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Greater Black-backed Sull and Bald Eegle • " Predation on AMerican Cooh

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Ste'an Sobkow1lJk

o Oepertl'lent. of Reneweble Resourcea

Macdonald College of f'tc6ill University, Montreal r

August, 1986

-A thesl s subl'l1 tted to the Faculty of~ Sreduate Studles elnd Reseerch

ln pertiel .f..ultllfllent of the' requirelllents for the degree of <>

H~ul t er of Sc 1 ence

, , '. @) Steran Sobkowtak, 1986 ...

, . Permissio~ has been granted-- LI autorisation a été accordée ... to ~he ~ëJ. tiçm~~ Library of à la Bibliothèque nationale Canada to rnicrt>filni ·... this­ du Canada de microfilmer thesis and to lend or sell cette thèse et de prêter ou~ copies of the. film. ~ de vendre des exemplaires du film.

,fi The author (copyright owner) LI auteur (titulaire du droit h as r'e s e r v e dot he r d' auteur) se réserve les publication rights, and autres droits de publication: neither the thesis nor ni la th~se ni ?e longs extensive extracts from it extraits de. celle':ci ·'ne may be printed or otherwise doi vent être impr imés où "reproduced without his/ber autrement reproduits san~ son written permission. autorisation écrite. ,

1 SBN 0-315-38226-0

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To Ply father,

whoae ~ncouragell\ent, love and generoaity 1'11 alwaVI treaàure.

r 'll Bee vou 1 n He.ven.

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SUQQested. short t1 Ue:

Avlan predation on eoot Il

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0 ABSTRACT 0 ~ Grea ter Black-b~.d Gull Larul! !!la'>, CUQ and 1,,"!!dure Bald Eagle •• Haliaoetul! leycoceoholuo' prodat'ion on Afllerlcon Coota Eul1co oeorlcono wo. /,' etudied '1"0/11 22 October" ta 18 Decel'lber 1983, on Laker. Hotta/lluakeet, North • Carolina. , '. \ t. During gull-coot-i-nhroctionll, coot behaviOl"s wh1ch rellulted ln 1 on '-;- • 0

1 1 _"" 1 interactions were: ru~h or fly <.44%), dive (33%),. lone coot ln l''IlKed

apecie5 "lock (14X) a.nd ci!itèr behaviorll (~X). Du~lng S0X of kllh, 0 coat

becaPlO"'. slngled ouf ·bec~lIe 1t d~ve. The l"Ioon tu"e' to aingle out.., coat wu low..er for kll15 (365'> than for atrlkell (103'~a). We obaorved 33

f.,' k.llla,t."\42 IItrikea ond 71 puraults. Gull prodafion aucceaa alPloat doubled

(23% to 44%) if a gull l'Ianaged ta atrlke 0 coot. DaUy act lVUy between

gulh and coots waa low~st frolll 1200-1500h. 5eallonaloctlvity increaaed

\ unUl the lIecond week of NovoMber, thon decrea!led. -Sulla Idlled between

4.e and 7.3X of tho observed coot populc',t1on 1n 2 Months. Pro y bohav1or 1

and gull age affected predotion 5UCCell!1, flock size dld not. Coots uII1ng

on "oggros51ve posture" and Mlunging at sitting gull" had a !11gnificantly

lower lncidence of predation th,," coot!1,which ueed,"on ih bock" end

Mdiving" behaviors. There wae no dlfferonco ln predation 5UCCOft~ betweon , dUferent coat flock Slzes. However, the proportion of interactiona on

eingle cootrs ln lIIixed,species flock.:p h creator th~n for any other flock c.'

eLze. Throo-year old gulla had hlchcr 5UCCO~1S thim odult and 1 and 2-yetsr

old culte.

We obaerved il'll'lature 8ald Eagles kill 4 Al'lor1con Coote And eted 8 ---,. ) of 33 coot~ ki.ll~d by Greater Black-bocked 6ulla. Coot cèS'rc~aaoe l'lada"up

\ at lets&t 79X of eagle dicte. E4g1oe obhined 94% of the1,.. food with 0 o gull CUt Up to 5 cul1s of ts11 lices would~",ob eacles trying ta otetsl coot CarC415l!105. Only 1 out of 17 plr4cy tsttOlllptt, WISII holtod by gull l'Iobblng.,

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~j o , Une ~tude de h pr~d8t1on de Foulql.!es d'A""~lQU~.: ful1ca ~QD.A par -des Go~l e'!!nds à Ma"t~e'!!u Noir ~J:..!.JJ rL~ et par de jeunes Aigles ~ 1. . '" Tête Blenche f:!ll!fteetu~ ~_ucoceJJ.h8Ius e été efect~e d~ 22 Octobre au 19

O~cePlbre 1983, sur'le lac MaHeMuskeet en ~aro1ine \du N9rd. . t· - Pendant les 1 nteractions co~lend- fo~ïque~ les cOMpodePlenh des

~'\ _'oulques-. résult!!lnt en des interactions d'une fou 1que e'!!lvec un go6land ~taient: la courtse sur l' eau ou le vol ( 44% ) ,la p1~ng~e (33%), une

'o~lciue seule f!!lisant ,partie d'une volée de dif ferentes espaces (14%)'" et

d'autres cOlllportelllente (9%). Pendant 50% des cas de pr'detion réussi, une

foulque est d~enue i st!flée perce quelle plongee'!!. Le teMps Ployen pou # "Q holer une foulque pendent une prédation réussi (36 6) était Ploindre que

celui pour un coup (103 5). NOLIS avons observ~5 33 cas de prédetlon . . r~u551, 42 coups et 71 poursuites. Le taux de r~uss1te a presque doublé 1 T (de 23% A 44%) lorsque le goél and parvenai t à frapper une foulque.

L'ectivitli quotidienne entre les deux espèces était à son p'oint le plue ) .. fI!Iible.,.... entre 12 et 15 heures. L'activité 5ai50nnier~ culMineit la dew<1~Me seMaine de NoveMbre pour, ensuite diMinuer.' Le5 goélands ont tué

de 4.6X li 7.3% de la population observ~e de foulques durant les 2 l'Ioie

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d'obeervation. le succ~s de prédatlon était influencé par le cOMporlel'1ent

des f-oUlque5 et l'âge des goélands, Mai ~ non par l' ll"lportance de le

vol~e.· lee foulques qui exhibaient les cOl'lport eMents ..

aggreeslve" et "se projetant 5ur un go~Jand posée" étaient l'Joins !louvent

VictiMe de prédation que celles qui affichaient les cOl'lporteMent5 "sur le

dos· et "de plong~e·. L' \rpor+.ance de di fferentes volées de - foulques

n'ava i t aucun rapport avec le tllUX de réU1551 te de prédat .1on. Cepandant 1 o le taux d'attaQue!l contre lee foulques isolés faisant -partie de volées de dH'erentes esp&cee étai t plus élevé ~ pour toute autre volée, Quelle

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qu'en Doit 1" dtl'leno1on. Le' goélando âgés de 3 "na "valent un taux da

~ j o succlh plus élevé que .1 es g~élanda adul rn et dsux ao's de 1 et 2 ona. ) , De Jeunes Aigles à T8te Bl&nche ont été obaervés a10r. qu'ih tuaient 4 FoulqU>ea.d'AI'I6rlque et qu'ih a'ePlparalent de'O. deo 33 'Oulquea tuées par les Goéland5 A Manhau Noir. L'a1btentat10n ,du aigle. étau

a 7!% au l'Ioi,"!5 de carcasses de 'oulq,uea et 115 ae procuraient , ". ---:...r: .. 94% de leur nourri ture par l' interPlédlaire' de ooélando. Une aeule if tentatives de pIraterIe our 17 a été 1nterrol'lpue par lell.;) goélands. Cel " .dernierll étai t de. tout âge. On cS pu observer un aigle harceU par 5 t

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.. T~8lE OF CONTENTS __ d. o . . . Page Liat 0' Tables ...... lx . Liet 0' r- i;u,.ee • • • • • • • • • • • • • • • • • f •••••••••••••••• ...... I\cknowledOll'lsnta •• ..-. •••••• , ••••••••••••• ',1' ••••• ......

Regulations on Thes'e p,.~~entat10n· ••••••••••• ~ ••• • • ~ ••••••• Il • )(v!J p,.eface ...... ? L1te,.ature.Rev1ew ...... ,. " ...... 2 V SECTION 1: Aspech of G,.eate,. Bleek~baeked Gull p,.edation on

A",e,.lean Cools •••••• fil ••••••••••• ,., ...... 10

Abst,.ect .. --" ...... ", .... -) . iii •••••••••••••••••••••• 11 Introduction •••••• ... , ...... # ...... ·...... ,. . 12 Study A,.ea and Methoda ..... -...... · .....,,~ 13

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• Resulta ...... • ••• 1 • 15

6ulle teating coots t? 16 ~ · .... · ..... ·..... '" ,Su1l pred~t1on sucees! ..... ·..... • •••••• e 17 Fectors aHecting predatic7n suceese, ·..... • ••• 1 ••• la ". Coot behavior 1 •••••• ·...... • • 1 ••• 1 • e,,1 •••• 18 Floek size • •••• 1 •• ·..... ·..... ·...... 19. .. Gull age • 1 •••• • • 1 1 ••• • ••• f .... 19 ...... , ...... ·...... ~ ... - -"- 01 scuulon • • 1 ••••• 1 ••••• ·...... ·.... •••••• i • · ...... 19

• Lite,.eture CHed ...... 'j' • ...... 1 ...... 25

~ ConnecÙ ng Ste t e",ent ...... "' .. ,...... ~. · ... , ...... 58 SECTION 2: Bald Eagles Killing A",e,.lcen Coote and Steal1nQ '" Coot Ca,.cesas! '1"01'1 G,.eater B~çk-backed 6ull s .•...••.••••••..•••• 59 Abat,.ec:t •••..•••••• ~ ••••••••••••••••••••••••••••••••••••••••

Introdud ion 1 •••••••••••••••••••••••••••••••••••••• .--. • Study Area and Methods ...... ,...... " ...... 61

Resulte ...... \ .... •••••• , ••••••••••••• 1 1 •••••••• 62

Food 1 te ... ! 'D •••• 1 ••••••••••• ...... ••• 1" •• 62 Mobbing ...... 63 o Eagle age and behavlor ...... 1 ...... 63

O'~cueUon ...... ••••• 1" ••••• 1 ••••••••••••••••••••••••• 64

vii .. . 1 Food lt8"8 t ••••• ...... -•...... ••.. ,64 "obb 1 ~g ••••••••••••• , ...... , ••• -- •••••••••• •"\ 6& o Eagle age and bohavlor ••••• '. . • • • • . . • • • • • • • • . • • • • • • • • • •• 65

Literalure CHad . ~ ...... -- .. 67

Conclusion ...... , ..... " .... '" 7.

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Table 2. Characterist1clJ of l'IaJor Larld predatlon studies Involv'lng . ~ 1 predato;' and 2 or ",ore prey durlno t'he non bre~~lng perlod 9

SECTION 1:

Table 1-1. Al'lerlcan Coot behavlors rnul t Ino ln select Ion by

Grearer Bleck-Backed Gul18 during interactions on Lake .. 4'2 ..... "et talltuekeet ...•...... •..•...... • "...... ) ...

" .. - ". ,Teble 1-~. 01sturbence til'le (OT), interaction tl1'1e (IT) and l'lean Bille to single out e 'c~ot 1 for k1l1,i\ str1kes and pursuih of - . . Al'lerican Cootl! by Greater Black-backed Gulls 29

Table 1-2. NUl'lber or Ulls 00, 'strlkes (S), pursuih CP) and

Greater Black-backed Gull predation succes! on Al'lerican Coots, / . j , accordlng to gull age. SOP· coots-Slngled Out and Pursued, • STR- coots Struck •••.•.••..•• ~ •••..•••• / ...• , , .•.• , , .•••• ' .••• ',' 30 . " ....

Table 1-". A/IIerlc"", Coot behavlors whJle interacting Id th Grea ter

Black-backed Gulls and associated predation !luccess during one cm

one encounters. 1(- 1<1lh, S· Strlkes, P- Pur!lults. SOP- cooh -. Slngled Out and Pursued, STR- eoots Stt'uck ...• , •...•••••.•.••••••• 31

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~I SECTION 2: - ; " o'" Table "2-1.. IfIIl'lature Bald El!gîe--,'o~d th"'l pleked up 'roll! Lake ' MaHafllu...-skeet, ",!th pl}esenee or !!b!5en~e 0' li 6retlte,.. Black"backed

. Gull eue' and/o,.. l'Iobblng ,-...... ~ ...... , ...... 69 "; \

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Page

,p SECTION 1:

J Fig. 1-1. Study area on Lake f1attal'lus.keet, North Carolina. OT • 2 , observation tow~rs, V - thi~ observation point troo", a vehicle - i 3, GR • gu Il roos t ...••.••.••.•••.••..•.. 33

\i Fig. 1-2. Situa'ion 1. 1) Greater Black-backed Gull approaches flock ..- of A"'erican Coot!!. 2) gull flies Iow over centre of coat flock

causing thel'l to !!~llt.lnto 2 groups. 3) guil turns an~ sJlits the

!!",aller ,of the 2. groups,, lsolating a sl'lall nUl'lber· of eoots. 4) gull' turns and isolates a single coot ,--rel'lalnder _9f coat! rUl,h or swt", to o •

reJoln the largest group. 5~ guII alane wlth o~e coat . ~ ...... 34

-' Fig. ;1-3. Situation 2. 6reahr<-8lack-baeked Sull ap'proache!l flock .

-" 01 A""erlcan Coots. 2) eoots begin ta rush toward shore. One coat ",a'y ,"

have been tipptng or dlving to feed, it ls· a 11ttle 510wer to takeuf'

than the others, lt.. _ ~ralls a'ter the rest of the 'lock. 3~ gull cuts

oft the, coot by Hylng ln Its path, forcing 1t çto stop. 4)' gull

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Fig. 1-3. Situation 3. Seille sequence o'.pictures, as 51tu4'ion 2 except ~ne coot dives durlng the.rush, usually the one closest .to

the gul1, causlng 1t to tall behtnd the rel'latnder of the flock. , The gui1 turns back and lands where the coot dove. Gull alone with

t he coat ...... , ...... , ...... '...... 37 o "

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Fig. 1-4. Situation 4. 1) Greater Black-backed GLH a~proaches

o flock of waterfowl (ln thi!! case canvasback) with'~ single Alllerican

Coot. 2) waterfowl flush, coot is IIIuch slower and faIl!! behind.

