Jesper Madsen

Impacts of Hunting Disturbance on Waterbirds-A Review

Waterbird hunting is a widespread activity in wetlands throughout Europe and constitutes one of the most significant sources of disturbance during autumn and winter. The biological evidence for effects of hunting disturbance on the behaviour and distribution of migratory and wintering waterbirds and its possible impacts on population dynamics is reviewed. Most of the literature has been concerned with local effects of disturbance, focussing on quarry geese and dabbling ducks. Comparatively little is known about effects on diving ducks and waders, while there is no direct evidence for impacts at the population level for any waterbird species. Hunting disturbance can cause temporary disruption of normal activities of waterbirds, alter their diurnal rhythms and increase escape flight distances. It can displace waterbirds from preferred feeding and roosting habitats at local or regional level and increase turnover, so that the carrying capacity of a site is not reached. Quarry waterbirds, and those occurring in large inshore concentrations, such as many dabbling ducks, geese and waders, are potentially most sensitive to disturbance. Hunting disturbance can disrupt pair-bonds and family structures which may affect reproductive output. Evidence is provided that many waterbird populations are limited by winter conditions and that the majority of studied waterbird species lose body reserves during winter. Because hunting disturbance causes under-exploitation of potential feeding grounds where population limitation is considered to occur, such disturbance will, by definition, have an impact at the population level. However, the magnitude of this impact has not been quantified and requires a modelling approach.

Estimating the total number of birds using a staging site

Assessment of the importance of migration stopover sites for waterbirds has typically been based on peak counts of staging birds, which underestimate the volume of the site (i.e., the total number of birds using the site over a season) because of turnover of individuals. However, volume is difficult to estimate. We present a new method for estimating volume based on a series of counts and concurrent resightings of marked individuals, with an associated measure of precision. Daily survival probabilities are estimated and used with counts to estimate daily numbers of recruits, which are then summed to estimate volume. We apply the method to 2 waterfowl data sets. The method has the potential to greatly improve our understanding of the true importance of staging sites to populations where resightings of marked individuals and accurate count time series are available. Identification of internationally important staging sites for waterbirds could thus be improved substantially by application of the method.

Energetics of Feeding on Winter Wheat Versus Pasture Grasses: A Window of Opportunity for Winter Range Expansion in the pink-footed Goose Anser brachyrhynchus

Traditionally, pink-footed geese Anser brachyrhynchus wintering in Denmark, the Netherlands and Belgium have used the Danish sites only during mild winters, rapidly moving southwards in case of cold spells. Since the 1980s, an increasing number of geese have remained on the Danish wintering grounds despite cold spells, foraging on pastures and winter wheat Triticum aestivum fields. We compare the daily time and energy budgets and the food quality in the two habitats during winter. Winter wheat fields were increasingly used by the geese as temperatures dropped. At temperatures around 0°C, the geese foraged in both habitats, spending on average 83.8% and 74.9% of the daytime foraging in pastures and winter wheat, respectively. The estimated daily energy expenditure was slightly higher on pastures than on winter wheat fields (1,076 vs 1,057 kJ). The estimated daily food intake determined using the ‘marker substance’ method was 148 and 157 g ash free dry weight (AFDW) in geese feeding on pastures and winter wheat fields, respectively, equivalent to a daily net energy intake of 1,109 kJ and 1,145 kJ. Daily food intake, estimated on the basis of oesophagus contents of collected birds, was 170 g AFDW in pasture feeding geese and varied within 159–229 g AFDW in winter wheat feeding geese. In the mild winter, the protein content in winter wheat and Poa did not differ, whereas in the severe winter the protein content remained high in winter wheat but decreased in Poa. During the winters of 1994–1996, the abdominal profile index, API, in individually neck-banded geese observed repeatedly, only changed significantly during late January 1996. Neither during cold spells was there any change in API. Since the 1980s, the area covered by winter wheat has increased markedly in Denmark. Because winter wheat represents a reliable and profitable food source even in severe winters, the recent change in agricultural practice has enhanced the development of a new wintering strategy of pink-footed geese, allowing a northward expansion of their winter range. Potentially, this will increase the crop damage conflict and may lead to further population growth.

