University of Groningen. Food finding. Prop, Jouke. IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to.
Chapter 4 Food intake, body reserves and reproductive success of barnacle geese Branta leucopsis staging in different habitats Jouke Prop and Jeffrey M. Black
Abstract This paper concerns the effect of habitat choice on the dynamics of deposition of body reserves in spring-staging barnacle geese Branta leucopsis. On their way to breeding areas on Spitsbergen, these geese reside for several weeks on islands off the coast of Helgeland, Norway. They use three distinct habitat types: managed islands, which are covered by Hay-meadows fringed by salt-marsh vegetation and where grazing by livestock occurs; abandoned islands, where in the absence of people vegetation on the upper parts of the islands has developed towards communities dominated by tall herbs; and agricultural islands, where pastures are the mainstay for the geese. In each of these habitats data were collected on intake and digestibility of food components. Habitat-mediated differences in the birds’ foraging performance resulted in large variation in the accumulation rate of fat and protein reserves. Total body reserves deposited by birds on abandoned islands were 11% less than birds in a managed habitat. Geese on agricultural islands deposited much larger fat reserves than birds in the other habitats, whereas their protein reserves were smaller. Fat deposition rates in the three habitats were related to different levels of digestibility and ingestion rate of the food. The probability of raising offspring through to autumn was positively related to the fat scores that individuals achieved by the end of the staging period. However, this was not the case for geese staging in agricultural habitat, possibly because the small amounts of protein accumulated may have prevented the development of a sufficiently strong muscle system. Creating reserves on agricultural land to accommodate geese in spring may therefore have negative consequences on the birds’ reproductive performance.
Published in: Norsk Polarinstitutt Skrifter 200: 175-193 (1998)
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Chapter 4
Introduction Food resources available during the non-breeding season can affect an individual’s fitness by influencing its reproductive success (Davies and Cooke 1983; Thomas 1983; Daan et al. 1989). The link between reproductive success and habitat choice outside the breeding season is particularly important in migratory birds that breed at northern latitudes but winter in temperate regions that are often influenced by man. This implies that management of the winter habitat could affect the productivity of the population. Geese breed in arctic regions and generally winter some thousands of kilometres to the south (Owen 1980a). During recent decades many goose populations have shown a tendency to shift from natural towards man-made habitats (Owen 1980a; Robertson and Slack 1995), thus becoming increasingly dependent on agricultural crops. Agricultural foods are highly digestible but contain fewer nutrients than natural vegetations provide. Foraging on food that has a high metabolisable energy content can be costly when the need for required protein is not met (McLandress and Raveling 1981b; Madsen 1985; Alisauskas et al. 1988). In our study of the barnacle goose Branta leucopsis population breeding on Spitsbergen, habitat choice and body condition during spring migration and the subsequent reproductive success could be established. This bird is therefore an appropriate subject for studying the relationship between pre-breeding habitat choice and subsequent reproductive success. The population used three distinct habitats: • Managed islands, where local people keep low densities of cows and sheep. • Abandoned islands, which were once inhabited by people. • Agricultural areas, where geese depend largely on pastures managed by dairy farms. Since the early 1980s goose numbers on managed islands have remained constant, whereas those on abandoned island decreased (Prop et al. 1998). The agricultural area was discovered by geese in the 1980s (Black et al. 1991). Seven years after the first flocks of geese had been observed on these islands, more than 30% of the whole population used this newly colonised habitat. Interested in studying the phenomenon and consequence of this habitat change, we posed two questions: (1) What are the implications of habitat choice for the reproductive success of geese? and (2) Could a difference in breeding success explain why geese progressively shifted towards the agricultural habitat? To find answers to these questions, we based our work on three levels of enquiry: • Determining the quality of the habitats in terms of food intake. • Estimating the accumulation of body reserves within each of the habitats. Fat and protein reserves were estimated separately by assessing the intake and output of energy and nitrogen. An independent measure of fat reserves (i.e. fatness score) was obtained from abdominal profile indices (Owen 1981). • Investigating the fitness consequences of the amount of body reserves deposited. This was done by comparing the reproductive success of individuals using different habitats.