\~ , 3) coot becol'les separated. 4) gull alone w1 tl") coot. Waterfowl Hy a

distance, then 1and ...... ,: ...... ,. 39 •

Fig. 1-4. Situation 5. Sa"'e sequence of plctures as situetlon 4

except a flock of coots ts l'Iixed wHh w~terfowl. Once wlIterfowl flush

8ituatlon 1, 2 or-3begins ...... , ...... 39

. Fig. 1-5. NUl'lber of interactions (HUs + str:-ikes + pursu!ts) 1

per hour of observation .betwee'n Greater Blaclc-backed Gull!! and ."

Al"lerican Coots throughout the day as recorded on Lake Hattal'lu!lkeet t

Il North Carol1na f,...ol'l 22 Odober to lB Oecel'lber 1993. (t<,S,P)· nUl'lber

of k111s, etrlkes, pursults ...... 41

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",Fig. 1-6. NUl'lber of houre of observat ion throughout. the day tor

Greater Blaék-backed Gull predation on A",er'tcan Cooh on La~e , Mattel'luskeet, North Caroline fro", 22 October to lB Oece",ber 1983 • 43

Fig: 1-7. Great er Black-backed Gull predation success on Al"ler!can

Cooh throughout the day, as recorded on,Lake MaHa",u!l~eet, Nonh -- Carol1na fro", 22 October to 18 Oecel'lber 1983. For sel"lple n-ze 0' kills, strikes, c!n..d pursuih eee Figure 1-5 • .., ...... 4S ,

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xii Fig. 1-8. NUl'lber of Hlls, !!trHes and pursuits by Greater ~

Black-backed 6ulls on A~erican Coot!!, by 40 'houre of observation, on o 1 .".. 0

Leke Mest tal'lr!!keet, Nort.~ Carol! ne as ob'served 'ro", 22 October

to 18 Oece-r:ber 1983 .• ' ••••..• ~ •••••.•.•••.•..••••...•••••. ; • • . . . •• 47

Fig. 1-9. Greater Black-backed Gull predation -t!H:Jccees on A",eric;:~n

; - Coots"by 40 hours 0' observetion,. on Leke MettaPluskeet,. North 1 ,Ceroline as observed rrol'l 22 October to 18 Decel'lber 1983. S~R- Strl~e

succëls, SOP- Slngled out and pursued suêcess ...... , ...... 49

. , Fig. 1-10. 6re~ter Black-bmeèl Gull preda'Uon "rate on AIII-e,rlcen Coot

population on Leke Matta~uekeet, North Caroline as observed (ro", 22 .. ~' <. '- October to 18Decel'lber 1983. Predetlon rate i5 calculeted by bloc~s

0' 40 hours 0' obse~vaHon. ( ) - nu",~er 0,' H 115 observed in each block , ...... ' ...... 51

Fig. 1-11. NUl'lber of Al'lerlcan Coots observed per desy. 'ro", 1 of ~ " .. observatlo~ Mette~u5keet, " points 'on Leke North Ceroline 'ro", 22, : 1 .- October to 19 Oece~ber -1983. ET- East To·wer, CT- Centrel Tower, ,le

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I3~ If'lpoundl'lent 3 ...... '1 •••••••••••••••••• • Il •• Il ...... ,. •• 53

t .. Fig. 1-12. Greatèr Black-backed Gull pr~ation eucceee on A~erican

Coots struck (STR) and coots slngled out and pursu~d (SOP), by coot

'- "- Hock sile as· obser"'ed on Lake Mattallluskeet, North Carol1na fro~ 22 ,0 b • -- ... Octeber to 180ecel'lbèr 1983. (K,S,P) ·'nul'lber of kille, st~ikee,

\. pursul~s for that flOck s1"ze ' ...... : •••••••••••••••••..••, SS 'l'Î ,

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Fig. 1-13. Percenhge of Interections (lctlls + strtkes +

pursuits) between Greater 8lack-becked Gull!! and AlItertcan Coot!

frol'l fIIixed spectes Hocks. by coot nock slze, 'es observ'ed on LtI~e

MaHa",uskeet, North Carol1ntl 'roM 22 Oct'ober to 18 Oecel'lber 1983\t \ *. · P < 0.01 (Fischer test). ( .). totel nu",ber of lnter'èettons recorded frol'l t hat flock st-ze ." \' .••.• ,. , ••.. , .. '.••. , , •.•• ". , •• 57 !6: •

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... .~ ;.tiv p - ACKNOWLED6EttENTS

o 1 thank Jil'l Lovv~rn for brlnglng to l'Iy a'Uentlon the gull-coot

lnteraction~ at HaUal'luskeet. rI al'l grate'ul.. to the Delta Waterfowl end ...

~ethnds R~search Stat ion for grent lng l'le I!I Web~ter Fellowshlp, durlng -- \

which Ul'!e the idees 'or tJ'1IS ~tudy were ,'orl'luleted. Bruce BaU put hls

trust' ln this Webster Fellow wHh l'Iany Interesh and gulded ",e.4nto the . fruitful path of thls ,tudy. Rodger Tit",en, l'Iy the,i, ,upervisor,. gave flle

a great deel of freedol'l to choo.,e a t9Plc of Illy Intere5t. Hi' Invelueble

COI'Il'!ents. encouragel'lent end patience will always be rel'lel'lbered. , -

1 el'! very gretefu'l to Larry Di Ho, re'~ l'Ianeger et Mattel'lU5~eet

National Wlldlife Refugê, for grenting l'le perl'lieslon to conduct the etudy l ' ~ - ~n the re'uge.~arry" cooperet1on provided l'le wHh acce,s throughout the

refuge, ceMUS data end l'laps of the refuge. 1 aleo than~ the refuge ,te"

f~~ thelr cooperation end ~elp ln dl'Illantling the observation tower,. Jtf'll

Lovvorn and Todd Arnold provid~ sUl'lulating idees during field wor'" e!ld

helped W'ith' ell fi~ld 10g1stlcs.' Todd,', help ln con,truct lng the towere - ,. ') / 'If .... h g~eatly appreciated. 1 will rel'lel'!ber- Mrs. \ Topping of Topping' e . , lakeslde Hote 1 for her k 1 ndne", co-operl!lt Ion and excellent Nort h J Carol1na seafood which l'Iade Illy stey ln North Carol1n~I'I05t plea,ent. li. - Mur~ln ~ll 1 thank Bruce Bett, Mike Anderson,, Henry , and the sta,t et \ ~ the Del ta Waterfowl' end Wet lands Research Stat ion for shM'ing thelrc ideae

l'Ia~lng l'Iy ,tays et Delta the l'I05t rewar-ding learnlng I!Ixperlence ot f'IIy

The ,tudents at Delta and Mecdonald College provtded 8 con,tant

f l~el!!l. 1 al'l gratetul {o Alain Tel'l!sie"';, Pl~rr:'e Cal'lpredon and

Jean-Yve!! plrot for ,h~rinlJ the!r observation, and exper.lence 0' pred8tor-prey interactions ln F:rance.

Roger Bider, Dave B1rd, Rodger T!t~an, Fred Whorlskey and an un~nown o • revlewer provlded uee'ul cOllllllents on earlter drafts 0' the thtê51,. Mark 1

• )(V : .. .,. "cCo 11 ough p;,ovided useful cOI'I",enh on ePl eer-ller draft ot section 2.

o Steve Tinker provided helpful c~l'I~ent; on 'lgure~l to 4. Marie Hervey

l'Iade ~any tl'elpful eOIll",ents on the French abstract. "

Il l { This study, aould. not have been possible without the' flneniclo?l , support of the North A~erl~an-Wl1dllfe Fou~datlon through the Della

" "Water'owl and Wet lends Research Sht ion who suPpJ ied tlii reseerch

fundinQ. Personai support was provided by Les Fonds F. C.A.R. (forl'lerl y

F.C.A.C.), le f11nlst~re, de l'Educetton du Qu~bec, the Oepertptenh. of Renewable Resources of Mc61-11 UnlverSi ty !!fnd "'y wUe Doreen. The Del ta

- état ion provlded l'Iost of the 'e_qulpfllent necéssary for- the study. A few " , 'ite"'l5 w~re suppl1ed by the Depart~ent of Reneweble Resources, t1ecdona Id r College.

. , My b~S ines~ padne,..., _ Ste:he~,~THe, encour!g~d l'le t~roughout thls

study: Finally but forel'lost, 1 themlc fil):' wIfe Oore~n for fle.r:. love,.

patience,' 'understandlng and lSupport throughout "'y graduate studies.

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" xvi .. -r­ REGULATIONS ON THE~IS PRESENTATI~N CI

The 'ollowinQ 15 lncluded ln accordance wH" the regulaUons of the

McSll1 Univer,tt.y Faculty of Graduete Studle,:

-The Candidate has the option, subject to the epprovel of -the

Oepart",ent., of Incltldlng as part of the the,ls the text of an orlg1nal

peper,Il or paper,!!, !5uitable for 5ubl'llss10n J.Q. .. le!!rned Journels for

pu~1tcat1on. In thl! case the the!!! l'IU!t etill confo,...", to 'e11 other

requlre~ent5 explalned ln Guidellne! Concernina ~Thest! Preparation.

AddiHonal l'Iaterial (experl1'1ent~l and design date a! "weIl as descriptions , 0' equlpl'lent) l'Iust be provided in sufficient detall to allow a-cIe!!r and preclee judge",ent to be l'Iade of the il'lpodance and origlnality of the

,research reported. Abetr!!ct fi full Introduct 10n end conclu!5içm l'IU5t be

Included, and where ~.D~e than one l'It!1nuscrlpt appear!'!, connecting t'ext,

end cOl'll'lon abetrach, introduction t!lnd conclusiom, are reQuired. A ",ere

c611ection of, Manuscripts Is not ecceptablei nor: can reprints of 0. published paper! be accepted.

Whlle. the inclusion o} l'Ianuscrlpt; co-euthored by the Candidate and

other!! "is not prohiblted qy Mc6111, the Candidate 15 ~/arned to l'Ialt,e an .. expl ic i t stateMent on who contr ibut ed to ,ue" \I/ork ~nd to ~Jhat. _ ~xtent , end Supervisore and other5 ~Jill have to bear ~/ltne!S!'!- to the aèeureey of , euch cleil'1s ,beforo the Oral COl'1l'1ittee. H ehould al!5o be noted thet the

ta!k of the ExterMI ExaMiner 15 /illide ",uch 1'I0ro difflcu!t ln such Co!!S~Sf t!- end 1t 15 ln the Candidate'! interest to l'lake author!hlp re!pon!lbil1tlee p

perfectl y c le!!!". • o '"

x\l11 • \ o PREFACE The Qj:JJecUve of thh thesls 1! to deecrlbe predatory 1nteractions , .". .. on A",erlcan CootlS by Greater Blàck-bac.ked Gulla and Bald Eag1es • • . ~ The thesis cOl'lprlaes two parts, both of whlch rOCU!.. on a centra 1 .. the~e; predation on A~er.lcan C00t~. ThIs tople .w~s chosen because 'ew al/lan stU(f1~s., have provlded basic deserlpt iVe data concernl no preda t or-pre~ Interact ions on çooh. The tiret section descl"lbel!l

1 1nteract ions between Gre,t er 9lack-backed Sulla and A",erican Coots, and .. considera three aspect,s of predation: prey behavlor, fl<1ck size, and

predator age. In the' second l!Ied ion, an unexpected aspect of guI t-coot

interactions i'!I exal'llned; (he pre:sence of Bald Eaglè which to' rely al~oet exclusive"ly.. on Qulls to ·obta1n and locale food for . ~" \ A brief flterature review 1s included to provlde background ln accordance

in'or"et ion ln' the .literature revlew 15 Intended ta COMp lelllent

inforl'lat Ion already present in the !-i1troduct 10n and d1scu!!s Ion 0' the 2

",anuscrlpts, although there 15 SOMe overlep.

Rodger ""fUMl!ln prov1ded cOI'Il'lent!s on the proposai and the study ln

. Qeneral, helped establ1sh prlorlt1es ln the field durlng " a v.1slt and

provided edItor1al COMMents on the l'lanuscr1pts. 1 planned the study and

the l'Iethods _ used, collected end- ~nl!llysed the data and cOMposed the

, ",anuscr, ipt a. . '. . _ ,:.,:... Th1s t~esis 1s being sub",l tted ln the 'orl'l of two Manuscr1pte .. ~ ~,~I.. • ",!·~.j..f;;'~~· "'ntended for publi'cat Ion ln the Auk. They confor", to the requ1re",enh 0' .(1.-,,;'1' the l'Iost. recent issue (July 1986>. The first section 15 intended as a

l'I~jor publ1cat 10n, ",hile. the s.econd wi 11 be subl'l1 Hed ~s a short 1(:-.' cOI'Il'lunlcat ion. My' t he!!i!! supervisor, Rol!ger Ti t",an will be the Junior

euthor or these l'Ianu5cr1pts. ) \ LITERATURE REVIEW

Predation of young blrds by gulh le weil docufllented

Dwernychuck and BoaQ 1972, Munro ~nd 8éderd 1977, Braun et al. 1980,\

Salath~ 1983, Mendenhall ~nd Milne 1995>, but predation on full grown

blrds 115 poorly studled to date (TaPlisler 1970, Sne,ll 1981, LaYlllon 1983",

Pierotti 1983, Stelllpnlewtcz 1983, Martin 1995>. Greater Black-backed

--C-ulh l"arus Plarlnus have been rep~rted k1111ng young b1rds tBe&l'len 1978,

Plerotti 1983, Burger Md Gochfeld 1994>, end full grown birds !Juch ae;

Shelduckl5 Tadorn~ tadorna (Svensson 19?1 l, Europeen Cooh Fultea atra

.. 1925). . 8ald Eagles Hall1Leetus leucocepbalus ere also predator~ of Afllerican

eoots (Baker ln Batc"elder- 1880), Grubb and Kennedy '1982). HOb/ever, Bald

- '" Q..1 \ Eaglee are opportuniste and feed on e variety of prey. They are not only . , , '1 predàtop{,&u,t also .pirates on several !5pedes (see Fischer 1985 for a / ",-/ f ---- ~ ~,). 8eld Eegles have. been reported :stealing fis" frol"lo Greater 8lack-backed Gulle (Erskine 1968), however they have never been reported

ete51ûlg APlorlcen Coots 'rol"l these gul1!J.

Cooh ere prey not only of gulls ond eegles. but al:so to Peregrlne

Falcons Falco ILf!c..eJ;lCj..!llL~ (Scott 1880, Oekker 1980>' Although peregrlne!5

and Bald Eagle!5 kl11 APlerlcan Coots without a !ltruggle, the :situation for . - " !JulIe h not alwaY!5 :so ea,lSy. Thol"lsen lInd Meltofte (1980) round Greater

Black-bc!lcked 6ulls took 10 to 20 f!l1nute5 to klll e European Coot.

