of carbon and nitrogen for blood cells and blood plasma, from each sampled individual were measured. Isotopic ratios for potential prey items were also measured. 3. Delta15N and delta13C for blood samples were both significantly different between sample months. Generally we found a decrease in both isotopic ratios during the course of the winter. All potential prey items were also isotopically distinct. Multisource mixing models (isosource) were used to determine the range of possible contribution to the diet of individuals. 4. During early winter, diet consisted almost exclusively of sea grass Zostera spp. The level of Zostera spp. in the diet dropped until mid-winter, and was supplemented by the utilization of green algae Ulva lactuca, and Enteromorpha spp., and terrestrial grasses. Terrestrial grass comprised an increasing proportion of the diet in late winter, representing virtually the exclusive food source by April. 5. By examining intrapopulation variability in resource utilization we highlight a number of ecologically important factors not addressed by previous population level studies.     

Minimal intra-seasonal dietary overlap of barnacle and pink-footed geese on their breeding grounds in Svalbard

We analysed barnacle Branta leucopsis and pink-footed goose Anser brachyrhynchus summer diets (May–July 2003) based on the proportions of different plant constituents in the faecal material of adult breeding birds in Sassendalen, Svalbard to assess potential inter-specific competition. Diets were highly restricted and overlapped little during pre-nesting and post hatch. During incubation both species showed greatest variety in their diet, reflecting site-specific differences in local food abundance. However, locally the diets of pink-footed and barnacle geese resembled each other most at this time (although still differing significantly). The conflicting needs of nest defence and maintenance of body condition constrains the extent of the feeding resource utilised by nesting pairs and explains slightly greater dietary overlap at this time. Hence, there is little evidence of inter-specific competition (interference or depletion) at present, but this is most likely to be manifest during the incubation period in the future if goose numbers continue to increase. More detailed investigations of the degree of spatial overlap of the two species and their effects on plant structure, quality and community composition are necessary to predict likely outcomes of expected increases in numbers of both goose species.     

Introduced Swedish Canada geese (Branta canadensis) have low levels of genetic variation as revealed by DNA fingerprinting

The Swedish, Finnish and Norwegian population of Canada geese (Branta canadensis), now amounting to some 30–50 000 birds, was founded by only five individuals. We used DNA fingerprinting to assess the level of genetic variability in minisatellite loci of Swedish Canada geese from two northern areas. For comparison, we estimated the minisatellite variability in lesser white-fronted geese (Anser erythropus), barnacle geese (Branta leucopsis) and a reintroduced stock of Canadian giant Canada geese (Branta canadensis maxima). The mean similarity between Swedish Canada geese was 0.76 ± 0.15, which is higher than recorded for any other natural bird population. The high similarity implies that a fourfold increase of homozygosity has taken place in this population. The probable cause for the loss of variation is the low number of birds originally introduced and a history of repeated translocations, leading to a sequence of founder events. As a consequence of the high similarity, it has not been possible to use DNA fingerprinting for determination of parenthood in the population studied.     

Fitness consequences of long-term pair bonds in barnacle geese: monogamy in the extreme

In barnacle geese Branta leucopsis, pair-bond members generally remain together every day, each year, often for life. Geesethat maintain long-lasting pair bonds during their lifetimeproduce more offspring than those with shorter pair durations.This result was shown while statistically controlling for thebirds' life span and the proportion of life spent without apartner, two variables that also influence lifetime reproductivesuccess. I argue that continuous partnerships are maintainedin highly competitive goose societies because of the constantneed for female—male cooperation, without which acquiringadequate resources for reproduction would be prohibitive forboth sexes.