- Top of page
- Materials and methods
- Management implications
1. In most situations that have been studied, the creation of refuge areas for migratory waterfowl has generally increased the number of waterfowl using a site. However, experimental evidence to show that the increases were caused by reduced disturbance and not by confounding factors, e.g. improvement of habitat quality of the sites or general population increases, is generally lacking.
2. In two Danish coastal wetlands, waterfowl hunting was the most critical source of disturbance to autumn-staging waterfowl (Madsen 1998). To test whether hunting disturbance displaced birds from sites, experimental refuge areas were established, with hunting-free areas manipulated annually during a 4-year period, followed by permanent refuge establishments, monitored for a further 4 years.
3. In both areas, experiments showed that quarry geese and dabbling ducks redistributed according to the position of the hunting-free areas, whereas protected species did not. In quarry geese, there was a 6–8-fold increase in numbers from before to after the experiments; in quarry dabbling ducks there was a 4–50-fold increase. Dabbling ducks extended the staging period into the winter. Protected waterfowl as well as goldeneye and coot (both quarry species) did not redistribute; their numbers either fluctuated or increased (2 to > 50 fold).
4. In quarry geese and dabbling ducks, changes in food supplies (of the herbivorous species), intensities of recreational activities outside the refuge areas, weather or overall population sizes could not explain the increases. In protected species, as well as goldeneye and coot, trends in local numbers were in accordance with overall population trends for some species; in others, the rate of increase in local numbers was higher than in overall population size.
5. In both areas, species richness increased from before to after the experiments. This was most pronounced in one of the areas where the refuge included shallow-watered areas in association with salt marshes.
6. It is concluded that prior to experiments, waterfowl hunting caused a displacement of quarry species, resulting in a species-poor waterfowl community. Refuge creation is an efficient management tool to improve the conservation value and biodiversity of wetlands of importance to waterfowl.
- Top of page
- Materials and methods
- Management implications
The experiments in both study areas showed that quarry waterfowl responded to the changing shooting regulations, both in terms of numbers and distribution. This is in accordance with the hypothesis that, if hunting was a disturbance source affecting the distribution and local numbers of waterfowl, it would be expected to affect most critically the quarry species. Some quarry species did not respond as predicted, namely goldeneye and coot (in Nibe-Gjøl Bredning). The possible explanation for goldeneye is that the species occurs in the deeper parts of the shallow-watered areas where it is not so affected by punt shooting; for coot, it has been shown that the species does not show as vigorous a behavioural response to shooting disturbance as wigeon (Madsen 1998) and, hence, are better able to cope within a site.
In the protected species which increased in numbers, i.e. cormorant, mute swan, light-bellied brent goose and barnacle goose, there was no evidence to suggest that the increases were due to restrictions on hunting. In lapwing and golden plover at Ulvshale-Nyord, the abrupt increases in numbers from 1989 to 1991 suggest an effect of the inclusion of the Nyord south marsh in the refuge; however, in the overall analysis of distribution, there was no significant treatment effect.
In neither of the two areas can the observed increases in numbers of herbivorous waterfowl be explained by increased food supplies. On the contrary, in Nibe-Gjøl Bredning, the decrease in Zostera marina was probably the reason behind the decrease in numbers of wigeon during 1994–96. The decrease in Zostera was partly compensated by an increase in Ruppia. However, Ruppia does not have the same shoot lengths and biomass as Zostera (Madsen et al. 1992b; P. Clausen, personal communication) and, in particular in situations with high water levels, wigeon probably encountered problems with food availability. Mute swan, light-bellied brent goose and coot may have partly encountered the same problems, but due to their longer necks and diving ability (coot), they are not as susceptible to reduced food supplies as wigeon.
Food stocks of species feeding on other items (seeds, zooplankton, invertebrates, fish and mussels) were not monitored. Therefore, it cannot be judged whether changes in these supplies influenced the experiments. However, the similarity in response by quarry geese and dabbling ducks, irrespective of diet, suggests that there was not a general trend in food supplies affecting the results.
