Western Sandpiper

Hypotheses about causes for shorebird declines being addressed by each working group:

Hypotheses to explain census declines

in North American shorebirds

Rob Butler, Ron Ydenberg, Garry Donaldson, and Stephen Brown

Pacific Wildlife Research Centre, Canadian Wildlife Service, 5421 Robertson Road, Delta, B.C. V4K 3N2 Canada; Centre for Wildlife Ecology, Department of Biological Sciences, Simon Fraser University, Burnaby B.C. V5A 1S6; Canadian Wildlife Service, Hull, Quebec K1A 0H3, Canada;
Manomet Center for Conservation Sciences, 81 Stage Road, P.O. Box 1770, Manomet, MA 02345 USA

Butler, R.W., R. C. Ydenberg, G. D. Donaldson and S. Brown. 2004. Hypotheses to explain census declines in North American shorebirds. Shorebird Research Group of the Americas Report 1 (On line)

There is ample evidence that many shorebirds counted in censuses in North America have declined in the past two decades but the reasons for the declines are unclear (e.g. Jehl and Lin 2001, Morrison et al. 1994, 2001). Censuses on the eastern seaboard and mid-west showed that 9 of 30 species were significantly declining with most pronounced declines among species with wide non breeding ranges in non-marine habitats (J. Bart, pers. comm.). Trends were most detectable in the north Atlantic and less so in the mid west. The geographic range and number of species reported to have declined in recent decades suggests that a general widespread cause might be at work.

Shorebirds breeding in North America migrate to winter quarters in North, Central and South America, Europe and Asia via traditional migratory routes or ‘flyways’. Many species travel tens of thousands of kilometres each year. Some species migrate in large flocks that assemble at a small number of stop over sites often populated by humans. Others migrate in small numbers stopping at many small sites and some species exhibit both migration behaviours. In nearly every case, shorebirds spend part of their year in habitats altered by large numbers of humans living nearby.

The collective wisdom of shorebird biologists and conservationists from Canada and the United States mostly drawn from North America was compiled through meetings of the Canadian Wildlife Service Shorebird Technical Committee in Quebec City, the Waterbird Society in Lacrosse, Wisconsin in 2002, and at the 7th Annual Western Sandpiper Research Group at Simon Fraser University in 2003. The view of these specialists was that the answer to why shorebirds were declining would require a coordinated international, multidisciplinary effort across the range of a few species which we call the Hemisphere Shorebird Project. Emerging from these discussions were five most likely causes for declines.

Possible causes for declines

1. Climate Change

Snow melt is thought to play an important role in determining the timing of breeding and reproductive success of arctic breeding shorebirds. The Eastern Arctic is showing wider swings in temperature than the western arctic. We predict that nesting success of shorebirds in the eastern arctic has become more variable than shorebirds in the west. We will test this prediction by examining meteorological records of timing of snow melt and air temperature at shorebird nesting areas over the past three decades. We will relate timing of nesting and nesting success in relation to snow melt and summer temperature at 10 sites spread from east to west across the arctic. We will then make an estimate of expected fledging success from archived data of snow melt over the past 30 years. The estimates for the eastern arctic will be entered into population models of recruitment to compare relative population change to actual change seen in counts made on Maritime Shorebird Surveys over the past 30 years.

2. Avoidance of predators

The recovery of birds of prey in North America has resulted in a shift in the migration routes, an avoidance of dangerous stop over sites, and briefer stop over by individual shorebirds at sites that historically held large numbers of shorebirds (Ydenberg et al. 2001, Butler et al. 2003). We predict that shorebirds have become more numerous in non-coastal areas that have fewer birds of prey. We also predict that small, enclosed sites where birds of prey can launch surprise attacks are used by fewer shorebirds now than in the past. Finally we predict that individual shorebirds are spending less time at sites that give an impression that the number of birds has declined.

3. Contamination

The ‘trophic contamination hypothesis’ posits that shorebirds accumulate industrial and urban pollution at stop over sites that are subsequently released in sudden high doses as fat is burned during migratory flights that then disrupt their ability to make migratory decisions. For example, large contaminant doses might hamper refueling by reducing the satiation signal in shorebirds so that they do not accumulate sufficient fat for migration. In addition, organochlorines deposited on mudflats in the 1970s and 1980s and now buried by sediments are more accessible to long-billed shorebirds that probe deeply for prey than short-billed species that forage at or near the surface. We predict that OC loads will be greater in long billed species than in short billed species. We will compare contaminant levels of birds collected along the migration route and on the breeding ground to determine if contaminant loads differ greatly between flights and sites. Within these samples, we predict that long-billed species will carry greater OC loads than short-billed species.

