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Foraging efficiency in Barnacle Geese 7

Foraging efficiency in Barnacle Geese Branta ieucopsis: a functional response to sward height and an analysis of sources of

individual variation

A. Lang12 & J. M. Black3

'C entre fo r Behavioural· Biology, School· of Biological· Sciences, U niversity of B ris to l,

W oodland Road, B ris to l. BS8 1UG. U.K. Email·: a lison@ burble land.com

in s t itu te of Zoology, Zoolog ica l Society of London, Regent's Park, London NW1 4RY, UK

’ D epa rtm en t of W ild life , H um bold t State University, Areata, C aliforn ia , 95521, U.S.A.

The functional· response and variation in foraging e ffic iency o f captive Barnacle Geese was m easured on pasture sward. Intake ra tes were ca lcula ted fo r a range o f grass sw ard heights using d irec t m easurem ents o f peck rate, bite he ight and num ber o f leaves rem oved w ith each bite. The functiona l response was found to be curved reaching a m axim um at 85mm and then declin ing. Each o f the three com ponents o f intake rate responded d iffe ren tly to sw ard height. Peck ra te declined stead ily w ith increasing sw ard he ight whereas bite he ight increased th roughout the sw ard heights used. The num ber o f leaves p e r bite showed a curved response to sw ard height, increasing to a m axim um at sw ard he ight o f 90 m m then declin ing. There were s ign ifican t d ifferences between indiv iduals fo r bite he igh t and leaves taken p e r bite. M u ltip le regression analysis using age, sex, body size and b ill size found that 73% o f the variation in leaves taken p e r bite could be explained by the age o f the goose, where m ore leaves were taken p e r bite by otder geese. The resu lts are discussed in re la tion to o the r stud ies on geese

foraging on sa tt-m a rsh sw ard and o the r grazers foraging on pasture.

Key Words: herb ivory, grazing eff ic iency, Branta leucopsis.

© W ild fo w l & W e tla n d s T ru s t W ild fo w l (2001) 52: 7 -20

mailto:[email protected]

8 Foraging efficiency in Barnacle Geese

There is a long h is tory of research

on the relationship between food intake

rate and some measure of food availa b i l i ty (known as the fu n c t io n a l response (So lom on 1949)). Recent

s tud ies of the func t iona l response have been cen tra l to the deve lopm ent of detailed popula t ion m ode ls to assist w ith the developm ent of effective m a n agem ent s tra teg ies (eg A rm s trong et al. 1997). Typically, an an im al's food in take rate increases w ith increasing food a va i lab i l i ty u n t i l a p la teau is reached at wh ich point food processing ab i l i ty p reven ts fu r t h e r increase (Holl ing 1965). In herb ivores food intake rates are frequently related to p lant s tand ing crop, w h ich is the aboveg round dry w e ig h t (Craw ley 1983).

Grazing by sheep, goats and catt le has been extensively s tudied on pasture. In a hom ogeneous sward, intake rate has been found to be closely related to sward height (Allden & W h it take r 1970; Jam ieson & Hodgson 1979; B lack & Kenney 1984; l l l ius et at. 1992; Gordon et al. 1996). S tudies on sheep also found that ind iv idual variation in intake rates could be, in part, explained by variation in the size and shape of the den ta l a rcade (Gordon et al. 1996). Indeed, the re is evidence of a l ink between inc iso r arcade and survival in

a populat ion of Soay Sheep during a popula t ion crash ( l l l ius ef al. 1995).

In birds, variation in foraging e ff i c iency has m a in ly been s tud ied in re lation to age classes (Greig et al. 1983; S uther land et al. 1986; Draulens 1987; Goss-Custard & D u rre l l 1987;

Jansen 1990). In these examples, juve

niles typically took longer to locate prey

or made m ore unsuccessfu l a ttem p ts at catching prey. In a repeated m ea sures study, Desrochers (1992) found tha t the forag ing eff ic iency of ind iv idual European B lackb irds increased as they aged. This increase could, in part, be explained by an increase in b i l l size, but was though t to be p r im ar i ly due to increased de tec t ion and hand ling sk il ls .