Weather conditions cannot explain the increases in numbers. The droughts during late summer 1994–96 had a negative effect on the number of waterfowl using the marshes at Ulvshale-Nyord; however, except for lapwing and golden plover, there is no discernable effect on the overall use of the area. Ice cover played a significant role in 1996, but only for the numbers of Canada geese and, to a lesser extent, mallard, which arrive during late November to early December.
Changes in recreational activities, in particular shooting, did happen in both areas. In Nibe-Gjøl Bredning, the slightly increased shooting activity in the areas adjacent to the refuge could have reinforced the treatment effect; on most week-days, however, the density of hunters was well below the threshold at which wigeon numbers will be affected on a diurnal basis (see Madsen 1998). At Ulvshale-Nyord, the number of hunters decreased when the experiments started, which matches the decrease in available hunting areas. The upsurge in activity in 1992 was probably a result of the release of a report (Madsen et al. 1992a, of which a summary was published in the hunters′ magazine) describing the first results of the experiments, i.e. that waterfowl numbers had increased inside the refuge areas, as well as in adjacent areas still open for hunting. The decreased shooting activity during 1993–96 may have facilitated an increased waterfowl use of the areas adjacent to the refuge. It is, however, difficult to separate the effect of the decreased hunting activity from the drought effects during 1994–96. For example, the decreased use of Nyord south marsh by greylag geese during 1994–96 could be interpreted as a drought effect or the effect of the general ban on goose shooting during August implemented from 1994 onwards. Thus, due to the ban, the geese would disperse from the refuge to adjacent areas in August. The general ban on goose shooting after 10·00 hours may be the reason behind the decrease in numbers of Canada geese at Ulvshale-Nyord from 1994 onwards, because the geese have found suitable roosts and feeding areas which were hitherto unattractive due to hunting disturbance.
In neither of the two areas did substantial changes in habitat management other than the refuge creations take place during 1985–96.
Some of the increases in waterfowl numbers may potentially be an effect of an increase in overall population size. From the monitoring of wintering waterfowl in north-west Europe, coordinated internationally by Wetlands International and nationally by the National Environmental Research Institute, trends are known for most of the populations of waterfowl concerned (Rose 1995, giving trends for swans, ducks and coot up to 1993; the Goose Specialist Group of Wetlands International, unpublished, giving trends for geese up to 1995).
The Danish cormorant population has increased steadily during the 1980s, 1990s (Bregnballe & Gregersen 1995); and combined with the establishment and expansion of a colony at Ulvshale-Nyord, this has been the primary reason for the increase in numbers roosting during autumn. The Baltic population of mute swan almost doubled during 1987–93 following a series of mild winters, with a subsequent decrease following a series of cold winters in the 1990s. This development is mirrored in both study areas. The eastern Denmark breeding population of greylag geese, of which the autumn-staging geese at Ulvshale-Nyord are part, has been stable during the 1980s and 1990s (Jørgensen, Madsen & Clausen 1994); whereas numbers at Ulvshale-Nyord have increased. The population of dark-bellied brent goose has doubled in numbers from the mid-1980s to the mid-1990s; a similar development was observed at Ulvshale-Nyord. The Svalbard population of light-bellied brent goose has only slightly increased during the 1980s and 1990s, and the rapid increase in numbers using Nibe-Gjøl Bredning has to be explained as a redistribution of birds within the autumn-staging and wintering grounds of the population, driven by declines in food supplies in the traditional autumn-staging areas (Clausen et al. 1998). The Russian-Baltic breeding population of barnacle goose has doubled in size from the mid-1980s to the mid-1990s, compared to a 12-fold increase at Ulvshale-Nyord. The Baltic population of Canada goose has increased by ≈40% from the mid-1980s to the early 1990s, whereas at Ulvshale-Nyord, numbers increased by a factor of eight. The north-west European wintering populations of mallard, teal, pintail and shoveler have been stable since the mid 1980s, which is in contrast to 4–50-fold increases in the experimental areas. The north-west European wintering population of wigeon has doubled from 1986 to 1993, in contrast to a 4–30-fold increase in the two study areas. Finally, the European population of lapwing is regarded as stable, while the north-west European population of golden plover is decreasing (Rose & Scott 1994); both increased in numbers at Ulvshale-Nyord. For the remaining species which fluctuated in numbers (whooper swan, shelduck, goldeneye, coot); the population trends have been either stable (shelduck, coot) or slightly increasing (whooper swan, goldeneye).