European Coot!5 de!end thel"lselve:s by gathering together in e f lock end

ralalng thet ... bill toward an overhead gull (Pastore 1984). These coot!5

can also be aggres51ve, ~ttacklng a Herrlng Gull ~ '~~nt8tue (Hibbert l . ~ "- o 1974) and killing a BIl!ck-headed Gull!...... LuH.b_4.llitU5 (Woffindln 1982). European Coot!5 are only sllghtly larger than A",erlc.sn Cooh (Cral'lp and

,,.A2 511'11'1ons 1990), but we can expect AMerican Coots to respond ln a sil'lilar

o wey to their European cou5ins. 'For e review of the gen~ral blology of

Al'ler::,an ~OO-h see F/er 1 chon < 1977).

Av lan predation can oc 51 tuatlons. A predator can

capturè young both at the Gochfeld 1984),

and gathered in cr~ches

grown blrds frol'l flocks (Kenward 1978, Celdwell 1986). Howe~er, the ,. ... rel'latr,der of this review will·be restrlcted to interactions on Bocks or

.' lone birds ~y 1 predator. - , Hany 'predator:s try to Single out 1 individuel fro", a flock. The ~ slngled out ind1'dual in general is beheviorally, physiologicellv 0'"

spatlally d1ffer~~=/rol'l the flo'ck. Behavlorally di Herent individuals

were noted by Stel'lpniewlcz (1983) i.e. -any ,,\ve~ie J~J~_u..lIU ~J..ilI which

behaved di Herent 1 y trol'l the rest of the flock becal'l~ the object- of

interest to the gu 11.' Dekker (1980) found that aU b irds wh{ch failed to

e cOl'lpletep dodQlhg a,ttel'lpt were selzed, by a peregr 1 ne.

PhY5iologicel1y different prey can be lighter than individuels ln e

popul~t1on.. (Kenward 1978), or cripples (Tal'lisier 1970) diff.erent prey lA/ere observed trall1ng behind a flock

and Whitacre 1975). Selection of prey frol'l a 'lock 15 o~ one of l'Iany

aspects of predation. J ' Mos~ publlshed reports of pred~tlon ~re 0' 1 or 2 observations of a specifie predator-prey interaction. Tables 1 and 2 are tabulations of

~eaulta trol'l Larid predation studies. The v~ables used as titles in the

tabt"es are the sc!Jl"le variable!! exal'llned In\Sect\on 1. 8ased on resul\s in j • the tables, the large Qulle are reaponsl~le fo~ Moet predation. Prey o epec les can be as sI'Jail "e the OoV'ek le, or "s large as a full Qrown Mallard Ana:s illtyrhYncho~. Gulh generally strlke prey on the head or

3 - '. neck. Gull feeding tiMe ranged between 33 and 41 Minutes. Ouration of an

o Interecton between a-Gull and its prey lasted betwee~ 30 seco~ds and 20

Minutes. Predation succes9 varled between 7.73% end 37%. The percentege

0' the' population kllled was reported dl"erently by c several lnvest1getora depe,ndlng on tHe' tusbUat and age of the young. The results

of these' 2 tables are ln no way exhaustive. but they represent gull

predation in generel.

\

..

\ ..... " . ...

• /

LITERATURE CITED , . "'V" Batchelder. C.F. 1890. The Beld Eàglb

• Braun, B.M..... P.A. Heinz, & 6.H. Heinz'. 1980. Herrlrig Gull predation on . , , Red-breasted Herganser ducklings. Wilson Bull. 92: 403.

~ Burger, J., & H. Gochteld. 1984. Great Black-backed Gull pred~tlon on KHt1wake tledgl1ngs in Norway: Bird 5tudy 31;. 149-151.. Caldwell, 6.5. 1986. Predation as a selective torce on toraglng herons: effec·h- 0' plul'lege color and flocking. Auk 103: 494-505. Cra",p, S. , & K.E.L. 51"'1'I0Q!I. (Eds.) 1980. Handbook of the b1rda , 0' Europe, the Middle East and North A'rica: the blrds 0' the western

Palearctlc. vol. II. Oxford, Oxford Unlv. Press.

Dekker, D. 1980. Hunting success rates, foraglllg habits, end prey

. [~ selection of Peregrine Falcon! ~igratlng through central Alberta.

Cano Field Nat. 94: 371-382. ., Dwernychuck, L.W., &' D.A. Boag. 1972. Ducka nesting ln association with ~

gulls-an ecological .trap? Cano J. Zool. 50: 559-563.

Erskine, A.J. 1968. ~ncounters between, Bald Eagles and other blrds ln

wlnter. Auk '95: 681-683.

Fischer, D.L. 1985. Plracy behavior of winterlng Bald Eaglea. Condor 87: '1 246-251.

Çorbush, E.H. 1925. Blrds of Hassachusetts·and other New England stat!s.

Part 1. Water b 1rds, l'Iersh blrds and shore blrds. l1a es •• Dep t . Agrle. .. ~ 0 ~

-- - \.

5 .. 1 o Freder1ckllon, L.H. 1977. A~e~en Coat

A"erica.

AasociaUon of Fish end Wlldlite Agencles • ...... Grubb, LC., & C.E. Kenn8dy. 1982. Bald Eagle wlnter habitat on

southwestern National- Foresh. USDA Forest Service Research Paper

RM- ,237, 13p. Rocky Mountain Forest and Rtlnge Experl",ent Station,

\Fod Collins, Colo. .... 1 . . ~ Hatch, J.J. 1970. Predat ion and plrec.y by gulls at a ternery in Maine.

Aut 81: 244-254.

'Hlbber-t, F.W. 1974. Coot attacting Herrln~ Gull. Br. Birds 67: 242-243.

Jyrkkanen, J .A. 1975. Glaucous-wlnged Gull 'predation on fera! Rock Doves. ',' Cano Field-Nat. 89: 77-78.

Kehward, R.E. 1978. Hawks and doves.: factors aUee-Una success and

select ion- ln Goshawk ~ttacks' on Woodpigeons. J., Ani",. Eco!, 47:

449-460.

Lay",on, "S.A. 1993. Californie Gull capturea flying Barn Swallow with lh

b 111. Wl han Bu Il.' 95: 295-297.

HarUn, f(. 1985 • .j:ferrlng Gulls prey upon 'e",ale r",iQan. Cano J • , , . . Zool. 63: 984-985. " Hendenhall, V.M. & H. Hilne. 1985. Factors affect na duckl1ng survival '9 0' Eiders SOMotria Mol11.s11'10 in northeast Scotl~nd. IbIs 127: 148-158 •

.... f1unro, J., & J. Bedard. 1977. Gull predaHon end crechina behavlour ln

~ the Co ","on Eider. J. Ani"'. Ecol. 46: 799-810.

Paga, 6., & D.F. WhUecre. 1975. Raptor predation on wlnterina shoreb lrds. Condor 77: 73-83. .. • Pastore, H. 1984. COl'lportePlent d~'enslf ~en h~rillson· des foulques a l"gard d'un goéland. Nos Oiseaux 37: 34Z.

6 .. -

o PlerotU, R. 1983. Gull-puttin interactions on l,Great , Island,

N"ew'oundland. Bio!". Conserve 26: 1-1". " Ru,debeck, G. 1951. The c~oice of prey, and l'Iodes of hunt ing ~ of pr,edatory , b-irds. wi t h spe.c:..1al reference to the1r selective effect. Dikos 3;' \ 200-231. -

Salath8, .T. 1983. La pr6dat ion du FlaMant Rose fh.<;tenlc_o...p..u.c...ta. !:..u.tuU: 1

Ecol. (Terre et Vie) 37: 87-115.

Scott-, W.E.D. 1880. On blrds observed ln SUl"lpter, Levy, and Hll1aborô'

counties, Florida.< • 8ull. Nuttall Ornlthol. Club. 6: 14-22.

Snell, R.R. 1981. Herring Gull aUacka and eats adult Male Dldsquew.

Wilson Bull. 93: i10-111..

Stel'lpniewicz, L. 1983. Hunting Methods 'or ~ Glaucous Gull an~ e~cepe

Maneuvers of ih prey, the Dovek~ J. Field,Ornithol. 54: ~29-331"

Sven5son, L. 1971. The Great B~-backed Gull Lac~~ ~_~Uw~ as a 1 "

o Tal"l1s1er: A. 1970. - Stgn1'1cation d~ or6gar151"10 diurne; et , de . W' l'aliMentation nocturne des 5arcellès d'Hiver Ana~ ,recea ereçc~~. , . Terre et Vie 4: 511-562.' , " Thol'lsert, J.B., & H. Heltofte. 1980. Great Black.-backed, Gui la , tdl:.UJ. ~. - Mrinua HUing and eating cooh Eul1cA Air:A. Danak. Orn. Foren.

Tldessk.r. 74: 147-149.

VerMeer, R.K. 1968. Ecological a~p'ect5 of dycks nesting ln high densltiea

aMong Larids. Wilson Bull. 80: 7B~83. of , .,JI .. Woftindin, I. 1982. Coots drownlng B~ack-headed Gull. Br. Bird.- 75: o o 535-5~6.

\ \

l , J 7 , '. • o ~-

Tabl. 1. Charac~erl.tlca of ~Jo~ ~~ld p~.d.tlon atué.,. lnvolv1ng 1 p~,d.tor and Z or ~ore prey an ~ne breedln; Qround. \ - 1 --~~--._------~------~------~------. \. 'redatar Pre.,' Locatlon Faeulng, tnteractlon P~ad.tlon 19opul.tlon 10CIUon Source of strl"e.. \1_ tlR.(~ln.) success " .. l1ed ------~~------~~------~------~--~------:------.------6r.at~r Black-backed fttlentlc Puffln Fr.t.rculo ~ 2.S: & 3.4% ScoUand ,Bea",an 1978

Gu ~ 1 1..AClI.!W:1lIIU. , Klt~lw.ke ~ SCldeS!yll fledgllnos

He,.rlng Suli Red-bral,ted ",,,g.nl'" ~ Dar"ISp" 11011110101 Llke "lch1gln B,..un .t .1. 1981

~ Arg'QtAS~a & "allard ~ glotYCbynsbg' due"llng. wn01. '" 6888 Klttlwake flldgllngs nec" 33 z --' Norway Bu"ge~ & 60ch'ald 1984 (X) 68B6 ta He,.,.l ng 6. ~rctlC te"n ~ p.r.di'••• & 0.42 to 1.2 M.lne Hatch 1'370 , ''''01101.1 Ca""on tern ~ ~ en1e'" whol. e'Hi="./pA1C o .. 6886 ta Ha~lng 6. COMO", elder So.. ,tr" !!01l1A1I00 ~uckl1ngs sw'allow 7.73-:H.Z: 12.32 Ulla' Seotl.nd "Ino:lball & ~~lne 1985 ... whola 100 dUckl~ng.'day

6BB6 & H,r~lng 6. Co~on' eider Sc.otcal "plll"'O' duc~llng~ swal10w 37~ St Lawrence "unro • eeda~d 1977 .' ~ & Harrlng 6; Pu" 1 n cblck hood N.w'ou~dland ~1,"ottl 1983 ",'" Pu'-fln adult ~ 1%

Vellow-lAgQad Herrlng tireater fla,nngo PhpOQlcpp$Oryl CYIar:.~II ••d & neek.-- 3.5 4ZX France ,CIalathe 1983

61.111 ~ c.chl0non, chlcks snflke & 0~6-4.0 blrds' Spitzberg_o Ste"pOlawlcz 1983 " 61ausous 6ull # Davek. ... ~A1a swallow 24b/lee,,2 c ~ / ~ hyRecb6reyl -...' .. r ""

'. ... , ~ t) < .,. \-. ~),"" ,- O' o ..,..

'~

.~ \ -. ... - v o Tlble Z. ChlrlcterlstlCI of ~IJor Larld pradatlon studles lnvolvlng,l predator and Z or ~orl prey'durln; the non-brelalng perlod. , ~

------t------~ PrldatClr Prey • ~ocatlon Feed\ng Interaction Predatlon Prey LacllLon Source of .. trille tll'le t'"e(~ln.) sucee ... b"heVlor \ ------~---~---~-----~------~------~------~------~-----~------

-. 6laucou. Gull Roc~ Dovi ~ llxlI back l'IClve lude Vencouver" JYr"kkanen 1975

Call 'orn~a 6u-H Bern Swallow ~ ~

~ ç.\a'Rcnlcys 5wellow~d 0.5 flylng Caillorn, .. Leyl'lon 1983 o Hlrring 6uU OldlqulW ClangulA hyeMo!l' Idult head & neck 41 66 .. IIwl~l'Ilng Toronto Snell 1981 \0

6r•• t.r Sl.ck-back.~ 6. Shelduck ~ ~ head Swed.n Sven"50n 1971

Hlrl"ln; ùull 3 Co""on tell 0nI1 ~ nead try tCl aWlIII France T.IIIluer 1970

Z P,nta,l ~ ~ 8. "W"Y (a11

1 "aUard neck crlPplod)

Grllter Black-backld 6. European Coots ~ A1cA nead & neck -- 10-Z0 8% floc"lng HoUand Thol'ls." 8. ~.lto't. 198e

'. ., ' . ':- \0

1 ~.- • , ( (" -. " , ,-

• t. - ,

~ o ... ,

. t

, .' , , \ o SECTI·OR 1:

,Greahr Black-backed Gull Predation on AMerica" eoote .\

) .

- --- ~

" ..