Van der Wal et al. (1998) estimated intake rates fo r sem i-capt ive Barnacle Geese Branta leucopsis feeding on the s a l t -m a rs h tha t is used in w in te r by the Siberian popula t ion of Barnacle Geese. Using a com bination of peck rates and dropping ra tes they found tha t short te rm intake ra tes fo r pairs of geese were lowest at high p lant s tanding crop and suggested tha t at high standing crop the increased dead m a te r ia l in the sward m igh t cause a decreased intake rate. It is possible to measure intake rates fo r geese feeding on grass sward directly, but it is necessary to measure severa l com p o n e n ts . Geese remove grass w ith each peck of the sward, and can make as many as 300 pecks per

m inu te . W ith each peck a va r iab le height of a grass t i l le r is removed (here te rm ed bite height), and a goose may

remove several t i l le rs w ith one bite. The product of these three com ponents gives the estim ate of intake rate. This d irec t m e thod of e s t im a t in g in take ra tes is very d i f f ic u l t in the f ie ld because each t i l le r m us t be measured repeatedly and pract ise is necessary to


count the high peck rates. Prop et at. (1998) used th is method on the spring staging area of the Svalbard populat ion of Barnacle Geese and found tha t to ta l intake increased w ith shoot size (the

sum m ed height of a l l t i l lers). Prop & Deerenberg (1991) and B lack et at. (1994) also used th is m ethod in the field. The effect of sward he ight on ind iv idua l in take co m p o n e n ts has also been measured in the field. In s tudies on the w in te r ing grounds of Barnacle Geese, peck rates increased as bio

m ass decreased during w in te r (Owen et at. 1992) and increased w ith decreasing grass height (B lack et al. 1992).

In th is study we investigate how Barnacle Goose intake rates are a ffec ted by sward height of the sam ple tu r fs f ro m the w in te r pas tu re th a t the

W i ld fo w l & W etlands T rus t (WWT) reserve at C aer lave rock , Scotland, m anage spec i f ica l ly fo r the geese (Owen et al. 1987). Three com ponents of intake rate [peck rate, bite height and leaves taken per bite) were m ea sured d irectly to t ry to detect the

m echan ism of the func t iona l response to a hom ogeneous sward. How these com ponents vary between individuals and w h e th e r varia tion between indiv iduals can be explained by goose sex, age, body size o r b i l l size, is investigated.

Methods

Preparation

Turfs m easur ing approx imate ly 30 by 40 cm were cut f rom two f ie lds at the WWT reserve at Caerlaverock in July1996 and t ranspo rted to the WWT centre at S l im b r id g e . The g rass w as mowed sho rt then a l lowed to grow to the required height, m easured using a foam disc sward stick. This method of regrowth was used to l im i t the effects of the m ow ing on the grass s truc tu re . The vast m a jo r i ty of the grass in the sward was perenn ia l ryegrass, Lolium perenne (>95%) and so only shoots of th is species were included in the tr ia ls . The n u m b e r of p e re n n ia l ryegrass shoots in 30 c m 2 of each tu r f were

counted so tha t any variation in sward density could be accounted fo r in the analyses.

T h ir ty B a rn a c le Geese f ro m the S l im b r id g e se m i-c a p t iv e f lo ck w ere caught during the m ou l t when they were unable to fly. This f lock was establ ished in the early 1960's f rom several w ild pairs [see B lack & Owen 1987 for details). The geese were co lour ringed, sexed, measured and then released into a large holding pen w ith grass, pe lleted food and runn ing water.

Trial format

The day before each t r ia l the leaves

of 60 shoots on each of three sam ple

tu r fs w ere measured and individually m arked using s m a l l num bered w ire m arke rs . The pair o r tr io of geese to be


used in a t r ia l were herded quietly into

the tr ia l pen and a l lowed to sett le w ith un l im ited pelleted food and w a te r fo r a m in im u m of two hours, then the food was removed fo r the n ight (see Figure 1 fo r pen layout).

Im m edia te ly before each t r ia l one pa ir o r tr io were herded into the obse rvation half of the t r ia l pen from where the geese could be observed and videoed from a hide less than 1 m away. The sward height was measured and then the tu r f was placed into the tr ia l pen. Each t r ia l was recorded on video. A l l m arked shoots were re -m easured im m ed ia te ly a fte r the tr ia l.

Figure 1 Diagram of the pen layout fo r the foraging tria ls . The flock rem ained in the ho ld ing pen (approx. 12 m χ 10 m] u n til required fo r tr ia ls . The geese used in a t r ia l w ere herded along the co rr id o r into the tr ia l pen (approx. 3 m x 4 m | . The tes t tu r f was placed in the le ft t r ia l pen as the tr ia l began. Observations were made from the hide.

Foraging Measurements

1 : Peck rates.The n u m b e r of pecks each indiv idual took in continuous foraging bouts were counted from the video. The average peck rate w as calcu la ted from a l l fo r aging bouts.

2: Bite height.The bite height was ca lculated as the o r ig ina l height when the tu r fs were marked, m in u s th e new height a f te r the tr ia l, plus the average growth rate ca lculated from unb itten shoots.


