Seasonal changes in habitat use of White-fronted Geese near Antwerp, Belgium

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1 Seasonal changes in habitat use of White-fronted Geese near Antwerp, Belgium T O M J. Y S E B A E R T, P A T R IC K M. M E IR E and A N D R É A. D H O N D T Introduction W ild geese ap p aren tly have a very inefficient digestive system (O w en 1972a). It is, th e re fo re, very im p o rtan t for these anim als to select a diet th at m eets their feeding d em an d s both quantitatively and qualitatively. W h en fe e d in g, w ild g eese n o t only choose betw een vegetation zones (O w en 1971 and 1972b) o r habitats (E bbinge et al. 1975; D erk sen et al. 1982; B oudew ijn 1984; M adsen 1985; A m at 1986), but they also select certain plan t species (O w en 1976; O w en et al. 1977; Y denberg and Pri 1981; D re n t and P ri 1987). H ow ever, m ost of th ese studies are concerned w ith seasonal p a tte rn s, o r deal w ith h ab itat shifts during w in ter o r spring m igration. L ittle is known ab o u t th e tem p o ral p attern s in goose usage w ithin a single m acrohabitat, such as a grassland com plex. In this p ap er, w hich is p art o f a study on th e feeding ecology of th e E u ro p ean W hitefro n ted G oose A n ser albifro albifro in F lan d ers, use o f a heterogeneous grassland area w as investigated. Study Area T h e study w as carried out on th e BA SFgrounds at Z an d v liet in th e north of the A n tw erp industrial area. T he area used to be p a rt o f th e A n tw erp polder landscape, b u t, as a result o f industrialisation, the area w as raised by infilling in the late sixties. T he study area com prises about 300 ha of poorly d rain ed grassland. It is crossed by a few ro ad s, railw ays and drai, but on the w hole it su ffers low d istu rb an ce from people. P art o f th e grassland is periodically flooded by rain during autum n and w inter w ith fo u r o r five bigger pools rem aining th ro u g h o u t the year. D uring th e sum m er th e p astu res are grazed by cattle and sheep, w hereas in w in ter only som e of them are used fo r grazing. B efore th e raising o f th e site the w hole p o ld er w as a trad itio n al w inter haunt. T he geese w intering in th e A ntw erp polder are p a rt o f a larger p o p u latio n, m ost of which ro o st on th e m udflats o f the R iver Scheldt, 54 W ildfowl 39 (19X8): b u t som e stay on th e study area. T h ere is som etim es in terchange betw een the geese o f th e study area and neighbouring haunts on th e left b an k o f th e R iver Scheldt and on th e D u tch polders. Methods G eese w ere co u n ted on average th ree tim es a w eek, b o th on th e B A SF-grounds and in th e su rro u n d in g polder. S cattered o v er th e w hole study area, 78 plots o f 2x2 m w ere placed at random. The p lo ts w ere w idely sp aced w ithin each p astu re and w ere m ark ed w ith one peg, to w hich a sq u are o f p o rtab le, telescopic plastic pipes ( 2 x2 m ) was ad d ed w hen studying it. Sam ples w ere m ade in each plot at the end o f N ovem ber. T his late date for vegetation analysis provided inform ation on the v eg etatio n o n w hich th e geese actually fo rag ed, b u t m ade th e determ ination of d iffe re n t p la n t sp ecies m o re difficult. T h e re fo re, b en ts A grostis spp. w ere lum ped :ogether as w ere m eadow -grasses Poa spp. The occurrence and abundance o f species were record ed using the T aley scale (T a n s le y 1946). T h e s e a b u n d a n c e - estim atio w ere later traform ed into num erical values (T able 1) and ierted into Table 1. Abundance-classes used in the description of the vegetation together with the numerical value allocated. Dom inant 10 Co-dominant 9 Abundant (a) 8 Between (a and f) 7 Frequent (f) 6 Between (f and o) 5 Occasional (o) 4 Between (o and r) 3 Rare (r) 2 Sporadic 1 th e v eg etatio n m atrix. E stim atio w ere ad d ed into th e vegetation m atrix of the am o u n t o f d ead m aterial in th e vegetation (th re e classes: > 6 0 % ; % ; < 2 0 % coverag e) and o f th e to tal cover (th ree classes: den se; in term ed iate; o p en ). T o classify the