'., o , "

-'--:/ . 10 o ABSTRACT . We atud1ed Grealer Black-backed Gull L....~,t.!U aCSDJ1!.Lt predat ion on . / A",erican Coots fuliea al'lericana, (rol'l 22 Oetober to 18 Oece~ber 1983, on

Lake Mattal'luskeet, North Carolina. We.sP~lC!srved 33-kills, 42 IItrikes and • f • , , 71 pursulh. Coot behaviors whlch resulted in 1 on 1 intera.;ctlons were: rush or fly (44X), dive• (33X), lone coot ln l'Iixed specie!l floçk (14Xland

\ ot her behavlors, (9% ) • Our i DI;;J 50% 0 f k 111 s, a coot becal'le 111n'O 1 ed o,ut.

because it dove. The t'tean Ut'te to single.out a coot was lower for UlIs

(36 Il) t han for strlkes <103 S). The nUl'lber of interactions betwean Qull s

and eooh wap lowellt frol'l 1200-1500h. Senonal aet iv1 ty increplled unt 11 • the second week of NbV'el'lber, then decreased. Gull!! ktlled belween 4.,6"and \

7 .3% of tlie ob!!ervèd coot populIItion in 2 l'Ionths. Gull predation !!ueel!lSS

all'1o~t doub!ed (23% to 44X) if a gull l'1anaged to ~trtke ft coat. Coots t using an "aggrelllliv~ post.u~· and "lunging ~l !!1tting Qull" had a

slgnl'f!cftntly lower incidence of predation.. than eoots whleh used ·on Us

baek" or "dlvlng" behavlors. There was no di Herence ln predaHon sucees!! '. di (ferent coot t ~ oek si zee. 'HO'I:iIever, the proport 1 on ~ 0' ~ interactions on Single coots in l'Ilxed spec1es flocks was 'greater than the

proport 10n of Inhract"ions 'rol'l l'Ilxed epec les Hocks for any other flock "" 5 Ize. Three year-old gul15 had hlgher succesll than adult and 1 and 2 'J

.. / .. " • o

11

...... INTROOUCTION

Predation 0' young birds by gulh \11 well docul'lented (Verl"leer 1!;168 ,

Owernychu~ and Boag 1972. Munro and é~dard 1977', Sdathé 1983) but l predat Ion on 'ull grown btrdll 15 poody studled to date (TeiPllsfer 1970,

PierotU 1983, Stel"lpniewicz 1983 '_., Greate" Black~becked 6ulh LaruS

M~~nul have been repoded kl111ng Atlantic Puf'ins ~ate~~~ §.t.Ç.li.M

( Bealllan 1978 .. Pierott 1 1983), Black-legged KlttiwaKe-s Rfssa t r LdJlJ:l~_la_ , .... ( Beal'lan 1978, ' 8urger and 60c:hfeld 1984), Shel ducks TaJ1.ru::,M a.®rna

(Sveosson 1971), European Cooh Fulict! ~tr~ CThol"lsen and Meltofte 1980)

and A",erlcan Coot! ~ AI!!.Ol:i.~lIl1!~. (Forbush 1925). Al"lerlcan Coots have dso i ' l ... [ been reported as prey of 8ald Eagles lfal1a~.:} ~J ~.p...Mlu!!. (Grubb and Kennedy 1982, SecHo" 2) and Peregrlne Falcons EAk.Q. peregrinus (Scott

1880, OaUe!" 1980).'

Studles 0' avian predation have Quantified certalr1 upech of , predatory lrfterec;t ions

invol ve end Aubry 1982, Sahthé 1983). However, 1'I0s~ eccount!! \ dellcriptions 0' 1 or 2 HUs. Descriptive inforl'l&t1on obtained 'rol"l, pradat 10n atudles includel5: how a preda'tor isolete!! 'an individual

(Stal'lpniawlcz 1983), predation, succe!!s

Selethé 1993), predation !lucce~!I throughout the day <8ird and ,Aubry 1982, , Salathl! 1983) and throughout the sea!lon (Sa ll!th~ 1993). ~nteracton' Ul"le (ThoPlsen end Moltofte 1980) and an estil'late of the percent age 0' prey'

.., k1l1ed froM ~a population

severa 1 behaviors durlng predation attel'lph COokker 1980, Stel'lpnlewicz

1983), but have not deterMined the predation !luccess assoctated wlth eech

behavior used by the 'prey. Page and Whitacre <1975) and Kenward (1978)

.0 found a higher predatlo'n succells on the sl'lallest flock si ze!l of prey

studled. Flnally, severai aspects of gull feedlng l5ucces~ Increase wlth

12 1 o guU age (Surger and Goéhfeld 1981, Greig et al. 1983). Slnce Iole heard 0' a ~ituation of Greater 81ack-backed Gull predation

on AI'\erlcaf'1\Cooh which could be easl1Y,and frequently observed (lovvorn J" '. , pers. comm.)f we undertook ta describe ver10Us a~pects 0' these ~red8tory

interact ions. ,

STUOY AREA AND METHODS r The study wa~ conducte~ on Lak~ Matte~uskeet, a 16,200 hectare lake

averaging 60 cm ln depth, located wlthin Matta~uskeet National Wildlife

'Rafuge ln east-central North Carol1n8 (Fig. 1-1>' Approxlmately 40% of

the eastern half 0' the lake h8s ~ubmergent vegetatIon, Most 1Y

PotaMoge!Qn spp. and Valllsne~L~ ~n~~lc~. (Lovvorn unpubl. data).

Emergent vegetation, pri~arily TYDhO spp., grows along the edge extending . up to 30 m 'rom shore.

Observations were conducted 'rQM 22 October to 18 Oece~ber 1983,

fro~ two 5 '" towers, loceted on points of land extending 600 PI inta the • lake 'ro~ shore, overl90ldng the south-central portion of the lake. A

vehicle parked next to an ImpoundPlent was ueled es e thlrd elt te (F tg.

1-1). Wc used a 1.S-60x spottlnQ telcscope or 10x binoculars for dawn ta

dusk observetions. Super 8 Movie fIl", (1.e. 120 1'1) of !JulI-coot . 1nteract Ions Qa .... e detail s of ail behav 10r!!. AIl gull-coot int eraot 10ns

were recorded on tape and later trenscribed. Date obtalned froM the taped

COI"IPIentary includedi dur.,tlon of tnteroct lon15" coat behtlvlors, nUMber 0'

tiPle~ a Qull etrucy- or grabbed a coot, nUMber of culls at an Internction

end Qull age. Other data lnc1ude cull feedlnQ UMe, whlch was calculated

a's the UMe frOM w~lch ,e kl11 occurred untll the gull left the c(\rctl~l5. \ o While ~onltorlnQ a feedlng cull ~e noted the nUPIber of occurrencel5 0' one gull dlspll!clng another and the I!ge5 of the Qulh • involved. Gulla

13 ~~~ ~-~~------

\ .. '- rooeted on the east end of the lake (Fig. 1-1). Six pre-du~k Qull counh \ o were Made tlt the roost over tt:le etudy period. Gul1~ were aged as adult

()4 years", 3 yt!ars-old, or 1 and 2 year-old

eccording to Robbins et lIl. (1966 f. The coot .,behavior ·on i t 5 ,back· WIU

ueed when a Qull landed or hovered l'55 than 1 M fr.ofl'l an lsolated coot.

The coot posit1oned Hs body",vertically, resting Hs w1ngs on the ",eter

for belance, scratching fiercely at the Qull .w!th Us feet. -Rushinc- wtlS

def,lned as a coot runnlng on the blater tlnd flftpping fts wLngs, with or

without the intention of fJight. A eoot was def ined as 2 or More " eoote eeparated froM ot her coot (s) by at 50 fI'I. The nUfl'lber 0' coots

end coot .f.locKs wi thln 1 KI'I each d~y . r . and subsequent counts were Made if flQcks "e'rged or ·split. We counted in

t '" ~ t blocks of 10 t9 S0 coots for '{ocks larger tha" 50 coots. Mixed specles

Hocks were deflned tle 1 or More coots alusociating with 1 or fI'Iore species

of weterfowl. Flock size of r'l1xed species Hocks consisted of the nuftlber

of coatIS.

Gull-coot interactions 'were CICfsslfied 415 k1lls, strikes or

pursults. A strike was defined tU! en interaction durinQ whic" a QuIl

atrik.es a coot..A pursuit 1s en lnteractlo" durlng whic" a gull singles

out Md pursues a coot but fails to strlke H. Predation success was . expressed in 2 ways: singlod out and pursued (hereafter r~ferred to as

SOP) and strlke (STR). T"ls division was used because coot survival

deereased once e coot 1.1<';15 struck. SOP success was cOPlp'\Jted as:

SOP success - K (100%) 1 K + S + P,

and STR succe~s wae cOMputed 6!S: . . r STR SUCCO!SB .. 1< (100X) 1 K + S,

where K, 5 end Pare tho nUPlbere of kill!S, 5trike~ and pursuih

reapedlvely. Calculatione for STR eucceS5 exc1ude pursuits s1nce no

14 , ~. o cont~t 15 l'Iade.between Qu1l5 and cooh durinQ pursuih.

R€SULTS There are 5 situai ion5 which lead ta a one on one, gull-coot

interactlon (Figs. 1-2,1-3 and 1-4). Bas~d on situation 1 (Fig. 1-2), e

çoot can be iliolated froPl a flock, by divine, rushlno or tlying, and • -;pther behavlor:;. 'In 6ituation5 2, 3 and 4, cootl! usine. ~ rush, dive or tly, \ and :single coat ln l'Iixed speci05 f lock, respectively, wi 11 be alngied

out.. Coat behavior~ wtuch resulted 1'0 1 on 1 interactione were: rush or o 'fi. ~ fly (44X), dive (33X), lone coot ln PI~xed ~pecie6 tlock (14X) and other

beliaVlors (9%)

one coat. If the flock. wa5 larger than 50 coots, the gull would u5udly

sph t the flock in 2 and proceed to Il1nole out 1 coot 'roPl the' el'll1ller group. .I(.1t wa:s.. sueces5ful, the Qull would haver av el' the 51-nole coot or land next to 1t dawnwind and try ta strike 1t wtth Hs blii. If it l'Ianaged ta strike or crab the coo.t, the !JuIl then attePlpted ta further' (

strike the coat unt i 1 tt was de

the head and neck. Gulls struck or grabbed eoots 1 to 41 tiMcS during a

-"111 (x"'11.6, 50"10.86, n""21 >. The nUMbcr of' stnkeo and grobll during a

kill by 3 year-old Quil5 i~ not s1gnlficantly dUforont l'rOM the nUMber

for i year-01d QuHs (P"0.757, Median tc"st. 00ni01 (978). Once the coot ~~ ~ . i wa5 dead, tho Qull then pulled at the abdorlon and beCèln feedtng on the ~ , villcera or breast. When undi~turbed by other Qull~ or Bald E491e~, gulls

fcd on fresh kl1h for li Mean of 35.2 Minutes (50-20.7, 0'-13), Gullll were a150 observed feedlng on a teMale Pintai1 finM o.aeuta, l'la le Canvallback. Oythyo YOlil!iocriQ and d Longnollo Gar Leph9!!teuP 9Q~eUQ. /\ Qull wa5

15 \ • obllerved flying wUh a Blue Crab Cali inectuo ,aniduo in îts bill, o èlthough 1t weil not seen feedlng on 1 t.

Gull.s testlng eooh

Before a kill, gulh oftenotested coat Bocks. A gull engaged in 1.3 disturbances o~\ average 150-1.48, n-f3 ) priaI'" ta a Hll. These \ ' dhturbanees inc1uded: pursuih (n~9), str'lkes (11), fluohin~ duek f locks

(5) and working another coat tloek CS). However, in 9 of 23 t39%) klt-ls

for whieh a guI! was observed fl"'ol'l ftrl!t arriving ln t~e vieini ty unt Il

kilUno a eoot, no prior dlsturbance::l were' reeorded. The _nul'lber of- /' \, dhturbances prior ta a kill was not signlfiedntly different between 3

and 4 year-old gulls (P-0.36, Median test). ..

Another way ln IIIhieh eoots were testelt, was by the durat Ion of an

interaction (Table 1-2). The l'lean Ul'le to Single out a coat was • 5igni f lcantly 10lller '01'" Hlls (36 15) than for strlkes.. (103 .)~ (t-t.. t. p\ . • 0 < '0.05, Steel end Torrie 1980)'. We c~u:uhted' the duration' of ki1ls,

str-ikes or ,pursulh ln 2 ways. < 1) Oi!lturbance tl1'1e, whlch was the ~i",e

frol'l firet dlsturbance of a «''lrt flock untll 6 coat, Illas killed, or until

the depadure of the guI!. (2) Interaction UPIe, whic" was the tll'1e fro",

'1r5t l'!!ol.ation of the coat until the'kill, or depadure of the Qull.

Mean disturbance t IMe was signl f icantly greater than !"Iean interact Ion

tt/'le for kl11s, 8trike8 end pur~ult5 (t-test, P < 0.05). Mean disturbance

tt/'le of kt1l5 <247 5) and etrlke5 (2915) was Ç)reat~r then disturbance

UPIe forpureuite (1485) (t-test, P <0.05).

Al"'ost a11 kilh (94X), strikes (95%) and purlluite (94%) were

cer.ried 'out by Single gu1l5. During 2 k1Ils, 3'ëlnd 2 IJulls, reepedlvely,

pertie1pated. In 2 strlk.es, 2 Qulls took part and ln 4 pureu1ts onë:e 3

Qulle and 3 til"lee 2 gulls pertlclpated. Once 6 kill occurred, the l'Iaxl1'1û'"

nUl'lber" of Qull S observed nea,. ft coat carcass wa5 5 QU1l5. \.. J6 - Gull predation 5ucce5S o Gull predation succeSs ah,ost doubled (Z3X to 44X) if e guH l''Ianaoed

to strike. coot . Our!"g 435 houre 0' observation, 33 1t11h,

42 '5trikes and 7l pursults were recorded. Dally frequency of gull-coot

-interactions was highest betla/een 0600-1200h end 1500-1700h whlle the

10we5t, frequency of interactions wes recorded frol"l 1200-1500h (Fig. 1-5).

Fr~quency la/a 5 expressed as the nUl"lber of 1 nterec tions per ho ur 0' observation ta elil"llnete aIL effects of uneven nUl"lbers 0' observetion

hours CFig. 1"'6). Predetion sucee!!!! la/as hlgher ove

1-7 >. The success (100%) et 1700h should t1e dillre,garded beceuse 0' an insutficient sal'llple size (n-l interaction). Gulls end 'cQth were , irst

seen on lB October while observation tOb/ers \&Iere being conatructed. Gull-coat interactione were flrst seen on 27 adober. When the nUl"lber 0' ,kills, strike!l and pursuits were plotted per 40 hourIS 0' observetion

(Fig. 1-8) a peak in activJ.ty occurred around the sec,ond week of Novel'llber /1 followed by a graduaI decl1ne. The begi""ing of the deéline coincided

with the arrivaI 0' Tundra Swans 910~ ~~Ju~bJ~Jt~ on the refuge. A "early

coneta"t !luccess r~te ar"ound 20% \&les recorded for SOP coote wU h a

Qradu~l decl1ne neel'" th,e end 'of the season (Fig. 1-9). The STR lSuCCess 15

~ore erratic, averaging around 501..

Besed on an averege 0' 725 coat 15 (50-545, "-55) observed per dey

fro~ an observation point, Iole observed Qul1s /1:111 4.6% of the coot

, pOP-l/letion in 2 l'IIonths. This velue was celculated es,

Percentage 0' · K (100%) 1 No. of coote observed. population killed

where t< is the nUl"lber of kille. When 4 kilh by i"'I"Ieture' 8ald Esgles Cp.

,0 S0) are included. the p,ercentege of the coot populet 10n preyed upon is.