1. the observed local trends in numbers of cormorant, mute swan, (whooper swan), dark-bellied brent goose, shelduck, (goldeneye) and coot largely reflect trends in population size;
2. in light-bellied brent goose, barnacle goose, Canada goose and wigeon, the overall population sizes have increased, but the rates of increase in the two study areas override the rate of increase in the whole population;
3. in greylag goose, mallard, teal, pintail, shoveler, lapwing and golden plover, overall population sizes have been stable or have decreased, while increasing in the two study areas;
4. changes in local food supplies (at least for the herbivorous species); weather conditions and recreational activities outside the refuge areas cannot explain the observed increases in waterfowl numbers;
5. for the quarry species, the most likely reason behind the increases was the creation of hunting-free areas, while for the protected species, it is not known whether the increases were due either to an attraction to the areas for reasons other than restrictions on shooting (e.g. in light-bellied brent, a discovery of a hitherto unexploited habitat) or to refuge creation.
In both study areas, but most pronounced at Ulvshale-Nyord, refuge creation resulted in a more species-rich waterfowl community, implying that prior to experiments, hunting disturbance caused an assymetry in response amongst different species. Specifically, the species hypothesized to be most vulnerable to hunting disturbance were displaced, i.e. the coastal quarry geese and dabbling ducks, while the more tolerant species remained, i.e. the protected species as well as quarry diving ducks and coot. This finding supports the disturbance ‘gradient’ hypothesis by Hockin et al. (1992), as well as the classification of species vulnerability to hunting disturbance (Bell & Owen 1990; Madsen, Pihl & Clausen 1998).
The reason that the species enrichment effect was most pronounced at Ulvshale-Nyord is probably because the refuge included marsh habitat in association with shallow-watered areas. The inclusion of the two habitats creates optimal conditions by which dabbling duck and waders can freely choose between feeding/roosting in the marsh or on the mudflats and shallow-waters during periods with low water levels in the bay. During high water periods, feeding opportunities in the shallow-watered habitat become restricted, but birds can still feed and roost in the marshes, provided that some water is available in creeks and pans.
The establishment of reserves in the two study areas has almost doubled the autumn and winter national totals for wigeon and shoveler. Ulvshale-Nyord has become one of the most important sites for coastal waterfowl in Denmark, and certainly the site with the highest species richness.
Where did the waterfowl, whose numbers have built up in the reserves so rapidly, come from? The speed by which the increases took place excludes the possibility that enhanced survival or fecundity of birds staying in the reserve areas played a major role. Two other explanations remain: (1) that the birds were attracted to the refuge areas from adjacent sites which have consequently lost their relative value, or (2) that individual birds have prolonged their length of stay in the refuges which cumulatively leads to an increase in overall numbers. The inference of (2) is that, before refuge creation, birds passed through the areas more rapidly on their way to more southern autumn-staging and wintering grounds; whereas now, birds are held back in the refuges and they are ‘missing’ further south in the flyway. Madsen et al. (1995) analysed counts of wigeon from key areas adjacent to the two refuges and found no negative trend in autumn numbers.