Tools in waterfowl reserve management: effects of intermittent hunting adjacent to a shooting-free core area

We explored how waterfowl in a large Danish coastal wetland responded to one day of marsh shooting at intervals of three weeks (1998), two weeks (1999), and one week (2000 and 2001). Shooting took place around sunset on a section of salt marsh that usually is part of a large shooting-free refuge. The behavioural response of waterfowl was recorded, and all waterfowl were counted on the day of the hunt and on the first and second day after each hunt. The wildfowl responded to the first 1–7 shots by moving into the open water parts of the refuge (dabbling ducks) or to sites > 8 km away (geese and waders). When the salt marsh was flooded early in autumn 2001, wigeon Anas penelope, teal A. crecca and lapwing Vanellus vanellus restricted their response to movements to other non-hunted parts of the salt marsh. On the first day after the hunts, mallard A. platyrhynchos occurred in significantly lower numbers both on the adjacent shallow and on the salt marsh. Wigeon numbers on the shallows were not affected by shooting, though they returned in lower numbers to the salt marsh on the first day after the hunts in 1998–2000, but not after the hunts in 2001. A similar pattern was observed in teal numbers on the salt marsh. We conclude: 1) that shooting lead to short-term displacements of dabbling ducks, 2) that the response of wigeon and teal varied depending on prevailing conditions on the salt marsh, 3) that intermittent regulation of marsh shooting was a management tool that ensured that waterfowl continued to exploit the area shot over on days when no shooting took place, and 4) that the relatively weak responses were linked to the existence of an extensive refuge adjacent to the area with marsh shooting.

Assessment of food intake rates in pink-footed geese Anser brachyrhynchus based on examination of oesophagus contents.

An alternative to the so-called ‘marker substance’ method used to estimate daily food intake in geese is presented. Based on the assumption that a grazing bird takes one leaf per peck, the daily food intake rate can be calculated by multiplying mean bite weight, peck rate and total daily foraging time. Wintering pink-footed geese Anser brachyrhynchus feeding on pastures and winter wheat fields were collected and samples of leaves in the oesophagus were measured, dried and weighed individually. We measured leaf lengths in unexploited areas of the same fields upon which geese had been feeding. Peck rates of winter wheat feeding geese were measured. Daily foraging time was obtained from observations of activity budgets of flocks of geese. Daily food intake of winter wheat feeding geese was estimated at 159–229 g ash free dry weight (AFDW) during late winter and at 188–212 g AFDW in early spring. For geese feeding on pastures in early spring food intake was estimated at 170 g AFDW. Averages were generally in accordance with estimates derived by the ‘marker substance’ method. Bite length was positively related to primary leaf length of winter wheat, suggesting that geese adjust bite size to available leaf lengths. There was a negative relationship between peck rate and length of all leaf types, but the relationship was only significant for primary leaves. Based on the assumption that bite length was identical to primary leaf length and the relationship between primary leaf length and peck rate, a quadratic relation between primary leaf length and instantaneous intake rates was derived, yielding a peak intake rate of 0.62 g AFDW min-1 at a primary leaf length of 8.4 cm. In the beginning of the winter, bite lengths were close to this optimal leaf length, but decreased during winter.

Can geese adjust their clocks? Effects of diurnal regulation of goose shooting

Since 1994, goose shooting in Denmark has only been allowed from 1½ hours before sunrise to 10 a.m. (since 1997 until 11 a.m.). The aim of the diurnal regulation was to provide autumn-staging and wintering geese with more undisturbed feeding opportunities, and hence to extend the length of their stay in Danish haunts. A field study was carried out during 1994–1997 to investigate the effects of the regulation on the behaviour and site use by geese, focused on greylag geese Anser anser and pink-footed geese Anser brachyrhynchus at three important Danish sites. Data from earlier studies and monitoring schemes provided baseline information. In one study area with low shooting intensity, greylag geese did not change the timing of their morning departure from the roost to the feeding areas. In two sites with higher shooting intensities, they gradually delayed their morning departure from the roosts over the years. In the two sites with intensive shooting, greylag geese redistributed themselves during the daytime, albeit in small numbers. In the site with low shooting intensity, greylag geese depleted the waste grain resources, the preferred food. In the two sites with higher shooting intensities, the geese left while food was still plentiful. Pink-footed geese did not change their roost flight departure and only marginally redistributed themselves during the daytime. In sites where shooting-free areas were established, numbers of greylag and pink-footed geese immediately increased. The weak reaction by the geese to diurnal regulation was not due to a lack of behavioural flexibility in response, but reflected the fact that staying and adjusting to the diurnal regulation was a less attractive option than moving on to less disturbed sites. In conclusion, the diurnal shooting regulation did not achieve the intended management objectives.

Decreased survival of pink-footed geese Anser brachyrhynchus carrying shotgun pellets

Pink-footed geese Anser brachyrhynchus caught during March–April 1990–1992 in western Jutland, Denmark, were examined for shotgun pellets by X-ray photography, marked by individual colour neck-bands and released. Based on resightings during 1991–1995, survival rates of pellet carriers (N = 114) and non-carriers (N = 230) were compared. Annual resighting probabilities were more than 99%, resulting in a high precision of survival rate estimates. No indications of differences in survival between sexes or age classes (first-year and older) were found, but pellet carriers had a significantly lower survival (0.765) than non-carriers (0.869), this difference being constant between years and cohorts. Since pellet carrying individuals could theoretically represent a non-random subset of the population, this is not conclusive evidence that infliction of pellets affects survival. The lower survival affects the frequency of adult pellet carriers found in X-ray samplings. Correcting for this, the evidence strongly suggests that pellets are inflicted upon at least 0.7 goose for each bagged one.