2 Habitat use o f White-fronted Geese 55 sam ples into distinct groups, th e Tw o-w ay In d icato r Species A nalysis o r T W IN S P A N was used. T W IN SPA N is a com plex, polythetic and divisive clusterm ethod (H ill et at. 1975; Hill 1979; G auch 1982). In o rd er to determ ine how th è geese use the different feeding sites, goose droppings w ere counted regularly in the plots. Since the deity of droppings is such th at counting over sm all areas is adeq u ate, and since digestion by geese is very inefficient, with a very short throughput tim e, it can be assum ed th at th e droppings found are from geese feeding at th a t lo catio n (O w en 1975a). D ropping counts are often em ployed in th e assessm ent of goose usage of various habitats in o rd er to d eterm ine seasonal and spatial patter (E bbinge et at. 1975; Y denberg and Pri 1981; D erksen et al. 1982). T hey can be subject to bias, due to the attitude of the observer, num ber of observers, size and shape of th e plot, effects of dropping deity, and effects of w eather and dropping disintegration (B édard and G au th ier 1984). T o m ake reliable counts, all counts w ere done by T JY, always in the sam e crouched position. T he sm all plots (2x2 m) allow ed a com plete overview. T he droppings w ere rem oved from the plots after counting. O nly single droppings p ro duced by grazing geese w ere included in the analysis, piles o f th ree o r m ore droppings produced by resting geese w ere excluded. D roppings w ere counted on 7 occasio. D ue to the w eather conditio (i.e. frost), the periods w ere unequal: 1: 14 N ovem ber - 5 D ecem ber; 2: 6-19 D ecem ber; 3: D ecem ber; 4: 31 D ecem b er - 26 January; 5: 27 January - 28 F ebruary; 6 : 1-17 M arch; 7; 18 M arch - 4 A pril. Results Seasonal variation in num bers T he num ber of geese in the study area (in relation to the average daily tem p eratu re) during the w inter is given in Figure 1. In N ovem ber, num bers increased slowly. A t the end of D ecem ber, w hen the tem p erature fell below 0 C, up to 5,000 W hitefronted G eese arrived, but m ost of them (X 1000) Figure 1. Number of White-fronted Geese (full line) in relation to average daily temperature (broken line) on the study site during the winter

3 56 Tom J. Ysebaert, Patrick M. M eire an d A n d ré A. D h o n d t left very rapidly. D uring the rest of the w inter num bers fluctuated largely in p arallel w ith the tem p eratu re. D uring the cold spells (often with snow-fall) geese left the area and w ent to the surrounding polder, w here th ey m ainly foraged on w inter c ereals and stu b b le fields, so m etim es together w ith B ean G eese A er fabalis. E arly in A pril the last goose left. Vegetation T he study area is dom inated by a few cultivated grass species: m eadow -grass, Poa sp.; ben t, Agrostis sp.; cocksfoot. D actylis glom erata. B ecause of the nature of the soil (a raised site), m any species typical of w aste places also appear, such as chickw eed, Stellaria m edia; com m on m ouse-ear chickw eed, C erastium fo n ta n u m ; yarrow, Achillea m illefolium. T he grassland on the B A SF-grounds is th erefo re probably a rath er m arginal feeding ground com pared w ith th e surrounding polder. T he T W IN SPA N resulted in a distinction betw een the tw o m ain groups (Figure 2). G roup 1 com prises 49 plots, subdivided into 5 subgroups ( l a - l e ), and is characterised by a high diversity, and a high abundance o f grasses, Poa sp.; D. glomerata; red fescue, Festuca rubra, but also w ith m any pioneer species, e.g. S. Media-, C. fo n ta n u m ; white clover, Trifolium repe-, A. m illefolium. G ro u p 2 com prises 29 plots, subdivided into 