For some species, the increases cannot be accounted for by even a theoretically complete ‘drainage’ of all sites within the two regions. In the autumns of 1987 and 1988, national aerial surveys of waterfowl were performed (Laursen et al. 1997). Even though aerial surveys probably underestimate the true numbers present, they give an overview of total numbers, especially of species such as wigeon and pintail which congregate in relatively few coastal areas. In the entire Limfjord and northern Kattegat region, excluding Nibe-Gjøl Bredning, 6600 and 9900 wigeon were counted in autumn 1987 and 1988, respectively. In comparison, in Nibe-Gjøl Bredning, autumn peak numbers increased from 5000 to 26 000 within a few years. Similarly, in south-east Denmark outside Ulvshale-Nyord, 2300 and 1300 wigeon were counted in the autumns of 1987 and 1988, which is only around 10% of the peak numbers recorded at Ulvshale-Nyord after the experiments. In south-east Denmark, fewer than 100 pintails were observed in 1987 and 1988, compared to 2000–4000 solely at Ulvshale-Nyord after the experiments. Hence, for several species, e.g. Canada goose, teal, wigeon, pintail and shoveler, the most likely explanation is that the increases were primarily due to prolonged length of stay. This hypothesis will be explored in more detail elsewhere. In other species that occur in large numbers in south-east Denmark, e.g. greylag goose and mallard, it is likely that redistribution from nearby sites also played an important role.
In dabbling ducks, it took 2–6 years between the creation of the refuges and the time when peak numbers were reached, and for some species, numbers had not levelled off by 1996. One possible reason is that birds are sampling environmental conditions in several areas. In this way, an individual bird will gain experience over some years of the reduced predation risk associated with the newly-established refuge areas and will then gradually prolong its length of stay. In Nibe-Gjøl Bredning, the parallel decline in wigeon numbers and Zostera coverage after 1993 suggest that the carrying capacity of the food supplies may have been reached. Similarly, the stabilization in numbers of some species at Ulvshale-Nyord suggests that food supplies may set a limit on the numbers; however, for many species it is premature to judge, even 6 years after refuge creation.
- Top of page
- Materials and methods
- Management implications
The study strongly suggests that waterfowl hunting caused a displacement of birds from important conservation areas and that the creation of hunting-free areas was an efficient management tool to improve waterfowl site usage, as well as biodiversity. The combination of shallow-watered areas and coastal marshes in a refuge provides an optimal design for dabbling ducks and waders. Shooting on the borders of a refuge affects distribution and probably habitat exploitation by waterfowl, and should be avoided, so that refuge borders follow natural borders in the landscape (see also Fox & Madsen 1997).
In areas designated to afford protection to migratory waterfowl, e.g. Ramsar sites and EU Special Protection Areas, sustainability is a management goal, which can be interpreted as enhancing conservation interests whilst maintaining some human recreational and commercial use. As shown by Madsen (1998) some activities are more disruptive to waterfowl behaviour and distribution than others, and activity levels and thresholds can be identified at which bird usage will not be affected. The finding was that mobile activities, e.g. mobile punt shooting, are more disruptive than activities at fixed positions. Such knowledge can be useful for the design of zoning of human activity in areas adjacent to a core refuge area.
In Denmark, as well as other countries which are either signatory to the Ramsar Convention or members of the EU, many conservation areas have been designated on the basis of the number of migratory waterfowl which were present, with no attention paid to the potential value of the sites. Certainly, the two experimental refuges have demonstrated that the potential for waterfowl was much greater than was suggested by the counts prior to experiments. The composition of the waterfowl community, especially the lack of species vulnerable to hunting disturbance, may give a clue whether hunting has a displacement effect, and this can be used as an indicator of the level of disturbance.
The results from the experimental reserves have served to form a basis for the designation and design of more than 50 new wildlife reserves within Danish coastal EU Special Protection Areas. The new reserves will be implemented during 1993–2000 (Madsen, Pihl & Clausen 1998). Because it is estimated that the number of migratory waterfowl passing through the country is an order of magnitude greater than what can be counted at any time in the autumn, it is anticipated that the new extended network of reserves has the potential to hold back significant proportions of the north-west European populations of dabbling ducks during autumn.
The wider implications of such refuge networks at population or flyway scale still have to be addressed. This question is, however, difficult to answer because, in migratory waterfowl, density dependence processes may operate thousands of kilometres away from the place where birds were disturbed (Madsen & Fox 1995). If refuge areas in the northern end of a flyway can hold back significant numbers of birds which would otherwise have been ‘pushed’ into wintering grounds with limited resources, then it is possible that the refuge creations will have a positive impact at a population level, achieved by increasing survival and/or fecundity.