4. Human Disturbance

This hypothesis posits that human disturbance at stop over sites has reduced the time available for shorebirds to accumulate fat for migration. As a result, the shorebirds stay longer to acquire the fat or depart for places where they can forage undisturbed. We predict that shorebirds at stop over sites with large numbers of people will have lower mass than the same species at nearby undisturbed sites. We also predict that the number of shorebirds has declined more rapidly at sites with high human disturbance than at sites with low disturbance.

5. Habitat Loss and Degradation

This hypothesis posits that the number of potential stop over sites has declined resulting in increased competition for food so that the poorest competitors moved to marginal foraging habitat where they suffered higher mortality than birds in good habitat.

Approach

We will decipher the cause for declines among the highly migratory species of shorebirds by synthesizing existing information from various shorebird networks and assemble teams to fill gaps in our knowledge to complete the synthesis for species in decline. The project would run for five years. The first year would be devoted to the synthesis of existing data, and establishing species networks including the enlisting of contributors. The subsequent years will be used to collect field data to thoroughly investigate the reasons for declines in selected species.

The Shorebird Research Group of the Americas (SRGA) is a collaborative, international, and multi-disciplinary group of biologists interested in researching conservation questions pertaining to shorebirds in the Americas. In the US and Canada, the SRGA advises on shorebird science to the National Working Groups of the USA and Canadian Shorebird Plans (Donaldson et al. 2000, Brown et al. 2000). Participants from other countries will be strongly encouraged and will report nationally according to their own specific needs. Its membership includes individuals from government, university and private organizations. Recent concerns about possible declines in some shorebirds in North America prompted the formation of the SRGA and the development of the following proposal to provide advice on the question of population stability of shorebirds in the Americas. Coordination of this project will be determined through consultation with SRGA partners. The Hemisphere Shorebird Project will be coordinated through the Centre for Wildlife Ecology at Simon Fraser University. Each country will implement the results as they see fit. In the US and Canada, results will be reported through the National Working Groups of the respective Shorebird Plans.

Shorebird researchers recognize that determining the causal factors for declining populations is a high priority for shorebirds but that there are additional research questions that must be addressed to build a solid science foundation on which to base conservation decisions. Thus, the SRGA will adapt to the science needs of the conservation community and address additional questions as priority needs arise.

References

Bart, J. and C. Francis. 2001. Quantitative Goals for Avian Monitoring Programs. Manuscript.

Brown, S., C. Hickey, B. Harrington, and R. Gill (eds.). 2001. United States Shorebird Conservation Plan, 2nd Ed. Manomet Center for Conservation Sciences, Manomet, Massachusetts. 70 pp.

Butcher, G. S., B. G. Peterjohn, and C. J. Ralph. 1993. Overview of national bird population monitoring programs and databases. Pages 192-203 in D.M. Finch and P.W. Stangel, editors. Status and management of Neotropical migratory birds: Proceedings of the 1992 Partners in Flight National Training Workshop, 21-25 September, Estes Park, Colorado. U.S. Forest Service Rocky Mountain Forest and Range Experiment Station General Technical Report RM-229.

Clark, C.W. and R.W. Butler. 1999. Fitness components of avian migration: a dynamic model of
Western Sandpiper migration. Evolutionary Ecology Research 1:443-457.

Donaldson, G. D., C. Hyslop, R. I. G. Morrison, H. L. Dickson, and I. Davidson. 2000. Canadian
Shorebird Conservation Plan. Canadian Wildlife Service Special Publication, Ottawa.

Jehl, J. R., Jr. and W. Lin. 2001. Population status of shorebirds nesting at Churchill, Manitoba.
Canadian Field-Naturalist 115: 487-494.

Morrison, R. I. G., C. Downes And B. Collins. 1994. Population trends of shorebirds on fall
migration in eastern Canada 1974-1991. Wilson Bulletin 106:431-447.

Morrison, RIG, Y. Abry, RW Butler, GW Beyersbergen, GM Donaldson, CL Gratto-Trevor, PW Hicklin,
VH Johnston and RK Ross. 2001. Declines in North American shorebird populations. Wader
Study Group Bulletin 94: 39-43.