19. December 2009

Census network

 

The Greenland White-fronted Goose Study census network

 

The first description of the numbers and distribution of wintering flocks of Greenland White-fronted Geese in Ireland and Britain was compiled by Major Robin Ruttledge and Malcolm Ogilvie and published in the 1979 volume of Irish Birds. For most Irish flocks, often wintering in remote bogland habitats, there were no regular counts to give a clear and accurate picture of the status of the population. Many of the flocks they described for the first time are incredibly difficult to access, let alone count simultaneously, making it difficult to compile counts at flock or national level.  Their pioneering paper showed that, based on available evidence, wintering numbers had apparently declined in the two countries from 17,500-23,000 in the 1950s to 14,300-16,600 by the mid-1970s.  Such was the concern raised, that the population was protected from hunting on the wintering grounds under legislation enacted in Ireland and the United Kingdom, effectively starting in the winter of 1982/83. 

This confirmed the need for adequate monitoring, if nothing else, to see if legislative change would be effective in restoring the population to favourable conservation status. To this end, the Greenland White-fronted Goose Study (an independent study group based at the University College of Wales, Aberystywyth) attempted to establish complete count coverage of all known wintering sites in Great Britain.  In Ireland, the then Forest and Wildlife Service (latterly Dúchas, with help from the RSPB in Northern Ireland) set up a count network, mostly comprising professional Ranger staff.  Thanks to these efforts, annual survey of all known winter resorts has continued to the present day, and we enjoy a far better perspective over the fortunes of the population than was the case 25 years ago.

The counts show the Greenland White-fronted Goose represents a conservation success story.  After removal of hunting mortality in Ireland and Britain (although the shooting moratorium was lifted at Wexford in 1985/86 and 1989/90 with strict bag limits in both years), numbers increased at a rate expected if the previous hunting mortality had been “additive”.  That is to say, that the birds killed were not some “doomed surplus” that would have died anyway (perhaps through disease or starvation), but their deaths added to those from natural causes.  Thanks also to run of good breeding years in the 1980s, numbers increased rapidly to peak of 35,500 in the late 1990s (Figure 1). 

Last winter’s counts confirm a rather dramatic recent decline in numbers over the course of just three seasons.  Numbers have fallen rapidly back to less than 27,000 last winter (2001/2002), disguised a little by the missing spring 2001 count (because of the Foot and Mouth epidemic, spring numbers were estimated on the basis of the autumn count).  Interestingly, numbers at Wexford Slobs (the single most important Irish wintering site) actually stabilised and began to decline from the mid-1990s, long before the overall recent declines.  The change in Wexford numbers can be explained by constant annual survival (based on resightings of collared individuals, thanks to the large Dúchas marking project on the site) and observed declines in breeding success. 

So why this spectacular recent decline?  Annual survival has been relatively constant, and based on the movements of collared birds, emigration from Wexford to other winter resorts is no higher now than in previous years. The international monitoring programme also samples the proportions of young in the population.  First winter birds can be distinguished from older birds by lack of white on the face and black bars on the belly, so it is possible to assess the numbers of young in the population in winter.  There has been a long-term decline in the percentage of young birds returning to Wexford, and a similar trend (although not statistically significant) on Islay, the single most important Scottish wintering site, since protection (see Figure 2).  Production of young at Wexford has been below average in 8 out of the last 10 years, with the result that the number of new recruits to the population has failed to replace annual losses.  It is this phenomenon that has caused the stabilisation and decline in numbers at Wexford, and the same pattern is undoubtedly responsible for the declines throughout the wintering range.  The average age of first breeding amongst known-age collared birds captured at Wexford has gone from just over 3 years in the 1980s to nearly 6 years in the 1990s, showing it is becoming harder for young birds to breed. Currently less that 5% of marked goslings survive to breed at all compared to over 20% in the early 1980s. It’s pretty tough being a White-fronted gosling: it is getting very difficult for new birds to reproduce at all.

So what has caused these declines in breeding success?  At present we cannot be sure.  They follow a period of increase in the population, so it may be a “density dependent” effect.  If some resource limits the numbers of geese able to breed successfully, the expansion in numbers may now have reached the capacity of available breeding habitat.  Weather plays a part – geese return with most young after an early spring thaw and a warm summer.  Five out of the last 6 spring and summers have been cool in west Greenland, and this has doubtless contributed.  This has a disproportional effect on geese breeding in the north of range, where the breeding season is shortest.  Ringing recoveries and satellite tracking has shown birds breeding in the far north tend to winter at Wexford and in the south of the wintering range, so this fits with the pattern of earlier declines there.  Scottish wintering birds (including those on Islay) originate from the southern breeding range, where longer seasons buffer effects of delayed springs, and where the declines in production have been less marked.  Finally, the colonisation of west Greenland by large numbers of breeding Canada Geese Branta canadensis should be mentioned.  These are birds of the interior race, probably originally from northern Quebec that have been arriving in increasing numbers to areas of west Greenland formerly only inhabited by Whitefronts.  Although both species coexist in large parts of the Canadian arctic, recent studies have shown that Canada Geese are behaviourally dominant over White-fronted Geese in Greenland, and that in one study area they have continued to increase and completely displace the native species.  We do not have a good idea about the scale of this effect, but the speed of this process gives cause for concern that inter-specific competition could contribute to falling breeding success amongst Greenland White-fronted Geese.

Whatever the solution to this complex problem, one thing is becoming clear.  The kill in Iceland has been increasing in recent years, the official bag has risen from 2,947 in 1996 to 3,685 in 2001.  Because of the decline in total population size, this means that the proportion shot has increased from 8% to 12%. Whilst it is clear that the autumn hunt in Iceland was sustainable during the period of population expansion of the 1980s and 1990s, the present level of kill cannot assist in returning the population to favourable conservation status.  While the cessation of the autumn hunt there cannot halt the current decline in the overall population, it is one tangible conservation action that will contribute to the slowing of the rate of decrease in numbers.

It is important that the management plan, drafted and agreed for this population in Wexford, Ireland in 1992 but never formally signed by the range states (Ireland, United Kingdom, Iceland and Greenland), be reconvened and updated, to establish research and monitoring priorities for action.  Actions must be targeted to secure the population for the future to ensure that the international investment in the protection of the population that occurred in the 1980s and 1990s was not in vain.

 

Figure 1.  Changes in estimated abundance of the Greenland White-fronted Goose, based on wintering numbers in Ireland and Britain.  Maximum and minimum estimates (horizontal dashes) given for the 1950s and 1970s come from Ruttledge & Ogilvie 1979.  Subsequent global totals come from the combined international spring census data from each of the last 20 winters (8).  The empty square symbol in winter 2000/2001 represents an estimated value, generated from the preceeding autumn count (the spring census was abandoned because of Foot and Mouth Disease that season). The empty square symbol in winter 2002/2003 is based on an estimate given the low level of reproductive success in summer 2002.  Spring counts from Wexford Slobs (filled circles) and Islay (open circles) are shown below.

 

Figure 2.  Changes in the percentage of first-winter birds amongst samples of birds at Wexford (square symbols) and Islay (circles).  Open symbols indicate pre-protection levels, filled symbols indicate post protection.  Horizontal dotted line indicates the average annual percentage for both sites combined.  Fitted regression lines show the modelled decline over time (solid thick line is a statistically significant decline at Wexford, the dotted thin line least squares best fit which fails to attain statistical significance from the Islay data).