3: Number of leaves per bite.Im m edia te ly a fte r the t r ia l the tu r f was examined fo r ind iv idual bite m a rks and the n u m b e r of leaves taken in each bite recorded. This method was checked by

counting the num ber of pecks made in a specia lly m arked area of tu r f and com par ing tha t to the n u m b e r of leaves taken in the area. The second method is less subjective, but only rarely did the geese take a s ing le b ite f ro m the required area. The m ethod of bite m easu rem en t was adapted f ro m Prop & Deerenberg [1991).

Functional response

Because these he ight t r ia ls took several days to comple te and involved repeated d is turbance, only one tr io of captive geese were used. Peck rate, bite size and leaves per bite were recorded fo r a range of grass heights f rom 4 to 12 cm. Only two geese provided su ff ic ien t data fo r analysis. Mean values fo r each p a ram e te r were ca lcu

lated and used in regression analyses.

Individual variation

When the geese w ere f i rs t caught m easu rem en ts of skull, ta rsus, c u l men, gape, nares and b i l l depth were made using Vern ier ca l l ipe rs to the nearest 0.01 cm [defin it ion of pa ram ete rs taken f ro m Dzubin & Cooch (1992)). Because b iom etr ic data is usu

ally h ighly corre lated, the data were analysed using p r inc ipa l com ponents analysis to provide one es tim ate of body size and one est imate of b i l l size.

Over a period of several weeks the foraging pe rfo rm ance of a l l 30 geese w as recorded, w ith each goose foraging on several sam ple tur fs .

Results

Sward details

Analysis of variance showed that there was no s ign if ican t d ifference in the perennia l ryegrass shoot density

between the two f ie lds , F-\ η9=2.36, P=0.141. The ove ra l l average shoo t density was 12,666 shoots r r f2.

Functional response

The mean values fo r each com ponent of intake rate fo r each indiv idual at each sward height were used in reg ression analyses. Bite height was found to increase l inea r ly w ith sw ard he ight

(R2=0.78, F 1J0=39.48, P<0.001), peck ra te decreased l in e a r ly w ith sw ard height (R2=0.79, F2,n=20.77. P<0.001) and the n u m b e r of leaves taken per bite increased w ith increasing sward height un t i l around 90 m m above wh ich there w as a dec l ine (R2=0.74, F2 iq=14.52, P=0.001). The data and l ines of best fit are shown in Figure 2. There were only s ign if icant d ifferences between birds fo r the peck ra te com ponen t. Combin ing the bite height and leaves per bite fo r each indiv idual at each height gives the bite size at each sward

height Figure 3. The plot and reg res sion analysis show tha t the am oun t of grass a goose can take in one peck

increases w ith increasing sward height


D Goose 1 Δ Goose 2

B 140

E 105

ω2oωû_

70

35 -

(a)

30 60 90 120

Sward height (mm)

ro3 0 1------------------------1------------------------T----------------------- T----------------------- !

0 30 60 90 120

Sward height (mm)

Figure 2. Variation in the com ponents of in take rate w ith sward height, (a) Peck rate decreased linearly w ith sward height, m ore so fo r goose two Ipeck rate=14-6-0.665 χ sward he ight-21.3 χ goose; n= 14, R2=0.79, F2,11=20.77, P <0.001). (b) B ite he ight increased linea rly w ith sward he ight (bite he ight=9.78+0.4-55

χ sward he ight; n=13, R2=0.78, F ί η0=39.48, P <0.001). (c) The num ber of leaves taken per bite increased

w ith increasing sward he ight u n til a round 85 m m above w h ich the re w as a dec line (leaves= -0.262 + 0.0524 χ sward he ight -0.000303x(sward he ight)2; n=13, R2=0.74, F2jo=14.52, P=0.001). Mean va l

ues are plotted w ith standard e rro r bars on bite he ight and num ber of leaves taken per bite.


to an asymptote above sward heights of 85m m (RML92, F2J0= 5 4 .U , P<0.001). The mean values were then combined fo r a l l com ponents to give an est imate of intake rate in m m .m in “1 fo r each ind i

v idua l at each sward height (Figure 3b). Intake rate increases w ith increasing sward height un ti l around 80 m m above w h ich the ra te dec l ined (R2=0.57, F2i9=5.87, P=0.023).