S.l~. The figure rlses ta 7.3% if a11 observat 10ns of Qulls (eeding on a

17 , 'resh coot c:arcalS 'or which the kl11 was net observed (n-16) are· o .included. A fresh coot carcass waa deftned es visible above the water up ...... to S00 1'1 awa'y ""on which a gull easlly tinds l'Ieat and whieh the gull . aggresI!I1vely de'ends 'rol'l other guI h., Gull predat ion rate, on cO,oh the

obaerved coot population averaged by 40 h04r observation perlods, ,ranged

'ro", 0 to 0.75% (Fig. 1-10). The nUMber of coots observed per day are glven ln Fig. 1-11. The nUl'lber of eoots observed fro", towers decreased. d~rng the season. This can be explained by a shi ft of birds toward IMpoundl'lent 3. Bt-weekly refuge counts perforl'led by refuge personnel,

show a l~ 6~ eoots on 18 October and a high of 3365 on 29 Novel'lber. Gulla roosted at the eest end o~~~_e (Fig. 1-1). The nu",ber of gulls

et the rool!lt renged troM 56 to '145 0<-92, n-6 counts),

Factôrs a"ecting predation success

Coot behavlor

Coot behaviors in response to the approech of gulis were divlded

Into 13 behaylors, end eath wes aSloclated wlth a dlffer~nt predation BuceelS (Table 1-4), CoOtl Uling an -aggres5ive posture· and "lunging. at. , sitting gull" suftered I!I-llgn1ficantly lower' Incidence of predation than

coatI whlch used ·on its back" or "diving" behaviors. Four behavlors;

aggreu1ve posture, JUMping at flying gull, lunging et sitting gull and

head dlpping were considered eggressive. AlI ~ther behavlors were not

G aggresslve and generelly hed the highest predation success 8ssoclated

wlth each behavior, e1though not aIl signifieent. Coots aMong Canada

Geese Brante çanadensis or Tundre Swene.were never killed. / The Mean nUMber of differént behaviors recorded for coots'during a 1111 (2.82) wes slgn1flcently dUferent fro"" the nUl'lber used durlng e ,0 str'1ke (3.38) (P-0.005, Median test, Daniel 1979). No slgnlflcant ditference was 'ound between the nu""ber of dlfferent beh;viors u"ed

, 18 o· during a kHI vepsus pursulh (2.68) and strik.es~ versus pursuih (P

0.05) • o) Flock slze

Figure 12 shows gull predatioJl""'1üccess on !Single cooh and flocks of

7 dU ferent s izes. Gull predation success on dif feret'lt coot f1ock~es

/' was not slgniflcântly" dl Herent for (STR) or (SOP) sucees!. The

proportion of interactions lnvolving lorie cooh frol'l l'Ilxed !peélea Ho~k!S

la. l5ignlHcantly grester than the proportion of Interactions frolll l'Ilxed

!Species flocks for ail other coot tlock sizes (P < 0.01 Ft~cher test)

(Fig. 1-13). Therefore in 23 of 30 (77%) interact'lons on aingle coots, r ~ ~ the coot was ln a l'Iixed apecie!l floek. Only 7 of 30 (23%) intera~~ons on

single coots began wHh e coot et least S0 '" fro'" eny other bird.

Gull age

Sulla 3 yeara of age hac( e- slgnif1cently hlgher predation lIuccess

than eUhr gulh )4 years-old (P-0.003 SOP- suceess, P-0~018 STR !!uccess,

.. Fiacher test,) or 1 and 2 yeer-Qld gulls (P-0.012 )(Table 1-3). Interaction

ttl'le dùring kilh la not slgn1ficantly different ~etween 3 and 4 year-old

gulls

year-old guI le had, the hlghest -predet ion succ.ess, but relllalned '1 • subordtnate to adult gulls when feeding.

DISCUSSION

Host.predators try to single out one individual frol'l a flock. T~e

singled out indlvld~el 115 generally physlologtcally, behavlorally or

spatlally di"erent t,..o", the group (Rudebeek 1951, Kenward 1978).

Behavioral selectlon wae observed when eoote dove. P~yslcal dl,terencea

19 '''-J

between coota ln a flock were never seen~ trie sssu ed that 10ne coat! ln

(» l'Ibed species tlocks were spdlally dH'erent. "'hen a flock of cooh 'ed

wHh water'ow1, 4ndlvldue1 eoots wandered '1"'0111 1 group of feeding , water' owi to another. In thl~SY, coots beca",e separated frol'l their,

'lock by too great a dlstai'-ce to cover when s dlsturbance flushed the

waterfowI, olten expo51ng Ion~ cootl!. Spatial di frerences were diffleult

to ",essure ln a group of' qutckly fleE:.ing coots ~nd lAIe're nbt recorded

uniess obvlous to the observer. In 3 Hils, 2 strikes end 3 pursulh we

noted 'the codt., was trail1ng the eseaping floek. P ge' and Whitacre (197 '1l 'ound that shoreb irds 1 to 3 III frol'l the f lock aUa lly di Herent)

those which hesU1.ted to lake off unHl after Hock dld _~,..~

l1kely to be aHacked by a pred~to"', However, wfth aIl pred8tor prey \ studies, the het that no difference Is detected 'in prey indivldua1s by , an eb50r~.r do •• not ••an th.1 Ihe pr.dater •••• n~ dl rr.rence.

OUI" observations on how eooh were kllled', ho~ gulls 'ed and 'or how

long agreed wlth other etudies. Sulls generally killed their prey wHh

, blows ta the head and neck (Tel'll~ster 1970, Tho",sen and Meltofte 1990,

PJ.erotti" 1983, Salat.h~ 1983). Once k,111ed, gulls' began feedlng on vilscera . ,

Black-backed Gulls et Hattal'luskeet was col"tpar~ble ta gu11s in Norway (35

versus 33 l''tlnutes) (Burger and 60ehfeld 1984) and to Herrlng Gulh k , 1 argentatus <35 versus 41 ",lnu,tes) (Snell 198U. ". The Grester Black-backed Gulle we obs'erved fed -all"lost excluslvely on

cooh, but we do not. belLeve that aU 82 gulls which roosted on the lctke'

red there. SOl'le gulls were observed flying south-east "rol'll the roost- ln

the l'lornlng and returnlng frol'l that direction ln the evenlng, presu",ably o ta Pal"ll1co Sound approxil'lately 10 kl'l to the .sout.h-east. Gulls prefesrred to lnteract with coatIS wi thout other gulls nearby. Thls was especlally • 20 1t. ---~~~------~~", 1 " ,

true 'Q: 1,2 and 3 year-old b\rds slnce :dUlts would dhplace thelll o any kllh ~lrld singled out cooh. Gull dlsplacel'lent saillat illles led ta Hghh which oHered 5ingled out coots a chance to ~scape\ and reJoin the

Hock. Gulls are not social hunters but ara opportunistic and would , quickly Joln eny interaction in progress wUh hopes ,of getting sOl'le 'ood

Uel'l. ~unro and .a~dard (1977) found that Herrlng Gulle would Join a disturbance ln progress ",hic" often -résulted ln the totel Ion 0' a .' . COl'll'lon Eider SOl'lateria ",ol1isl",a brood. It took a gull longer to Single out.. a coot durlng a strike than a kil!. There are 2 pOSSible explanatlons-, (1) Singllng out a coat ls ,an

energy costly actlvity sinee ,it requlres hoverlng above a floek untll a

coot is lsolated. ih'erefore, t"eogu.ll ",ay have been tao exhausted by the I- I longer chase of a strike to l'lake the kill. (2) The gull l'lay have been

assesslng . ctlot Hocks; w!th"cooh of sil'lllar physical capacltles durlng ( .. strikes. Concurrently, 1 f one coot ln a tlock was ",~aker than the others 1t could be slngled out and" killed qUicker than a coot 'rol'l a ...l'lore i \ ~ ".~ ho",ogeneous flock, therefore counted as a kill, not a strtke.

Gull predation success all'lost doubles <23% to 44%) once a coot ls , str'uck, t"us success was consldered eeparately for coots slngled out and 1 pursued ana for t hose laI"ich \/Iere. etruck. If a coot laIas struck, the gul1 l'lay have Infl\ictad an InJ!:,ry t~, the coot "'hit per",t t te~ further rhol'lsen and ;,Melto,te (1980) 'ound Greahr lack-backed Sulla had ex

lSuccess on Europ'ean Coo~5 in Ho lland. However, t hayon 1y ob served 5

kllh. il ,

Dally-- interactions ·~tlaleen gulh and coots le 10lalest et IIIld-dIlY.--

Salath' (1983' found a sll'111ar trend laIith Yellow-legged'Herr1ng 6ul1~ !u., " , o çach1nnans kll11ng 6reahr Flal'lingoes Phoenlcoplerus r;.u1ltU: ,.o,eYI. Salath~ found a nearly constant success 0' 42% throughout the dey. "e

21 r observed a deerease ln preaation aet Ivity with til'le dur~the autur'i, ../ however, sueees!'- relllained nearly constant throughout. The: decrease ln

aeUvUy during the season was attrlbute Tundra ·Swans .

on the lake. When in large nUlllbers, ould, spread out evenly ln the

sout~ part of the lake. Coots would l'Ilx wlth the swans, and gulls rarely

dlaturbed thelll 'then. Ojsturbance low flylng aircra ft, ft resultl~ o vehtele, hUl'Isn acU .... t ty or hunt tng would cause !Swane to l'love offshore.

These dlsturbances fs.(!.d gull-coot In:erad tons sinee i t 1'I0yed

waterfowl offshore leavlng the eoots "ear shore ln l'Ionospecific flocks.

We est1",ated that between 4.6 and.7 .3% of the eoot population was

~ ktlled ln 2 1'I0nth!. Beal'lan (1978)Jestl~ated 2.5 and 3.4% of 2 puffin .

populations ("'05 Uy adults) ",ere kil! ed by Greater Black-backed Gu Ils ln " Scotl"nd whHI7 Plerott 1 J,-1983) found less than lX of adult puH 1 ris II/ere - k1l1ed by _ guI15 ln New'ou~dland. Stel'lpnlewi cz (1983) esti1'lated that

Glaueous Gulla ~ hyperboreu5 kill 0.6 to' 0.7 adult Dayekles Plautus

alle, 1.5 to 1.7 fledgqngs and 4 young testing II/lngs per-day pel" k",2 of

cliff nestIng habitat on Spitzbergen. ,.Therefore, ~ulls can exert ft !drong .' selective pressure ~n a population.

AMerican Coot behavior affeded the outcoflle-- of a~ interaction. , Aggresslve behavlors and'coots afllo~g Tundra Swans and Can~da Geese tended to ·p.roduce·'1 ,.. high evae10n sucees!. European Coots have been observed l '. -~, attacking ft Herring Gull (Hibbert 1974) and usihg a -defenslve" behavior 1 by gatherlng together and ra15lng their bills toward an attacking gull

(Pastore 1984) • !Je neyer .,observed such ra i sed bl11 behevior ~by an f <1\ • Al'lerlcan Coot flock, however, lon,e -aggres5ive coots ralsed the1r bill

toward low flylng gul15. One coot was e~n observed chaslng a gull which , \ 0 landed next to 1t ~ 'the coot grabbed onto the gull' s taU and wes 11 Hed 3 III off '"the w~ter.

22

,., '\ ~ .. Cools used a greeter veriety of behevlors durlng strtkes then durlng

i o kills. This that cooh could al ter the success of a gull by Oslng e greeter veriety of behavlors. It ..,ey aleo suggest thet coot!!!

kl11ed \&Iere weeker then those invol,ved ln :Jtrikes.

F lock 51 ze did not hltvè an effect on predeti on success. AHhough no

dtfference ln p.redation success"based on ftock size exlsts, it W8S to e

~oot'~ adventage, to be in t~~r:est' flbCk possible. Kenward (1978) , found G05hawks Accipiter gentiUs had the highest !5uceess on lone / woodpigeons, Col~fI1be palumbu5. C!!lntrery to Woodpigeons, whose 'OnJy option • when pursued by a p;edc'!l,tor 16 to flee, cool!! reslst belng kllled ",hen ( sl,ngled out by aguIl,. Therefof'e,.onee~ingledout, coots frofl1 dtHerent

floek sizes hed eqliel suceeS5 avoiding being killed. Mixed spec:les flocks

proved to be an unexpeded aspect of the sludy. Mixed speete!! flocle.!! are 1 often con!!id~red 85 a ",ay. of decreasing predation due to the Increesed

'loek size (Morse 1970). When coots ,.,ix w\Jh other specie!! a gull ,Clin

cause!! the ·other bf..rds to leave exposing the eoots. In• euc,h li way, Jone cooh ,which stray 'fro", their flock cen be di,scovered by e gull--- di5turbin~

'r'lixed species flocks.

Gull age aIso showed unexpected result!l. Thre~ YMr-old gulls had e

higher suceess than Qulls ~4 year!l-ol~ c'!Ind 1 c'!Ind 2 year-:old guI h. There • -are at least two posslb le exp lanat ions. ~ge l'Ic'!1y have been l'IisJudged. Th! s

eppesrs unlikely although t n 1 or 2 cc'!Ises when l1ghtlng w... !! poor a~d the

gu11 far awey, th~s ':'c'!Iy have occurred. Secondly, a 'ew experlenced 3

year-old gu115 l'1ay have been responslble for the ",aJorlty of activlty by

thi! age clas5. AduIt gulh were l'lore nUfl1e!"ou,s at the roo.st"end tharet ",ay

have been a turnover 1n the gul h of thl s ege c lass whtch disturbed "

coot!!. Jyrkkanen .( 1975) bel1eved thet only a 'ew edult Glaucous-w1nged'" '''1(1) o ,/ Gull'! k alaucescen5 ware reeponslb1e for pigeon Colul'llbe !.!.~.!Jf pr,edatlon

23 Jn a' 'relght yard. 61ra1deau and L~'ebvre (1985) found pigeons o spec lal1zed ln food Ue",s 'or"'lng. a ·skill pool· • .Therefore the ldeas of Increased Uock size decrees Ing pr.edatlon !$uccess, - lInd Increas lng

predation BUtCen wlth age, did no~ hold nI' thls situation.

.. - ; -\

"

... •

" .

, ,

,\

o , .

24 ?I~~~,~RE CITED ... o Bee",an. H.A.5.· 1978. The feedlng 'end populetlon ecology of the Great

-slick-backed Gull ln "orthern 5cotland. Ibis 120: J26-127-:' - - Blrd. O.H., 8. Y. Aubry. 1982. Reproductive and hunting behaviour in

o Peregrlne Falcons. f'dco peregrinus. in southern Quebec. Cano \ Fi:el~-NaL 96: 167-171.

Burger, J., '8, Gochfeld. 1991. Age..!'related di fferenc'es in piracy M. /

b&havlour of four spec1es .of gulls, ~eru..JS.. Behavlour 77: ,242-267.