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Body stores and habitat choice Study area Data were collected in the coastal area of Helgeland, Norway (65º45'N, 12ºE). Over 10,000 small islands are scattered off the coast, extending up to 40 km from the mainland. Many islands are steep and barren, but islands that are flat and close to sea level usually provide vegetation suitable for geese (Gullestad et al. 1984). Throughout the area, small settlements, where fishermen/farmers and their families live, are located on so-called Home islands. Each of these islands is surrounded by a scatter of Outer islands, together forming a cluster of managed islands. Home and Outer islands are grazed by sheep and cattle, and the vegetation is cut for hay-making. Typical for managed islands is the presence of Hay-meadows, which are characterised by a high density of grasses (mainly Poa spp.) attractive to geese. In the 1970s and 1980s many of the local people moved to the mainland, and the traditional management came to an end. Haymeadows are almost lacking on abandoned islands (Prop et al. 1998); the upper parts of these islands are covered by a vegetation predominated by herbs that are inedible to geese. Managed and abandoned islands are fringed by salt marshes that are heavily used by geese. The marsh zones are dominated by Puccinellia maritima, Festuca rubra and Agrostis stolonifera. The agricultural areas are located on larger islands close to the mainland. Main crops grown on the fields are Phleum spp. and Poa spp. In each of the main habitats, a study island was selected: Sandvaer (visited in 1988-1992), Laanan (1987, 1989-1993), and Herøy/Tenna (1988-1993), representing managed, abandoned and agricultural habitat, respectively. The islands of Sandvaer and Laanan are within the traditional range of the geese (Gullestad et al. 1984). Most of the observations on Laanan and Sandvaer were collected on the Home islands which are visited by geese from dusk to approximately 8 a.m. During the remaining part of the day geese feed on surrounding islands. Data on body condition and the identity of ringed geese were collected in all the years the islands were visited. We included data collected in 1980-1982 on Laanan, which in those years had the characteristics of a managed island (people abandoned the island in 1980). Other data were mainly collected from 1990 through 1993.
Methods Analyses on plants and droppings At intervals of 3 days, samples of the main plant species were collected by carefully imitating goose grazing with finger and thumb. Within Festuca rubra we made a distinction between two different types. One type dominated most of the Festuca-zones along the shores, the other - which we called Festuca-low - was more patchily distributed closer to the shore line. Samples were immediately dried at 70ºC and stored for later processing. The amount of material collected varied between 5 and 15 g dry weight per sample. Samples were ground in a mill to pass through a sieve of 1 mm. They were then analysed for total nitrogen (Kjeldahl, modified to include nitrate), acid detergent fibre (ADF, one of the cell wall components, Goering and Van Soest 1970), and ash (by incinerating samples for 8 hours at 500ºC in a muffle furnace).