Individual variation

Two p r inc ipa l com ponents analyses using covariance m a tr ices were pe rfo rm e d using the b io m e tr ic

m e a s u re m e n ts taken of ind iv idua l geese. A p r inc ipa l com ponent incorporating the ta rsus length and s k u l l size tha t expla ined 84% of the varia tion between indiv iduals was used as a genera l size p a ram e te r (0.857 χ ta rsus

n G oose 1 Δ G oose 2

0 30 60 90 120

Sward height (mm)

.E 12000 £E 9000 H E,

6000 -ωΠ5

3000 -

30 60 90 120

Sward height (mm)

F igure 3. (a) The average bite size of the geese increased w ith increasing sward he ight to an asym ptote around 80 m m (bite size=-93.3+3.83 χ sward height-0 .0183 χ (sward he ight)1’; n=13, R =0.92, Ρ210=5A.M.

P <0.001). (b) The average intake ra te of the geese appears to decline above 80 m m (intake=-8986+4-14 χ sward he ight-2 .47 χ (sward he ight)2; n=12, R;=0.57, F2 9=5.87, P=0.023l.

14· Foraging efficiency in Barnacle Geese

+0.516 χ skull). A second b i l l size para

m e te r in co rp o ra te d the rem a in ing m easurem en ts and explained 57% of the variation (0.476 χ cu lm en 1+0.668 χ gape+0.547 χ nares+0.165 χ bill depth).

There were no s ign if icant w ith ingoose d ifferences fo r repeated tr ia ls and so data fo r each indiv idual from

several tr ia ls were combined. Analyses were only pe rfo rm ed fo r ind iv idua ls w ith more than a m in im u m n u m b e r of sam ples [50 pecks rate m easurem ents , 20 bite height m easurem en ts , 20 leaves per bite m easurem ents). Analysis of variance showed tha t both the average n u m b e r of leaves taken w ith each bite

and the bite he ight varied s ign if icantly

between indiv iduals (Figure 4). Table 1 gives a s u m m a ry of the average value across indiv iduals fo r each intake c o m

ponent. G enera l ised l in e a r m ode ls were then used to investigate how the mean values fo r each com ponent of intake varied w ith age, sex, body size and bill size, toge the r w ith in teraction te rm s . A s u m m a ry of the s ta t is t ics from the basic m ode l fo r each com ponent of intake is given in Table 2. The only s ign if ican t m ode l was of age predicting the n u m b e r of leaves removed w ith each bite (Figure 5, R2=0 .73,

F 114=37.72, P<0.001). No s ign if ican t

Ind iv idua l

2.5

2

1.5

1 -

0.5 -

* * * "

1 2 3 4 5 6 7 8 9 10 11 12 14 15 1S 17

Ind iv idua l

F igure U. Two com ponents of in take varied s ign ifican tly between ind iv idua ls : a) mean leaves taken per bite (n=17, /“ i 5,g93= 10.16, /-’<0.001 ) and b) mean bite he ight (n=12, Fl0712=7.2, P <0.001). Bars indicate the

standard e rro r of the mean.


Table 1. S um m ary of re su lts fro m varia tion between ind iv idua ls tr ia ls .

mean SEM n

Leaves taken per bite 1.7 0.4 16

B ite he ight (m m l 34 1.38 11

Peck ra te (pecks/m in) 104 4.91 14

Table 2. S um m ary of m u lti pie regression s ta tis tics fo r the varia tion between ind iv idua ls tr ia ls .

P red ic to r C oeffic ient t P

Mean peck rate Constant -338.5 -0.34 0.737

Sex 48.91 0.6 0.557

Age 6.113 1.31 0.214

Body Size 16.06 1.59 0.139

B ill Size -25.13 -1.36 0.2

Mean bite size Constant 31.51 0.78 0.463

Sex -2.377 -0.58 0.585

Age 0.36 1.49 0.186

Body Size 0.387 0.77 0.471

B ill Size -0.713 -0.82 0.441

Mean num ber of leaves Constant 3.903 2.58 0.026

Sex -0.0652 -0.59 0.568

Age 0.0519 6.32 0

Body Size -0.0196 -1.74 0.109

B ill Size -0.0038 -0.14 0.895


Age (years)

Figure 5. The mean n u m ber of leaves taken per bite increases w ith increasing age (mean leaves=1.43 +0.052 χ age, n=16, R?=0.73, f 1|U=37.72, P <0.001).

re la t ionsh ip was found between the intake rate com ponents and the goose b iom etr ics o r sex.