______, 8, ______• 1994. Great Black-backed Gull predation on

l

r Daniel, "W.U. 1978. Applied nonparal'letrlc statist1C5. Boston., Houg~ton

MUrlin Co. ..

Oekker, O" 1980. Hunting success rates, forag! ng hab! h, and prey "

lIelect10n of Peregrine' Falconll 1'I1grat ing, through ~entral Alberti!.

Cano Field-Nat. 94: 371-382. 1

Owernychuk, L.W., & D.A. 80ag. 1972. Oucks nellting in association wlth

gulls-an ecolQgicaI trap'? Cano J. Zool. 50: 559-563.

Forbush, E.H. 1925. B!rds of Massachusetts and Other News England states.

Part 1. Uate.r bi~ds, "'arsh birds and shore birds. Mess. Oept. Agr1e.

61reldeau, L-A •• ·& L. Le.febvre. 1985. Indivldual feedlng preferences in

feraI groups of Rock Doves. Cano J. Zooi. 63: 189-191.

6relg, S.A •• J.C"Cou15on, 8, P. tfonaghan. 1983 .. Age-,.~l~ted dl"erences

ln 'oraglng success in the Herring Gull Larus ~..menJJttus. Ani"'.

Behav. 31: 1237-1243.

'6rubb, T.C., 8. C.E. Kennedy. 1992. Beld Eagle winter ha~1tftt on

eouthwestern National Forests. U5DA Forest Service Re5eerc~ Pape~ o 'RM- .237, 13p. Rocky Hountàin Forest and Range Experlfllent 5t;tl,on,

Fort Collins. Colo.

25 ) H~fft. J.H.J. 1971. Herrlng Gûll aUaclus Eared'Srebe. Condor 73: 253.

~ Hibber-t,- F."'. 1974. Coot attacklng 'HerrinQ Gull. British Birds 67: 242.

Jyrkkanen, J .A. 1975. Glaucoue-winged Gull predation on ferai Rock Doves.

'\ Can Field-Nat. 89: 77-78. Kenward, ,R.E. 1978. and doves: factore affect lng success and se lecèi on in Goshawk aHacks on Wdodpigeons. J. Anil'l. Ecol. 47: 449-460. / • Morse, D.H. 1977. Feeding behaviot and predator avoidance in

heterospecific gr-oups. Bioscience 27: 332-339.

Munro, J., & J. ei:dard. 1977. Gull predation and cr~ching behaviour in

the COl'lllton Eider. J. Anillt. Ecol. 46: 799-810.

Page, G. , & D.F. Whitecr-e • 1975. Reptor predat ion on' winterlng " . ') ehoreb)rds. Condor 77: 73-83.

Pastore, M. 1984. COl'lportel'lent défensif -en hérisson- des foulques 0 ~ f

l'Sgerd d'un goéland. N05 Oiseaux l 37: 342.

P iaroU 1, R. 1983. Gull-puffin interactions on Great Island,

Newfoundland. Biol. Conserve t6: 1-1.4.

Robbins, C.S., B. Bruun, 8. H.S. lil'l. 1966. A Guide to Field

Identif1cetion: 81rds of North Al'lerice. Golden Press. N.Y. "'1 Rudebeck, G'. 1950. The choice of prey and l'Iodes of hunt 1 ng of predatory

blrds ",!th special reference ta their selective effect. Olkos 2:

65-88.

. 1951. Th~ cholee of prey and l'Iodes 6f hunt lng 0' predatory

blrds ",!th epec ial reference to their select ive ef fect. Olk05 3:

200-231. o Selathé, T. 1983. La prêdat ion du Fhl'lant Rose Phoenicopterus ruber ra se..!! s par le Goéland Leucoph~e Larus cach1 nnens en Cal'largue. Rev.

Eco!.

26 o SC4jJtt, W.E.D. 1880. On birds observed in SUfllpter, Levy, and Hllhboro. count1es, Floride'., Bull. NuHall OrnitholoQicel Club. 6~. 14-22 • Snell, R.R. 1981. Herring Gull aHecks and eeh adult Melee Oldsquaw.

~. Wihon Bull. 93: 110-111.

Steel, R.S.D., & J.H. To,"rie. 1980. Princip1es and procedures of

statist1çs: A bloflletricel epp.... oac·h. 2nd ed. New York. McGraw-Hl11

Book Co. . .' _. . Ste",pniewlcz, L. 1983. Hunting l'Iethods 6f the Glaucous Gull and escape

. fIIaneuvers of i h prey, the Oovekie.· J. Field Ornithol. 54: 329-331. , Svensson, L. 1971. The Great Black-backed Gull LeJ:'_u_~ ru-.tl'JJ!.!i es a

predator. Var Fagelverld 30: 125.

Ta", 1 s ler., A. 1970. Sign! hcation du grégarisflle diurne et de , l'al1",enhtlon nocturne des Sarcelles d'Hiver Anas ~r~c~~ ~~ L.

Terre et Vie 4: 511-562.

!hol'lsen, J .B., & H. Meltofte. 1980. Great Black-b'eclc.ed Gulls LeruJ!.

l'Ierinus killing end eating.coots Ful1ca etria. Denek Orn. Foren.

T~desskr. 74: 147-14~.

Ver",eer, R.K. 1968.'Ecological aspecte of ducks nesting in high denslties . - c!ll'long Larid.lS. Wilson Bull. 80: 78-83. •

'/ o (

27 Ct Table 1-1. Al'lerlcan ,Coot beh~vlors result l~g ln selection by Greder- Slack-baekect GuI ~s dYf-..ing interactions on Leke Mattal'luskeet. * .'

---~------~---- '- Coot behavlors Interact Ions

KUI Str1ke Pursul t ..... Tohls .. ". ------~-~------

O'ive 13 7 25 45 (33%) \ Rush or fly 9 . 25 25 59 (44%)

1 coot t'tSF a 4 2 13 19(14%) # Othar 0 4- 9 12 (9%) , ------~------~------~ Totah 28 38 71 135 , ------~~-~------

"et Lone· coot tound in l'Ilxed specit!!I Hoek.

..

o r

:.

28 -'

~ o ~eble 1-2. Dlsturbance tl~e (DT), Interac\lon tl~e (IT) end Meen'tlMe to single oût & coot, for k1~ls, strikell and purs}Jih of AtoI~rican Cootl! by

Greater 91eck-backed 6ul15. •

---.------~------~----~------~------.. , . Kilt Str i ka Pursult

OT a ITb OT IT DT I-T

------~------,------:------:"''''".-- 'If \ ,. Sal'lple slze 29 29 35 '32 61 6t . ,° 0 .J 1 Mean ( se.conds) c 247 211 ,291 ," 183 149 94

Ml n1 l'lUlli 55 46 21 3 5 1 .. Maxil'lulII 840 ·735 1090 1020 SG0 635 c \ Stânderd dev1ation lBS 207 270 2~7 150 124 ," .. _ Mean tll'1e ta single 36 103 , 62.

out a, coat " ------4-• 1 "'- e DT.!" duratlon '1"0'" first ethck· of coot flock to kill (or depl!rtu,..~ 0'

gull ) •

b IT- durtltion rrol'l first isoletion of'coot frOM flock to kl11 (or

depar t ure 0 f gu 11 >.

c Co",perlsons between Pleans slgnificant et the 0.05 level (t-test) e~cept

for tbe '0110w1ng: .... I5-K VS 15-0 (Slrike) NS

AT-K VS AT-O

29 -1 "4 / o

Table 1-3~, NUl'lber 0' k,HIs (K), strlkes CS), pursults CP) end Greate'r

Blac~-backed Gull predatlon success on A",er1can Coots, eceordlng to gul1

Cl, - age (SOP- eODh.Single~ Out and Pur8u~d, STR- eooh Struck).

------.. Gull age (years)

, ' --~~------~------

1&2 • 3 . • ~4 , Unknown . ------~------~------~------~------1(1'11 33' 1 11. ,il\ ' 20 l' .. • Str1ke 42 4 4 32 2

Pursult 71 8· 7 55 1 • . Total 146 13 , 22 107 4

. , ------~------

Suceess SOP a 23~ 7.7 b 50 e 18.7

Sucees!!! STR d 20 73 e 38,S

~ ~ ... ------

e accord1ng to the equation, succe~s SOP - K <100%) 1 K + S + P.

b 1&2 vs 3 P-0.012 (Fisher test).

c 3 vs 4 P-0.003.

d accorc:Uhg to the equotion, ~ucce55 STR - K (100%) 1 K + S.

e 3 vs 4 P-0.018.

( o '.

" 30 / . '

Table 1-4. A~ericen Coot behavlors whlle 1 nterac t 1ng

Black~backed Gulla and àuocl~ted predation succeas d",rlng one on one - ~'

encounters. f(II!' Kllh, S- S(rlkea, P- Pursuih (SO~- eooh SI!'1g,led Out end \ Pursued, STR- coot,s Struck)' -----~------~------~-~------~------

-Coot behavlors (Nu~ber of K,5,P) - SOp

------~---~------~-;------~--ï-----~------­" On Ih baek 27,16,12 60 X S4 " Dlvlng . 14,12, 54 X

Rushlng 21 ,3S, 64 19 + 37 . • Swil'll"ling to vegetation 2, S, 3 18 + 25

'g Flylng 3" S,13 14 33 ~ Alert 6,14,22 14 + 30

~ 5wl1"11"1Jng to flock 3,1O,13 12 + 23

Aggressive posture 5,19,20 12- + 22 + JUl'lping at flylng gull 1, 4, 4 11 20

lunglng at 51 tt 1ng gull 2,11, 9 9 + 15 +

Head' dipping 0, 2, 2 0 0.

Al"long Tundra Sillans 0, 1 t 2 q, 0

~ Al"long Canada Geese 0, 3., 1 0 0 ------\ et o X values di"e"r fro'" + values (Fisher test, P < 0.05) r

31 1"" , ... . , /, 1> -

(,

"

l ' •

-- /

Fig • . 1-~. Sfudy area on Lake Hatte~oskeet, North Caroline COT • 2 . . obllérvàtiofl towera, v • third observation point vahicle .... overlooking i"pound"ant 3, GR ~ Qull roost> . •

. . r

1

..

, ..

\

, . • 32 - .J - . '0 o

~ t. ...

• 76°10' MA~TAMUSKEET \ NAT~NAL WILOLIFE 'REFUGE N

~ t "... . , . /' o 100 200 300 400 km ,1 "- ~.1 ... . __-r--./ __- --u.-'~. ~ '\ w . ùs~'t. U) 1 ) o , " ,\ 1 "·~··l L .4 k ë f>.~ cq ,. ~~ -GR

3~·30· • .1

,... ~

o 1 2 S~. • • • 1 7rt'IO'

/' j ~ o- • ~ ,

• Vi

~

....

. .)

Fig. 1-2. Situation 1. 1) Greder Black-backed Gull approaches flock of

API.riesn Cooh. 2) gull 'lies low over centre of eoot flock cL',\slng the",

to apl1 t Into 2 groups. 3) gull turns and l5'pl1 ta the. sI'Ialler of the 2

groupa, lsolaUng a 51'11811 nUl'lber of eooh. 4) gU~.... tur"! and Isoldes a,;"

, .. lng~a coot, re"'41nder of eoots rush or swil'll to reJa!" the largest group.

S) Qull Ilona wlth one coat.

\

(

34 \

-' , ~---­. "

.. o

... / /

, .. .: #.:. . . . \ /' ~. .. ~\ ' ~ ~ !

,/ ..

~ , r!' U / C\J rt') - ~ .. . ~

\ .

~ \ 1 ~

1 •

.: J~~"I~" .. ~. .. .. ".... " ) ' .. .f' " ;'...... " ~", .. '~ ,,'t",'\ ...... ',,.,,' ' ..... ; "~ ...... ) ..... ~ .... ~ ... , .. '1.r~]~ ~~ .., ta'" , ~ I,~'t' ''l'''' .. !t'" 1 • .• ~ • ~4' ," _ tI ~ •. .• ,~~an~.. '~"- • • , ~ .. '..... ," " ...... ~, .... ' . '\.0 ...~

"

...... ~ ". -'- 35 tl ,.'" -".. " o ,

/

"

"

Fig. 1-3. Situation 2. Greater"Olack.-backed Gull approechell flock, of ~ ~ ~ ~

nllteriesn Cooh. 2) coota begln to rush tOialard !shore. One coot lI\~y have

", , been Upping or dlvlng to feed, ft 15 a l1ttle slower to takeoff than the

others, ~ trall s' after t~e rest of the f)ock. 3) gull C~5 off the coot , . :' by flylng ln -1t~ path, forcing' it to 15tap. 4) g~ 1 ~ alone wU h cO,at,

l re/'llalndar of Bock lesves.

"

, .,11>'

Sl tuatlon 3. Sal'le sequence of pletures .SII' sHustion 2 e>!cept, one' èoot . divell dùrln9 the rush, usu,ally the one closes! ta 'th,e gull, caùsing. ft ta'

, ft , " behlnd the rel'lainder of the floèk. The guÙ tQrns, back.' and", 1~nd5 'all . ,;s: •.

where the coot dove. Gull ~Slone wUh thé coot. "

J'

J'

, '. 1

, .' 'i , J.

i ,

Q r:. o ~ •"

.. t

3G , .

, , " <. 1 o

.. ~ 1 .. ~ - ", A' ..~ , .. el' ." -'- J. ."...... ~ .. ~ 1 \..,. .. al' ~ ,. ta

"'--...... :: ...... ,.···ol~·~ - ...... ~ .... ,. " ...... '\II ...... :'- ... __...... ~. ~ ...... 2 f "-: .. ~ ...... "'-... '

__ ...... { ...... "-'...... -.. r, • , ...... -.' ,.,...... _. t, ,..i..!...... -~ ~ .. ',. -"WIIIa .. ,.Il ...... , -.. - ....-...... rI&- ,-, ••. ~ ...... •• ; ••• 3 ...... • • . ."...... -...... _. . -,-: ...... "1'"'. • •• " • ' ~ . ., .- ...... ~ ' .. : ...... '...... ~­ --- ;...... 1-'...... 4' " :- :: ',- ...... ~•. : ~.::: ...... r·"I~ ..... -, ... :...... - ...... 1" .. _

"

." 37 . '. " '" ,\.

j ,

. , o --

• Q

\ 1

•.

1-4. !Htuatlon 4. 1) Orester Bisck-backed-Oull spproaches Bock of Fig. o • r

waterfowl (1n th1s-- csse 'Canvasback) -~1th a ~lngle n/llerican Coot. 2)

waterfowl flush, coot 115 Pluch slolller end faIls behind. 3) coot becol'les

Beparated. 4) gull alone with coot. "'stado..,l 'Iy li distance, then land. \ n ., ,

, Situation ~5. !ial'le sequence of ~ ictures es,s.l tuatlon 4 except a flock of

cob'ta ilS ,IIIixed.with waterfowl. Once waterfowl fluah situation 1, 2 or 3 \ beglns.

