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

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

Foraging efficiency in Barnacle Geese 9

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

10 Foraging efficiency in Barnacle Geese

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.

Foraging efficiency in Barnacle Geese 11

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

12 Foraging efficiency in Barnacle Geese

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.

Foraging efficiency in Barnacle Geese 13

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.

Foraging efficiency in Barnacle Geese 15

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

16 Foraging efficiency in Barnacle Geese

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

Foraging efficiency in Barnacle Geese 17

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).

18 Foraging efficiency in Barnacle Geese

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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