3 subgroups (2a-2c), and is characterised by a low diversity, and a very high abundance of only a few grass species, Poa sp.; A grostis sp.; m arsh foxtail, A lopecurus geniculatus; p e re n n ia l ry eg rass, L olium perenne. T he occurrence of the m ost abundant species in each subgroup is sum m arised in Figure 3. T he differences in vegetation com position are due to abiotic factors (T able 2). O ne of the m ost im portant is clearly the hum idity gradient. O thers are the nature o f the soil and its m anuring. T hese factors to gether w ith the species com position d eterm ine th e structure and cover of the vegetation (T able 2). G oose usage T he num ber of droppings per plot per period varied betw een 0 and 135. The num ber of droppings p er vegetation subgroup, expressed as a percentage of the total num ber o f the droppings p er period, C b 2 a 1 e 1 d 1 c 1 b 1 a Figure 2. analysis. Dendrogram showing the different vegetation groups as determined by a TWINSPAN-

4 H abitat use o f W hite-fronted G eese sp ecies Figure 3. Average abundance of some common plant species in each subgroup. / = Poa sp.; 2 = Agrostis sp.; 3 = Atopecurus geniculatus; 4 = Dactylis glomerata; 5 = Festuca rubra; 6 = Lolium perenne; 7 = Cerastium fontanum ; 8 = Stellaria media; 9 = Trifolium repe\ 10 = Achillea millefolium; 11 = Taraxacum officinale; 12 = Moss is show n in Figure 4. F or each subgroup (except subgroup Ie ), th e average num ber of droppings p er plot per day w as found to d iffer significantly b etw een th e seven periods (K ruskal-w allis test, T able 3). T he lack of significance in subgroup Ie is due to th e very low dropping deities. F or each period (except period 7), the average num ber of droppings p er plot per day w as fo u n d to d iffe r significantly b e tw e e n th e 8 v e g e ta tio n su b g ro u p s (K ruskal-w allis test, T able 4). T he lack of significance during period 7 is again due to the very low dropping deities. D uring period 1, m ainly th e plots of group 2 (especially 2c) w ere used. T he sam e

5 58 Tom J. Ysebaert, Patrick M. M eire and A n d ré A. D h on dt Table 2. Characteristics of the vegetation for each subgroup by the TWINSPAN-analysis. H M S C D 1a b Ie Id Ie a b c H (Humidity) 1 = dry 3 = wet M (Manuring) 1 = low 3 = high S (Structure) 1 = rough 3 = fine C (Cover) 1 = open 3 = dee D (D ead material) 1 = less 3 = much trend was found during period 2, but subgroup Ie also becam e an im portant feeding site. T hus, early in w inter, w hen num bers are still low, there is a selection tow ards the w etter and m ore m anured plots. T he vegetation of these plots coisted m ostly of cultivated grasses and had a fine structure and a dee cover w ith less dead m aterial (T able 2). D uring period 3 (w hen large num bers of geese arrived on th e study area) and period 4, a decline in the use of group 2 was seen, w ith a positive change tow ards subgroups 1a and 1b. Subgroup Ie rem ained as im portan t as it was during the first tw o periods. T he decline in use of th e w etter plots of group 2 (especially 2 c) was partly due to the average tem p eratu re often falling below zero, and resulting in the freezing of som e of the w etter plots. This m ade them uuitable for foraging, so the selection was then for the drier plots, w here birds w ere able to find m ore unfrosted grass. T he preferred plots still had a high state of m anuring and a dee cover. D uring periods 5 and 6, vegetation subgroup Ie and to a lesser extent 1 a w ere the m ost preferred. P erio d 7 w as difficult to in te rp re t, because o f the very low dropping deities. H ow ever, th ere w ere indicatio th at the plots of group 2 w ere used again. By calculating the Spearm an R ank correlation coefficients for the num ber of dro p pings per plot betw een th e 7 periods (T able 5) we see th a t the geese shift sequentially betw een feeding sites. Indeed, period f is