..

, '

" .; ., ! • 0 v

l" '.

30 • 1 " - '" 1 ~ ·r o

.'

~.,~~ .... . ~~"" .. ~ .. . ~r ...... :..... ~...... 2 T ··.~...... ~" ...... ~"~ ... ~::::. '"? ..... ' T ~: ..... ~ ...... 1. ~.. . . '" r·' .... , · .AIID::·. ,- ... '...... ~... · ...... " ...... ' ~ ..... ~ ...... "- .. ~...... , 3 rr...... r'··1·~..~ .•...... ". '. -.... '" ..~: .... , .. ,. ~ 1 . ... . , ', .. - · . · · '''J'. · "~ ·· . . 4 · . - .., · .. ·ro . ·. . ·· .. 1

39 ------~, . --~--

l

.. " \

l'

rig. 1-5. NUlllber 0' interactions (kil1s-. strlkes of pUl"5uits) per hour of obllervation bet'ween Greate,. Dlmck-becked. Gulh and ''l'Iericen Coots

throughout the day es recorded on Leke· Mat tal'lusJ..eet, North Carol1 na 'ro",

22 Qctober to 100ecel'lber InaJ.

pursui ta:

, (

.- t,. b o

40 \ r . ; 0 ~ o -- .57 (1,9,6)

.513

.458

.3aa A

.342 Q:: o:::> ... :I: :s ...... 285 ci Z

.171 1,.

• 114

.oS7A

4 '0 r ~a r 'i a tdt ~4 ~ vy1 to/1 t:y1 ~ ~ t?,:(1 t?,:(1 ";io. 6 7 8 9 10 11· 12 13 14 15 16 17 TIME OF DAY '.' 1.

~ - \ 0

'.

~

,

J

" \

Ftg. 1-6. Nu~ber 0' hour! 0' observation throu~hout the day for Creater

Olack-backed Gull predat ion on I\Merican Coots on laJ,.e MattaPiuskeet, North

Ca,.ollna frol'l 22 Octobe,. ,to 10 Oece~ber 1 n03.

s

..

42 ~ ~ i .. t ~lt"t .. o \, -,' o

150

4~ ,-

40

3~ ,. Z 0 i=;

~ 30 V/A V/A V/À 1//./ f//.J f//l r././1 r.//I ".//1 - .::- W VJ (J') " CD ~ 25

(J') 0::: 20 :J 0 :I: 115

10

5

/ 0 (,a r,.a K

..

, '"

ng. 1-7. Oreahr Black-backed Gull predation succelll5 on Al'lerican Coots

throughout the dl! Y, tU! recorded on La~e Metta",u5~eet, North Carol1na 'ro",

22 October to 10Decel'lber 1:g03. For sal'lple size of kllls, I5trikes, and . \ pursui t!l see Figure 1-5.

o .• *' 44

~t ~ t.:;J~'_ -~ ~ n. a (f) o

. .

/'

o o o o 0 o -o • ~ a ~ ~ g ", .. (x) SS3JJnS, NOUV03Hd, 45 ,..,.,...... ------~--~---_._--_ .. _-_ .. _-~ - --_._-- _.p------,-:------~~.::r"

r

.... o

, .

• r

'Fig. 1-9. NUl'lber ot kUlIs (K', strlkes CS) end pursuih (,r> by Greater

Oleck-b&cked Gulls on A~e~icen eoots, by 40 hours of o&servetion, on Leke

Hettal'luakeet, North Ceroline'ea observed 'rol'l 22 October to 10 Dece~ber .,

1903. /'

"

.e

40

" ~i,&~. _ o z y w o §~

1 •

10

1') u C• "N W ~~ 0 co...

l'4 ....-7

10 .... " - > 0 IOZ N +1 U o 0 ., ... ft ...o • ~ 0 • , - ," - • - SMnOH al' / SNOUOW3lN1 ·ON 47 o

"

, - ....

"

r:ïg. 1-9. Greater Dlack y backed Oull predation sueceu on ''l'Ierlcan eoots,

by 40,~~~hourlS 0' obâer~et1on, on Lake Mathl'luskéet, North Carol,lna as obaerved 'ro", 22 October to 10 Deeel'lber 1!)03 (STR- Strlke suceess, sor- i, S1ngled out and pursued l5uc~e5lS). 1

. " "

o

:;' " • 40

.. , , - o œ: a.. - Z 1- 0 .. W Ul (J)

C) ~- ·w u o ,f' -1 , . ..

'1 ,ro... , . ., . - 1 _ -. N'

, . . ;~

, .,p , ."

c) 'U o.. l' , ~~~~~~NJ N.f­ , - l'I '« ~~~~~~~~~~~~~~~~~ 0

" ' co.,... . , ,

" , . -'

, - , , 1 c:......

, "

,., ,~

p ",0 , .> o IO Z 'N , " o .. -.". . '.0 o o . . 0, 'Z ~ g g , ... " • ". (Z) SS330nS NOIlV03Md· o 49 ' i,

r o .'

J

,., 0, -'. ~ U ~ ..

Flg. 1-10. . Greehr BleC'k-becked Gull predet 10n rete Wn APleriea"' Coot , , , popula11on 'On, . Leke Met tal'luftkeet, North Caroline, ~s ~erved fr,o", 22 \ October ~o 18 DecePlber 1903. Predation ret~ ls calculeted,., by blocka of 40 .. hours of' obaérvetton. ) • nUPlber'of ~il15 ob~erved ln each block. ,.

'.

'"

-, , '

, "

\ .

-. • , , .' , ,

. , ~~~~~ 51 ..

o

( .. ..

,..

D

.. rig." 1-11. t4U1'1ber of nPler1cari eoots observe.d per day, fro/ll 1 of 3 observation pointa on ,Lake Mathl'lu5kee,t,.. North CarpI!na 'ro/l1 22 October ,to 10 Decel'lber 1903. ET- Cast Tower, CT- Central Tower t 13- l'PIpound/llent

3.

o.

.Ir

52

, : ' .. ,; , . Q r .. Z 1-w u· t') W . " .C> , 'W x 0 c· 0 '-l -q

0 D D III '",\ '0 0 ... 0 . 0 D - \ 0 ~ 0 o o. CO 0 0 0 .. 0 ., Q 'X -0 x )( x 0 x N x ~ " X' 0 0 X W f X t- 0, .,« X ... 0 X . 0 0 X )( 0) X ." .. 0 ..v > 0 0 z 0 ( 0 0 X • x· 'X 1 ... X t') X X X

N N X -tJ 0 0 0 0 0 I,g 0 0 0 0 o U 0 0 a 0 0 0 0 0 0 0 • \If ~ 0 ID ~ CIl •...... '" .. • '" SIOOO ~O ·ON i .. .~ ..... J ••: ._" .. , 53 o

ng. 1-12. Gre4ter Ohck-backed Gull predation succes! on I\",erican Coots

etruck (STR) and eoota 51ngled out and pursued C~OP), by~coot t'lock IIlze

ail observed on La .... e MaHal'luskeel, Nor:th Caroline frol'1 22 Odober to 10

Oecel'1ber 1903. (I(,S,r') • nUl"lber ~f 1\1115, etrlkee, pursuits for that

'1ock size •.

" . 54 ~ o e ( " o

70

(2,1,10) ), , LEGEND '~ STR

80 ~. sop

~o ,...... N -(1') (1') LaJ4Q (..) \,Il \,Il C,.) ::;:) (1') :z 0 30 ~ 0 LaJ a::: v/~ Vhl V/l ~.A v/~ V//f v/~ c:>"7/t , CL. 20t ~ ~ ~ ~ ~ ~ ~~ ~ J -1 .. 1 ~. V~ ~~ r'l""~ W~ ~~ WAIII W~ 10

KG<, O i 1 11-25 51-100 201 ...... 400 2-10 26-50 101-200 401+ FLOCK SIZE .. -0

- ---

\

l'Ig. 1- 13. rereonlage of int.JlO~' Cklila .trlk.. pursuils) between Creater Olack -backed' Cul h and "/11er Ican Coote 'rol'll l'liA,ed epecies

Hocks, by coat 'lock 5ize, as observed on Lake MaHal'llusJ..eet, North .,., . Il (Fischer Carolina f'roPl t.L. October to 10 Decel'lber la03. - p < 0.01 • .. exact t~5t). ( ). tot.al nU/IIber 0' Interllctionll recorded t'rOM that flock

eize.

-

5G • o. + ~ ~ CD -0 \ - ~ 0 .,0 1 ~\), 0 N .. / ~ ...1 0 ... " Il "'w .. N (J)-

.~ o~ IOU .P 10 U)..J l'fLa..

", - o .... ' 1 l'f x- l. ' *0 *~~~~~~~~~~~~~~~~~~~~~~~,

o ,...0, o -, . ~ / g ~ ~ 2 2 o (X) >13011 S31:l3dS 03Xtn "O~ SNOU3W31NI-

57 \. CONNECTING STATEI1ENT

o In the precedlng section, 1 Inveetlgated verious aspects of Greater 1 Black-backed Gull predation on Af'1erlcan Coots. ft /Jas shawn that 3coot

beh~vior and gull age have an effèct Oh predation success while flock

eUe doee not. However, gull-coat interactions were not the only aspects

of !edatton worth 1 nvest igatlng. f In the next section, 1 exal'1tne Bald Eagles Killing coots and

eteal1ng coots Ulled by gulll5. My goal 1.s to Bmlwer two questions: 1-

what proportion of the food obtained by Bald Eagles i5 obtained using

Qulls li!! cues? 2- does 1'10bbing by gulls significantly reduce the success

of eagle piracy attel'1pts on gul1s? These questions 65 well as a

description of how 11'11'1ature 8ald Ellgles obtaln food by st-eallng Al'lerican

Coot cürcasses killed or found' by Greater Black -bBcked Gulls, will be exal'11ned. , ... r

o /;" se ....

o \, ..

SECTION 2;....

...

Bald Eagles Kl11(ng" A~erlcan eoots and S~eall"g Coq! Cercasses frol'l Greater Black-backed Gulls ,

.~ \

l ,

o

59 , r ~. '", .'

ABSTRACT () \ We observed iMMature Bald Eagles Hallaeetus leucocephalus kHI\ 4

\ "Alllerican Coatis Fulicl! alllericana and steel 8 of 33 coots k i lIed by Greeter

Bleck-backed Gulla larus ~erlnus fro~ 22 Oclober to 18 Oecelllbe~ 1983 on

Leke "e.th~uskeet, 'North Caroline. Coot cercasses l"Iade up at least 79% ot

eegle dleta. Eegles obtalned 94% of thelr food wlth a gull eue. Up to 5

gull"1I of aU c!rges would ~ob eagles trying ta steal coat carcasses, bot

~ th1~ did not affect 5UCC~SS of eagle plrac~. Only 1 out of 17 piracy -. atte",pts wall helted by gull,l"IobblnQ.

",

, ,

"

" ••

60 INTROOUCTI ON o Bald Eag1es Hâl1eeetus leucocephalus ~re opportun! sh and feed on a v5r-lety' of prey. Sinee l'Iost prey are Identl fied frol'l pelleta, 1 t 15 not. ! - ., always known how the1r food 1s obtalned.. Bald Eagles have' been reported

kill1ng Al'lerlean eoots Ful1ca al'llerlcana (~rubb and Kennedy 1992) and

steallng prey 'ra", l'Iany specles (see Fischer 1995 'or a ,revlew). Ersk lne

(1989) observed Bald Eagles stealing fish frolll Greater 8lack-backed GuI lis

l Larus l'Iarlnus'--. however they have never been reported steallng Al'lerican

Coots.

Whlle observing Greater B18ck-backed Gull predat Ion on Al'lerlcan

Coots (Sect Ion 1) we noted Bald Eagles k I111n9 coots and stea11ng cools ... ~)J kl11ed by gulls. This led us to ask 2 questions: 1- what propor-tton 0' the observed. food Hellls ,obtained by Bald Eag'les 15 obtalned wlth the use ~ 0' guÙs as eues?; 2- does IIIobbing by gu11s slgnlf1cantly reduce the success of e8gle p.lraey atleJ'llpts on guI h'7. Here we answer these

r Quest Ions, ,descrlbe how 11'11'1ature 9ald Eligles obhin food by sleallng

'-'Al'lerlcan Coot carcas~e5 kl11ed or round by· Greeter Black-backed Gull s,

./'--'-and docul'lent Bsld Eagles k 111ing çooh • 1 ...... \ / " STUOY AREA AND METHOOS

The study was conduded on Lake Mat tal'luskeet, a 1&,200 hectare lake

averaglng 80 CIII ln depth, located w!thtn M"Uallluskeet National Wlldll te

'Refuge ln east-central N.orth Carol Ina. Appr~xl111ately 40% of the eaatern • half of the h,ke l'las sublller~ent vegetat1;~" IIIostly poJJ!lf'I'Iogcton spp. and

Valllsnerla "IIIerlcc'!Ina (Lovvorn unpubl. data>. EMergent vegetation,

prl1'1arl1y Typha spp., grOW!5 along the edge extendlng up to 30 1'1 tro"

shore.

Observations were conducted 'r9111 22 October to lB Decer/lber 1983,

SI .'

'rol'l two 5 III towers, localed on points extend1n~ 600 III frolll shore,

-0 ov~rlooklng the south-central portion of the lake. Pt vehlc1e 'parked next

let:' an '",poundIIIent was used as a third site. We used a 15-60x spoU Ing

,teleacope and 10)( blnocu larll 'or dawn to dusk observations. Interactions

were recorded on tape and later t ranscrl bed. When an eagl e lAIas spoU ed we

recorded a11 of 1 te act 1 vit y ln the are a unH 1 i te departure, usually

towards a prelluI'led nlght roost south 0' the lake. Every t 1"'15 a Bald , . Eagle-gull Interaction was seen, date, t 1"'15, eagle age and behavlor, gull

age and behavior, presence or absence of gull IIIobblng and nUl'lber of gulls

IIIabblng ..,~~recorded. A gull, cue lA/as deflned as e gull Iandlng with!n 2\ t- '. III of a carcass or a live coot whleh subsequently aHracteoan-eaglë.--

Eag1es w!th white heads lAIere aged as edul te and those with dark

heads were conai dered 11'll'lature. Gulls were aged as adult (~4 ye~r-s), 3

years-old, or 1 and 2 years-oid (poQled slnce felol were seen) accordlng to

Robblns ,et el. (1966). A gu11 strlke was de ft ned as a gu11 st,.fking Il .. ' coot. A 'pursult . lA/lUI an iflteraction durlng whlch e gull succe!la'ully 5 .. ' . - IIl1J'1g1ed , out and pursued a coot but hlled to strik.e H.: An eagle ~te~pt

lIura deflned as an eagle flyfng 10101 over a careass, or live coot "..,1 th

talons lowered ln an attel'lpt to capt,ure 1 t.