Shotgun pellet loads and infliction rates in pink-footed geese Anser brachyrhynchus

Pink-footed geese Anser brachyrhynchus caught in western Denmark during March–April 1990–1992 were X-rayed to detect shotgun pellets. Among first-year and older geese, pellets were detected in 25% (N = 69) and 36% (N = 286), respectively. A simple theoretical model is proposed, relating frequencies of pellet carriers in different age classes, adult survival and the annual rate at which pellets are inflicted upon geese. This model resulted in estimates of pellets being inflicted upon 1,000 first-year and 800 older geese annually. However, annual bag is ca 1,000 first-year and 2,000 older geese, and thus, expectedly, pellets should be inflicted upon approximately two older geese for each first-year. From this it is concluded that the model leads to unrealistic estimates because the observed frequencies of first-year and older pellet carriers are inconsistent; if 25% of the geese carry pellets after their first hunting season, more than the observed 36% of the older geese should be carriers. One explanation of this could be statistical uncertainty of the observed frequency of first-year carriers. A frequency of 15% pellet carriers (lower 95% c.l.) after the first hunting season could account for the inconsistency. As an alternative explanation, lower survival of pellet carriers is proposed. A more general model including differential mortality predicts that survival rates of 87% and 78% for non-carriers and carriers, respectively, could explain the observed carrier frequencies. Use of lower 95% c.l. leads to estimates of pellets being inflicted upon a minimum of ca 0.5 goose per bagged one. If differential survival exists, this ratio will be somewhat higher.

Exposure of spring-staging pink-footed geese Anser brachyrhynchus to pesticide-treated seed

The Svalbard population of pink-footed geese Anser brachyrhynchus is concentrated in western Jutland, Denmark, from early March to early May. During spring, the geese shift feeding habitat from grasslands and stubble fields to new-sown fields. To avoid crop damage, grain bait is provided at five sites. The aim of this study was to quantify the exposure of geese to, and the ingestion rates of, pesticide-treated seeds, and to evaluate the potential effects at the individual and the population level. During spring 1994, approximately 7% and 1 % of the total number of goose-days were spent on new-sown cereal fields and new-sown pea fields, respectively. After the commencement of sowing, about 25% of all goose-days were spent in new-sown fields. Late-departing individually marked geese made more frequent use of new-sown fields than early-departing individuals (P < 0.001). Geese foraged intensively in new-sown fields early in the morning and sometimes late in the evening. Due to high feeding profitability of the new-sown grain compared to grass, the geese obtained half or more of their daily energy intake by feeding on new-sown cereal fields, even where bait grain was provided. Spring-sown barley is treated with the fungicide Imazalil and peas are usually treated with Thiram. The daily Imazalil ingestion rate by an average goose was estimated at 9–15 mg active ingredient (a.i.), or 3–-5 mg a.i./kg body mass, which is two orders of magnitude below reported LD50 values for various species of test birds. Imazalil may have sub-lethal effects, especially on geese using the new-sown cereal fields for consecutive days, but the low toxicity and high mobility of the compound suggest that effects are minor and short-lived. The daily Thiram ingestion rate by the geese was not quantified, but it is calculated that a goose would have to eat about 100 g of peas to reach a level of Thiram ingestion (200 mg/kg diet), which could have sub-lethal effects on reproductive parameters. A goose foraging on new-sown peas can accomplish this within less than one hour. Special management precautions should be taken to deter geese from exploiting new-sown pea fields.

Hunting migratory geese: is there an optimal practice?

Since the mid-20th century, many European and North American goose populations have increased dramatically in numbers, causing conflict with agricultural interests in their staging and wintering areas. In some cases, to mitigate such impacts of rapid population increases, population control has been attempted by increasing harvest rate. In this study, we investigated how autumn-staging pink-footed geese Anser brachyrhynchus responded to hunting, with a view to determine hunting practice that would lead to an increase in the hunting bag. There was a significant increase in the distance between the hunting site and the goose flocks, on comparing goose distribution on the day before the hunt up to one day after the hunt. The effect was significant when at least 10 shots were fired per site but not when 1–10 shots were fired. The timing of shooting in relation to migratory phenology did not affect the time taken by the geese to return to the hunting site, but after a hunt in the early part of the staging season, the number of geese in the study area increased more rapidly than towards the end of the season. The maximum number of geese shot per hunting event was obtained when hunting events were separated by three days. Our results indicate that hunters can increase local harvest by temporal and spatial optimisation of practices. These results may be used as a tool in wider-scale regional and international processes to regulate the population size of pink-footed geese by shooting, depending on the willingness of landowners, hunters and managers to coordinate hunting practices.