strongly correlated w ith period 2, so the goose usage is sim ilar during these two periods. W e find sim ilar significant correlatio betw een periods 2 and 3, betw een periods 3 and 4, betw een periods 4, 5 and 6, and betw een periods 5, 6 and 7. D iscussion In general, in the A ntw erp polder the E uro p ean W hite-fronted G oose fed mainly on cultivated grassland and poorly drained pastures. O nly during cold periods did the geese shift tow ards stubble and w inter cereal fields. A sim ilar attraction to relatively w et and m ostly cultivated grassland has been observed in Sw eden (M arkgren Table 3. Differences in the number of droppings per plot between each vegetation subgroup for each period. Period N = ** ** ** *** * * Kruskal-Wallis: Chi-square + significance ***= P<0.001 **= P<0.01 *= P<0.05 = not significant Table 4. Differences in the number of droppings per plot between each period for the 8 vegetation subgroups. Subgroup 1a 1b Ie Id Ie 2a 2b 2c N =70 N=63 N=56 N=105 N =49 N =89 N =42 N = *** *** ** *** *** ** *** Kruskal-Wallis: Chi-square + significance ***_ P<0.001 ** = P<0.01 *= P<0.05 = not significant

6 Habitat use o f W hite-fronted Geese ie O) c C 1d 40 ' 2c I B Æ j Udii II., i p e rio d p e rio d Figure 4. Average number of droppings per vegetation subgroup in each period, expressed as percentage of the total number of droppings in that period. 1963), in Belgium (K uijken 1969), in the N etherlands (L ebret et al. 1976), and in E ngland (O w en 1971). W inter grain fields, clover fields, stubble fields w ith w aste grain, ploughed o r harrow ed fields w ere rarely used. H ow ever, now adays m any pastures and grasslands are traform ed into arable land, forcing geese to feed on these fields. T he basis for habitat selection in wild geese is very com plex. Small herbivores, such as geese, require m ore nutritious and digestible foods th an large herbivores, because of th eir lim ited ability to digest fibre and cellulose (B uchsbaum et al. 1986). G eese have very low digestibility coefficients in contrast to th eir m etabolic needs. It is th erefo re necessary for them to se le c t a re la tiv e ly h ig h -q u a lity d ie t. H ow ever, feeding selection is cotrained by the need to avoid plants w ith high levels of secondary m etabolites (B uchsbaum et al. 1984). T he im portant characteristic of the vegetation is the energy which can be m etabolised per unit w eight ingested. This

7 60 Tom J. Ysebaert, Patrick M. M eire an d A n d ré A. D h on dt Table 5. Spearman rank correlation coefficients (+ significance) between the different periods, based on the number of droppings per plot per period. Period / / * * / / / * * * / * * 0.11 / ***= PcO.OOl **= PcO.Ol = not significant N = 78 energy is determ ined by the nutritive value (O w en 1975b and 1979; Y denberg and Pri 1981), and the digestibility of the plant species (E bbinge et al. 1975; D rent et al. 1979; M adsen 1985). T hese are in turn determ ined by the species com position of the vegetation and its state of grow th, and by the fertility of th e soil. D uring cold spells, the geese leave the m ain study site (the B A SF-grounds) for the surrounding pold er to forage on stubble fields and w in ter cereals. A p p aren tly, energy intake on the B A SF-grounds is iufficient at such tim es. N evertheless, under norm al conditio geese prefer the B A SF-grounds. From this it appears that feeding preferences are also influenced by factors such as isolation from disturbance (N ew ton and C am pbell 1973; O w en 1979; K uijken and M eire 1987), distance from a ro o st, and w e a th e r c o n d itio n s (O w en 1971). T he B A SF-grounds not only serve as feeding grounds but also as a roost; the disturbance is low as com pared to th at on the polder, w here shooting and farm ers activity is high. T he geese also show a tem poral and spatial preference for distinct vegetation subgroups w ithin