Discussion

The resu lts presented in th is paper show the intake rates of the geese increased w ith increasing sward height to a m a x im u m at sw ards of 85mm , then declin ing at h igher sward heights. The com ponents of intake rate tha t were

measured each showed different va r ia tion w ith increasing sward height. Bite height increased th roughou t the sward heights used whereas leaves taken per bite decreased at the h ighest sward heights. The resu lt is tha t bite size increases to an asym pto te wh ich when combined w ith a decreasing peck rate

gives the observed decreasing intake rate at high sward heights. These f in d ings correspond to s im i la r resu lts from experim ents on sheep, goats and ca ttle. Several s tud ies have found tha t bite depth (s im i la r to bite height used in goose studies] increased [M ilne et al. 1982; B a r th ra m & Grant 1984; Burl ison

et at. 1991 ; Laca et al. 1992; Flores et al. 1993; Gordon et at. 1996) whereas bite rate declined w ith sward height (Black & Kenney 1984; Gross et al. 1993;

Gordon et at. 1996). However, the m ethod of feeding is very d if ferent in geese compared to m am m a lian g raz ers. The b i l l a l low s the goose to scythe grass from the side, ra the r than bit ing f rom above. It appears tha t at very low sward he ights the geese have d iff icu lty


gather ing more than one Leaf in a s ingle bite and so rely on a high peck rate. At in te rm ed ia te sward he ights the geese take more t im e to ga ther leaves toge th er in one bite and so peck slowly. At high sward heights a s ingle bite takes several chews to ingest leaves tha t are

longer than the b i l l and so peck rate decl ines fu r the r : the average b i l l depth of the geese was 20 m m and yet they were removing up to 70 m m to ta l grass in one bite by tak ing more than one leaf pe r bite. The d i f fe rence in feed ing m e thod be tw een geese and m a m

m alian grazers could explain why the indiv idual variation in b i l l size was not found to be related to ind iv idual va r ia

tion in foraging perfo rm ance . Rather than the physical size of the b i l l d e te r m in ing the am oun t of food in one bite, it appears tha t it is the increased abil i ty of o lde r geese to ga the r several leaves at once, wh ich enhances th e i r foraging perfo rm ance. This explanation fo r the func t iona l response observed assum es tha t the geese were m ax im is ing th e ir intake rate th roughou t the t r ia l and so peck rates decreased as a m echan is t ic response to long grass ra the r than to main ta in a certa in level of intake. This seem s reasonable because the feeding t im es of the tr ia ls were short.

There are several l im ita t ions to the genera l ity of the cu rren t study. Firstly, a func t iona l response w as genera ted- f ro m two geese. U n fo r tuna te ly , because of the frequen t d is turbance

necessary to comple te th is t r ia l only captive geese could be used. Secondly, the specific func t iona l response is also

likely to be a funct ion of the hom ogeneity of the grass sward used in the tr ia l. Sheep tr ia ls have shown tha t a lte r ing the density of the sward can s ign if i cantly a l te r the po int at w h ich intake asympto tes (Black & Kenney 1984] and th is is l ikely to be the case in goose fo r aging because in dense swards the geese can take more than one leaf per bite.

As w ith the study by van der Wal et at. (1998), ou r resu lts suggest tha t the 'best' habitat, in te rm s of maxim iz ing intake rate, may not a lways be the habitat w ith the h ighest food ava i lab i l i ty. In the tr ia l, geese achieved op t im a l intake rates when foraging on swards

of between 60 m m -1 0 5 m m in height, peaking at 85 m m . Above 105 m m and below 60 m m , in take rates were m uch reduced. If geese choose th e ir food based on a m in im u m achievable intake rate (see Ebbinge et at. 1975; Drent et at. 1979; Owen & B lack 1990), they may avoid pastures w ith ta l l grass fo r the sam e reason they avoid pastures w ith sho rt grass - s u b -o p t im a l intake rates. In addition, ta l le r grass is also known to contain less protein and to be less d igestible due to an increase in the am oun t of phenolic compounds, and dead m a te r ia l (B lack et at. 1991; Fox 1993; Prop & de Vries 1993; Prop & V u link 1992). Geese probably favour actively growing sw ards of in te rm ed ia te height because they provide nu tr ien ts tha t are easily acquired and digested

(sensu Owen 1975, 1979).


Acknowledgements

This w o rk fo rm s part of a N.E.R.C. Spec ia l Topics P ro jec t (P r inc ipa l Investigator J. Goss-Custard). A.L. was supported by a s tuden tsh ip w ith in that project (GT24/95/LSPE/4-). Thanks go to the many staff at WWT S lim bridge who helped to capture the geese and set up the tr ia ls area. Also, thanks to Melanie Kershaw and Freddy Woog fo r patient help w ith the daily round up of the geese. A lasda ir Flouston and Freddy Woog provided he lp fu l com m en ts on an ea r l ie r version of th is m anuscr ip t .

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