RESULTS

Food 1 telll!

Whlle loo~~g, 'or' and at gu l1~-coot Interact'1ons for 435 hours, .' Wei

obeerved Bald Eag1es kili 4 eooh and steal 8 of 33 k Ille >'rol'l Greater , Black-baeked 6ul1s. Based on these observations, coot carcellses co~pr18ed

d least 79X of eagle diets durlng the fall et ttetta~uskeet (Table 2-1). o Flah accounted 'or 2X of the eagle dlet by OCCUrrence. The proporUon 0' observations of eegles. obtalnlng food with el gull CWet, ,versulS wlthout a 62 f -

~,;_:j.' .;~ .. ,-~'. cO .. , ••• - ,

eue 15 81gn1f1cantly greater t-han 75~ "(B1no"'1al test, P-0.0001). Based on o ~ o observat 10n'1I for wh1ch pres'ence or absence of a gull eue could be . . deter:'lIIlned_~ 94% of food !tellls were obtalned 1111 th a' gull . c~ .', .. • Mobblng 1

, , Th~,..e was no 5'1gn1 fiç:ant di fference be·hle~n the proport 10n of

observat 10ns 10.11 th gul1 s 1"I0,bb 1n\1l"1J'1\atur.e c~Ç)le!l versus observations

1111thput ",obblng at coot carcasses (8ino"'181 test, P-0.86S) and for total

food Ite",s ptcked up\ by eagles (P .. 0.881>

"'9bbed··~agles. faglès wer~ ",obbed by a ",axiMUI'I of 5 gulls 0(x-2.B, " 50-1.13., n"2B). Twenty eight observat1on~ of eagles ",obbed Included 22

observat10ns of eagles obtaining a food He", ,

during. gull pursu1ts, 3 obsërvations of an eagle flylng over li glif1 thllt " landeà on the water and 1 observation of 5 gulls which prevented an eegle .... t , ,

'ra", steal1ng a 'food itelll. The latter wes the only observation of 9ulh

succesafu"Uy preventing an eagle frQPI steallng a food He",.'

Eàgle age end behavldr

Q ,Only 3 of 1~2 observations of eagles involved adults. ~11 eagles

observed securlng food !tePls lIIere i",,,,atures. In 2 ~n!ltances the eagles r seel'led to be Inexper1enced hatch-year blrds beceuse ",ore 'than 5 attePlpts

to p"lck up a dead coot were needed before 1t Illas suceenful. On 14 and "15

Novel'lber we observed 3 i"'Plature Bald Eagles !51J'11ultaneous~y. The

nUl'lber uslng the lake at one tll'le was esti",ated 'ta t;Je 5, based on the

above slght1ng~ and lone eagles aeen regularly' ln the eaat and west ends

of the hke. Uslng Fi scher' s (1985) deflnit Ion we observed 5 ·plracy

attelllph· betw.een il'l"'ature BaJd Eagles, of whlch 2 were succesafut.

Eag1es kllled 4 coots uaing 2 capture ",ethods. Two kilh began as a Qull o 'st,.1ke on a coot, 2 others as gu11 puraulh. Eaglea kl11ed 3 eooh by flylng low over the coota, low~rlnQ their talons and grasplng the". The

63. (l fourlh kill beglJn as e coot dQve in 50 CI'I of,water. An eagle landed in

the weter. where the coat dove and wc:.i ted hal f subl'lerged for 1 l'Iinute, \ then flew,to, ft" perch,with tbe coot. , l':-,,,ature Bald Eagles aho attel'lpted unsuccessfully to capture coots

",slnoled out by gul1s before the eaglea' arrivaI during 2'of 31 strikes by

gulla. Eagles interrupted and 'stopped 4 of 41 gull pursuih, during' 2 of q " thel'l the eagle l'Iade, Ua pwn unauccessful attel'lpt while in 2 cases an . '. eagle flew over the eoot and guU'without attel'lpting to capture the coot.

~ "

DISCUSSION

Food 1 te",!! " ." .. Baoed on o.bservet Ions of ee'gle~ obhining 'ood,' c~ots were the "'~Jor

food i te", of il'l",.e~ur' Beld Eagles et Matta,:"uek.eet. A dependence on-' one type of .food is not unusuel for winlering eegles. .Baid Eagles relied ,.prll'larily on road 'and hunter kl11ed Black-tatled'. Jackrabbi'\. te \ " , .. caf&'Qrnlcus. ln a Ui-eh desert (PlaH 1976), "'on. dead tJatérfowl on a

MleÏRIuri refuge' (Griffin et al. 1982) and

, , , ' Fclr~sh 'of ~ri zona' and New, Mex lco (Grubb and ooKennedy 19~~ >..

Greàter Slack-backed GuI la were et'sy to locate fl6r observation o . because ot their size and coloration. We assul'le that eagles have l1ttle

! d,l fficulty locatlng gulls and learning the Ir feed 1 ng behav iors. Eegles

'-le. 10)perCh ,O~d .olled for 0 gull 10 !end or begln clrcling obov. one

location be,fore flying to t·haf"area. ay u"ing ft gull eue, eagle" found

,,,,ost of their food ite"'!l. Along the Mississippi river, eagles stole fi~"

frol'l Al'leric~n Crowe ,Çorvu!! hrachyrhynchoe by wa1ting untll they had" freed fj li fien tro", the ice, then swooped in and robbed the prey (Fischer 1985). '\~ ! ~

Graeplng prey on the water and diving after pre-y as obeerved ln this

• !tudy, were J10ted by Grubb and Kennedy (1982). Baker (ln 8atchelder 1880)

64

J, 4 , o observed an eagle dive efter e divlng ~oot end rel"laln 5ubl'le~ged for a hw seconds before surfaclng and slo",ly taklng off with the coot. Kenyon

(1940) describes an IP1P1.!1ture B.!Ild EagJe ktll1ng a coot rushlng. ta t.!lke

off., lendlng with the ,coot and abandon! ng it for no reason., We dld not

\ observe an ellgle kill1ng 8ny flylng coot. or ebendoning 8ny food.

Eegles were not el ways successful. We observed lIttel'lpt5 by e8glea

which feiled becauee coote dove at the lallt Plol'lent. At'ter ft l'Iaxil'lUI'I of 4

pesees over the diving coot an eagle would return to ite perch. Bende~

(1980) obaerved li coat escape capture by en adu lt Bel d Eagle by l'IOV ~ ng

into el'lergent vegetation and divlng.

Mobbing

Mobbing did not algniflc~ntly reduce the succe~s of eeg,le pir-"cy on

gu11s. Gulls 'l'Iobbed ea91e.15 wh1ch: stole 8nd killed coots, dlsturbed gulls

engaged ln pUl"'sul ts w!th 8 coat antl those thet.f le", over gU115 whlch were -.Î_ , w1 t-hout food. The nU/'Iber of gulls Plobblng, gul1 age o!Ind al'lount of 'ood

!eH on the carcess !ieel'led to inf luence 8 gu11' 15 decislon to rlob en eagle

but we lack data to confirr'l thel'l. Contr8ry to Lien's (1975) ob:5ervatlon,

gU118 were never ob5erved chasing an eagie eway frol'l a prey 1 tel'l. A rlsk

ex1sted for eny gull whlch stayed at e C8rcass whi le lin eagle 8pprollf-h,d r

because Herring Gulh k .!Irg~ntatus have been athcked by 8ald Eagies , .. 1.. "". (Ye8ger 1950, Poor 1936) and Gre8ter Black-backed Gu11s have been round

in food rel'1ains of eegle:5

Black-backed GU115 alwaYI5 1 ifted off the water before the eegle lIr,.lved

lit the carcarHL

ElIg 1e 8ge and behav i or

1 AlI e,;,gles obaerved feedlng in thl:5 IItudy ~ere If!'1l'1l1turee. Gr1' ftn , o '- ( 19B 1 ) indlcated that Plany 11'11'111ture elIg1e15 l'lay" seeK ,wlnterlng erell8 not ( 1 used by adu lts. Sprunt and Liges I( 1966 ) round a tendenc y for 11'1Mature 651 \ 1 eagles to ... inter fadher south then adulh. In Maine, t"''''ature Bald Eegle

lIurv 1 val 1 ncreased when feedi ng stationa, were elltab 1 ished (HcColl ough

1986 ) • At Mettel'llJ akeet , il'll'leture Bald Eaglee 'Play have hft'd a f:tigher

i lIurvive! rete because an eaey food source wall provided by the gulla and beceuae of 1014 use 0' the aree by potentlelly cq",petit1ve edult Bald

Eaglea,

Three explenetlonll eccount for the observed piracy by Bald Eagles.

First, 5a prey size increalles, handl1ng til'le Increa!les, therefone

opportun1tles for plrecy Increese (Grubb 1971, Fisçher 1985>' This trend

Crows, When e fish required 1'10re thl.ln 5-10 seconds of l"Ic!lnipulation by a

lIIergen5er "e Bald Eagle attePlpted ta steel if (Grubb 1971), Crow!! ... hich

could not sWlIllow a filSh within a fetal e.f!conds al!io had 1t stole" by

eaglel5 (Fischer 1985), Such cOl'lpari!5ons are poe!!!ible lA/ith Greeter

Bleck-backed Gulle. The,y are weIl known predators of nestJings lJnd

ducklingll whlch can be 5wal10wed quiekly. However, guil feeding UMe on li

coot averagea 35.2 ",inutes (50-20.7,n-13) (Section 1>, there fore

increaelng the chance of piracy. Secondly,w!th population increllse!5 of

Greeter 81ack-backed Gulh (Orury 1973) the probabil1ty of the!r ( 1 wHh eaglel! and consequent Iy, eag1e piracy, he!!! Increased. ~cJunter!lri - Finally, bur obliervations of predation by gulls and etlgle plracy My be

nothlng new, Just a phenol"lenon rarely obscrved and recorded. Th15 type or

pradet ion 15 not easy to observe' because i t u!5ually occurs 1 n a l'Iar ine - environ."ent or on li large body of weter, durlng l'Iigration or wlntering

periode when hw rcsearchers ere in the field. Wherc et.lgles, gu11s and , \ , j coots overlep ln etaglng end wlnter Ing areee, we believe an observer wJih o pet lence ahould lIee al",i lar event s. \

66 _ l''fi- LI TERATURE CI TEO

Batchelder, C.F. 1880. The Bald Eagle (HaI~~tus l~tl)J:..ruL~he.l.us) as a

ff hunter.. Bull. Nuttall Ornithol. Club 6:58-60.

Bender, R.D. 1960. Arwaerlcen Coot success1'ully escapes 1'ro", a Bald El!Igle.

Wil son Bu 11. 72: 404-405.

Orury, W.H. 1973. Population changes ln New England settbirds.

1) Bird-Bandlng 44:267-313.

Erskine, A.J. 1968. Encounters between Bald El!Iglee and other blrds ln

wlnter. Auk 85:681-683.

Fischer, D.L. 1995. Piracy behavior of wlnterlng Bald Eaghe. Condor

87_: 246-251 •

Griffin, C.R. 1981. Interactive behavior arwaong Bald Eagles winterinÇ) ln

North":Central Missouri. Wll!!on Bull. 93:259-264. o GrifHn~ C.R., T.S. Boekett, & Il.0. Sperrowe. 1982. Ecology of' Beld Eagles,' winterlng nellr a waterfowl concentration. Special Scientif1c

Report-/ Wildlife.' no. 247. U.S. Dept. of the Interior, Fleh and ,,'

Wildlife Service.

Grubb,T.C. 1971. Bald Eagies eteal1ng fish frOlll COIII",on Mergoneers. Au'k

B8; 928-929.

Grubb, T.C., & C.E.Kennedy. 1982. Bald Eagie winter habitat on

southwestern Nat lonal Forests. USDA ForMt Service Res(llIrch Paper

RM- 237, I3p. Rocky Mount!!!n Forest end Rtmge ExperiMcnt Sttltion,

Fort Collins, Cola.

Kenyon, K.W. 194~' An ob..5ervetlon on the feedtng of the southern Bal d

Eag 1 e. Condor 42: 265-266.

'Lien, J. 1975. Aggre65ion between Great Blec~-backed Gull!! and Bald, (, 0 Eaglee. Auk 92: 584-585. t

67 {il MCCO~Ough, H.A. 1986. The poet-fledging eeology and populetion dyna~ies

~, Bald Eegles in Maine. Unpublished Ph.O. dissertetion, Orono,

Maine. Univ. Haine.

PlaU, J.8. 1976. Bald Eagle5 winterlng ln e Ut~h de .. ert. AM. Birds

30:783-788.

Poor, H.H. 1936. A Herrlng Gull eHMked by e Beld Eegle. Wilson Bull.

48: 220-221.

Rabbins, C.S., B. Bruun, & H.S. ·ZiI'1. 1966. A guide to field

identification: Birds of North AMerica. Golden Press. New York.

Sprunt, A.,IV, & F.J. ligas. 1966. Audubon Bald Eagle studies 1960-1966.

Proe. Natl. Audubon-Soc. Conv. 62'25-30.

Todd, C.S., L.S. Young, R.B. Owen Jr., & F.J. Gral'111ch. 1982. Food habits

of 8ald Eagles in Maine. J. Wildl. Manage. 46: 636-645. C " Yeeger, L.E. 1950. 8ald Eaglcs ettack crippled gull. Wl15o~ Bull. 62:210.

. .

1 o

68 Table 2-1: IPiPIature bald eaglc food Itel'l5 picked up FrOM Lake

o MettaPiuskeet, w1th presence or abeence of greater bl~ck-backed cull

(GBBS) eue andl or haras5P1ent.

Hel'! SSB6 ,cue GBBG h/5rllssel'!ent

. picked up n

WHIl tHthout Unknown

Coat 37 27 0' 10 19 17 1

cercaS8 (b ) , Fieh 1 0 1 0 0 0

Unknown a 3 1 4 3 4 1

carcaslI (c )

Totale 46 30 2 14 22 22 2

------~--

e Not known if carcaes found or kllled, or 1f gull neerby, only saw

iMMature Bald Eegle flying wtth it . .. b Doee not~ include 4 Al'lericon coots killed by eegle~.

c Carcllss which could not be posttlvely Identlfled lUI a cOQt due ta

distance frol'! observer and or 6Mount of carcess rCMelning.

\

..

o 1 •