a grassland com plex. M oreover, th ere are indicatio th at geese shift feeding sites sequentially during w inter. H ow ever, this selection is not absolute. A lthough the vegetation is often sim ilar over larger areas, quite im portant changes can occur in a short distance. C oequently, while geese are m oving during foraging (on average 2 0 steps/m in), they can cross several vegetation subgroups. H ow ever, the tim e spent in each vegetation subgroup can differ. E specially w hen large num bers are p resent, the geese are feeding over the w hole study area and m ake use of m ost of the plots. T he geese seem to select for a vegetation w ith low am ounts of dead m aterial (O w en 1971) and w ith a dee cover and a fine structure. T his vegetation is found on m ore m anured soils w ith a high hum idity. F urtherm ore, the species com position of the vegetation is also im portant. Som e plant species are p referred, others are avoided. In this study a significant positive correlation is found betw een the num ber of droppings p er plot and the abundance of Poa sp. for period 3 (rs:0.35 P co.o l), period 4 (rs:0.36 PcO.OOl), and period 6 (rs:0.26 PcO.O l); the abundance of T. repe for period 2 (rs:0.24 P c0.05) and period 5 (rs:0.26 PcÒ.05); and the abundance of A. geniculatus for period 1 (rs:0.35 PcO.Ol) and period 2 (rs:0.25 PcO.05). A negative correlatio n is found w ith the abundance of A grostis sp. for period 3 (rs: P c0.05), and for period 4 (rs:-0.35 PcO.Ol). Faecal analysis could confirm these correlatio. Indeed, it was show n in the o th er studies that wild geese do select for distinct plant species. T h e e x p lo ita tio n o f p re fe rre d fo o d sources by grazing wild geese results often in their depletion to a level beyond which exploitation is no longer profitable. The utilisation of seed crops o f A grostis stolonifera by G reylag G eese A er aer in sum m er is one exam ple of such a finite food supply. O n th e o th er hand, B rent G eese

8 Habitat use o f White-fronted Geese 61 Branta bernicla w ere able to visit a saltm arsh every 3-5 days, resulting in a rem oval of th e w hole harvestable fraction of the food stock, because the food source. Plantago m aritim a, shows regrow th in spring (D ren t and Pri 1987). In the present study the geese shifted tow ards o th er (less p referred) vegetation subgroups during w inter. Possibly, the p re ferred vegetation was depleted because regrow th was very slow in w inter. W eath er conditio seem to have accelerated the shift tow ards o th er vegetation subgroups because of the freezing over o f som e o f the w etter plots. In a m ild w inter the geese w ould forage for longer on these plots. W hen grass grow th starts (M arch), there are indicatio th at the geese shift back to the vegetation subgroups first used. Acknowledgements We wish to thank Mr. Fichtelberger and Mr. Clotman (BASF) for their permission to work on the pastures of the plant, Lut Dem arest and Johan Heirman for the vegetation analyses and helpful discussio, and Ludo Benoy, Raymond de Man, and D r Eckhart Kuijken for their interesting advice. Summary Distribution and habitat use of European W hitefronted Goose Aer albifro albifro in winter were studied in relation to habitat selection near Antwerp (Belgium). Geese were counted on average three times a week. The vegetation was analysed in 78 plots, placed at random. Eight vegetation subgroups were distinguished, based on a TW INSPAN program. Goose usage was determined by counting droppings in the plots in 7 periods throughout the winter. Geese shifted from the study area towards the surrounding polder during cold spells. Geese also showed a tem poral and spatial preference for distinct vegetation subgroups within the study area. M oreover, there are indicatio that they shifted feeding sites sequentially during winter. The different use of feeding sites was explained by the fact