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Chapter 4 Each day 5-10 samples of 20 droppings were collected. In the agricultural area this was done at intervals of 4 days. Care was taken to collect only fresh droppings by selecting sites where geese had been observed the preceding few hours. Samples were dried at 70ºC and stored. Later in a laboratory samples were re-dried to constant weight, and weighed. For further analyses samples collected the same date were pooled (two bags per day), and homogenised in a blender. First, these pooled samples were used to assess diet composition by microscopical analysis. Identification of fragments was based on the form and structure of the epidermal cells (Owen 1975), which allowed to identify separate genus, species or Festuca-type. An adequate amount of material was taken to cover most of the surface of a slide. Preparations were non-permanently mounted in water, and no additional procedures were required to improve the identification of fragments. The relative proportions of food components were determined by systematic point sampling (Prop and Deerenberg 1991). Subsequently, the samples were ground in a mill to pass through a sieve of 1 mm, and they were analysed for ash, total nitrogen and ADF. To avoid the complications of nutrients leaching from the droppings, only samples collected under dry conditions were analysed. Dropping rate Dropping rates were assessed by following foraging geese and timing the production of consecutive droppings. Only birds within close range (less than 100 m, and most often within 50 m) were followed, and observations on a particular bird were stopped as soon as the abdomen was out of view. By using markers set out by the observers as reference points, in addition to micro-features in the terrain, the exact location of droppings observed at the time of production was noted. After the geese had left, droppings were recovered and dried for later re-drying and weighing. Abdomen profile The abdomen profile (AP; Owen 1981) was used as an index for the amount of body fat deposited. AP’s were assessed on a scale from 1 to 7. Consistency in observations between years and habitats was achieved by placing a set of dummy geese with different AP’s on the main observation sites as a reference. Only observations on birds that were individually recognisable by leg rings with inscriptions were used. Studies on pink-footed geese Anser brachyrhynchus and Hawaiian geese Branta sandvicensis showed that AP classes (similar to ours) are linearly related to fat reserves (Madsen et al. 1997; Zillich and Black 2002). As we assume this holds for barnacle geese as well, we treat AP as an ordinal-scaled variable. Reproductive success Parents and their offspring usually stay together for at least 6 months (Black and Owen 1989a), and this enabled us to determine the number of goslings that survive up to winter as a measure of reproductive success. Observations in the wintering grounds were collected at Caerlaverock, Scotland. For further analysis, pairs were classified as successful (observed with at least 1 gosling) or unsuccessful (no goslings). Data of geese observed
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Body stores and habitat choice both in spring and in subsequent autumn allowed a comparison between reproductive success and final AP prior to departure for the breeding grounds (i.e. average AP after 14 May, rounded to the nearest integer). Calculations Protein content in plants was calculated by multiplying total nitrogen by a factor of 6.25. Apparent digestibility of the food (%, on an ash-free basis) was calculated following Van Soest (1982): D = 100 × (1 - Mf / Md) where Mf and Md are the concentrations (ash-free) of a marker in the food and droppings, respectively. We used ADF as a marker. As a small proportion of ADF may be digested in spring (Prop and Vulink 1992) estimates of digestibilities are conservative. Mf was calculated from the regressions of ADF in food plants by date, where the relative importance of each species was weighted by the occurrence in the diet. Ingestion rate of organic matter (g min-1) was calculated as: IR = (W/I) × (100 / (100 - D)) where W= average dropping weight (ash-free) of the sample; I= average dropping interval derived from the regression of intervals by date (see Results). The quotient of W and I estimates the egestion rate during foraging, and observations were therefore only collected while geese were active (feeding plus short vigilant spells, as opposed to periods of loafing lasting for at least several minutes). Digestion rate (g min-1) was calculated as: IRD = IR × (D/100). Retention efficiency of nitrogen (%) was calculated as: AN = 100 × (1 - (Mf /Md) × (Nd /Nf)), where Nf and Nd are the proportions of nitrogen (ash-free) in food and droppings, respectively. Nf was calculated from the regressions of nitrogen in food plants by date, where the relative importance of each species was weighted by the occurrence in the diet. Ingestion rate of nitrogen (g min-1) was calculated as: IRN = IR × (Nf /100). Retention rate of nitrogen (g min-1) was calculated as: IRAN = IR × (Nf /100) × (AN /100). Excretion rate of nitrogen (g min-1) was calculated as: ERN = (W/I) × (Nd /100). The excretion of nitrogen is a continual process, and we estimated excretion rates during rest (ERN0) from the regression of ERN on IRAN by extrapolating to a nitrogen ingestion rate of 0.
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Chapter 4 Accumulation of nitrogen (g day-1) was calculated as the difference between ingested and excreted nitrogen: SN = (IRAN × Active) - ERN0 × (24 × 60 - Active), where Active is the number of min per day geese were foraging: 990 min in managed, 1044 in abandoned and 888 min in agricultural habitat (F2,340=15.38, P