that the geese probably had to deal with finite food supplies of different qualities, and often with severe w eather conditio and disturbance. References A m at, J.A Numerical trends, habitat use, and activity of Greylag geese wintering in southwestern Spain. Wildfowl 37: Bédard, J. and G authier, G Assessment of faecal output in geese. J. appi. Ecol. 23: Boudewijn, T The role of the digestibility in the selection of spring feeding sites by Brent Geese. Wildfowl 35: Buchsbaum, R., Valiela, I. and Swain, T The role of phenolic compounds and other plant cotituents in feeding by Canada Geese in a coastal marsh. Oecologia 63: Buchsbaum, R., Wilson, J. and Valiela, I Digestibility of plant cotituents by Canade Geese and Atlantic Brant. Ecology 67: Derksen, D.V., Eldridge, W.D. and W eller, M.W Habitat ecology of Pacific Black Brant and other geese moulting near Teshepuk Lake, Alaska. Wildfowl 33: D rent, R., Ebbinge, B. and W eijand, B Balancing the energy budgets of arctic-breeding geese throughout the annual cycle: a progress report. Verh. Orn. Ges. Bayern 23: D rent, R.H. and Pri, H.H.T The herbivore as prisoner of its food supply. In: J. van Andel et al. (E ds.), Disturbance in Grasslands. Dr. W. Junk Publishers, D ordrecht. Pp Ebbinge, B., Canters, K. and D rent, R Foraging routines and estim ated food intake in Barnacle Geese wintering in the northern Netherlands. Wildfowl 26:5-19. Gauch, H.G M ultivariate analysis in community ecology. Cambridge University Press, Cambridge. 298 pp. Hill, M.O., Bunce, R.G.H. and Shaw, M.W Indicator Species Analysis, a divisive polythetic m ethod of classification, and its application to a survey of native pinewoods in Scotland. J. ecol. 63: Hill, M.O TW INSPAN, a FO R TR A N program for arranging multivariate data in an ordered two-way table by classification of the individuals and attributes. In: Ecology and Systematics. Cornell Univ. Ithaca, New York. Kuijken, E Grazing of wild geese on grasslands at Damme; Belgium. Wildfowl 20: Kuijken, E. and M eire, P Overwinterende ganzen in Belgie: lessen uit bescherming. De Levende Natuur 88: Lebret, T., M ulder, T h., Philippona, J. and Tim m erm an, A Wilde ganzen in Nederland. Thiem e-zutphen. 183 pp. Madsen, J Relatio between change in spring habitat selection and daily energetics of Pinkfooted Geese Aer brachyrhynchus. Ornis. Scand. 16:

9 62 T om J. Ysebaert, Patrick M. M eire an d A n d ré A. D h on dt M arkgren. G Migrating and W intering Geese in Southern Sweden: Ecology and Behaviour Studies. Acta Vertebratica 2:299^118. Newton, I. and Campbell, C.R.G Feeding of geese on farm land in East-Central Scotland. J. A ppi Ecol. 10: Owen, M The selection of feeding site by W hite-fronted Geese in winter. J. Appi. Ecol. 8: Owen, M. 1972a. Some factors affecting food intake and selection in W hite-fronted Geese. J. Anim. Ecol. 41: Owen, M. 1972b. M ovements and feeding ecology of W hite-fronted Geese at the New Grounds, Slimbridge. J. Appi. Ecol. 9: Owen, M. 1975a. An assessment of faecal analysis technique in waterfowl feeding studies. J. Wildl. Mgmt. 39: Owen, M. 1975b. Cutting and fertilising grassland for winter goose m anagement. J. Wildl. Mgmt. 39: Owen, M The selection of winter food by W hite-fronted Geese. J. Appi. Ecol. 13: Owen, M Food selection in geese. Verh. Orn. Ges. Bayern 23: Owen, M., Nugent, M. and Davies, N Discrimination between grass species and nitrogen fertilised vegetation by young Barnacle Geese. Wildfowl 28: Taley, A.G Introduction to plantecology. Allen and Unwin, London. 260 pp. Ydenberg, R.C. and Pri, Th Spring grazing and m anipulation of food quality by Barnacle Geese. J. Appi. Ecol. 8: Tom J. Ysebaert and André A. Dhondt, D epartm ent of Biology, University of Antwerp (U IA ), B Antwerp, Belgium. Patrick M. Meire, Laboratory of Animal Ecology, Zoogeography, and Nature Coervation, State University of G hent, Ledeganckstraat 35, B-9000 Ghent, Belgium.