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<br />.~.
<br /><tA'" ,. .
<br />. .: 1.
<br />
<br />:l Ph.D. thesis. Oregon State Univer.
<br />
<br />~~d 'K. jefferts. 1984a. Distribution
<br />nee of the eady life stages of .'qui?,
<br />lOatidae (Cephalopoda, Oegopslda) In
<br />1 North Pacific, Part 1. Bul~eun of th~
<br />eoce Museum (Tokyo) Senes A Zool:
<br />106.
<br />and K. jefferts. 1984b. Distribution
<br />.nee of the early life stages of squid,
<br />:matidae (Cephalopoda, Oegopsi~aet
<br />ern North Pacific, Part 2. BuIlelln of
<br />1 Science Museum Tokyo) Series A~
<br />, 165-193. ,!
<br />l. W. Frost. H. P. Batchelder, M.j.;- c
<br />d R. E. Conway. 1984. Life histories"
<br />lzing copepods in a subar~tic oce~(
<br />mw plumchrus, Neocalanw cnstatw and,
<br />ngii in the Northeast Pacific. Pro~~:
<br />aphy 13: 201-243, '''l
<br />A. Hatch and C. J. Lensink. 1~.~8,;
<br />lasbn seabird colonies. U.S. Fish3./lt "__~
<br />vice Washington, D.C. ."C
<br />184' The diet and food consump .
<br />assi'n's Auklets during summer, an'.
<br />with other plankton-feeding at.'
<br />,65-77. ,
<br />B. Fulton and S, G, Sealy. 1985,'.
<br />of zooplankton by Ancient Mur
<br />Auklets in the Queen Charlot!~
<br />al of Plankton Research 7: 443-45
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<br />Ocean Sciences No. 87. .:'
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<br />in Hecate Strait and Queen C~!t
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<br />ography and Ocean Sciences
<br />Robertson, R. W. Campbell, q
<br />on. 1983. Distribution and
<br />.ds on the Canadian west c
<br />'e Service Technical Rep?'~
<br />sh Columbia. 74pp. -;W~.
<br />G. Sealy and G. A. Sange_\iJ.'_
<br />logy of the Alcidae in th~;\
<br />c. Pp. 189-227 In Seabirds!~'
<br />role in marine ecosyste~
<br />. Cambridge Univ, Press,~, b
<br />
<br />A. Vermeer, K. R. Summ~
<br />979. Numbers and habit~r
<br />LUklet breeding on Tria,'1g e
<br />nbia Auk 96: 143-15L~:>.~
<br />'7. Notes on behaviourJ)f:"
<br />uth Polar Skuas off th~ ~-r'-
<br />let 58: 47-49. '~;;
<br />D. Heinemann. 1979<S,5~
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<br />196, . ::;
<br />Biostatisticai analysIS
<br />:Iiffs, New Jersey.
<br />
<br />~
<br />
<br />NOTES
<br />
<br />Bald Eagle Incursions and Predation at Great Blue Heron Colonies
<br />
<br />DONALD M. NORMAN', ANDRE M, BREAULT' AND IAN E. MqUL'
<br />
<br />11nstitute of Wildlife Toxicology, Western Washington University, Bellingham, WA, USA 98225;
<br />'Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada V6T 2A9
<br />'Deparunent of Animal Science, University of British Columbia, Vancouver, British Columbia, Canada V6T
<br />2A2
<br />
<br />Abstract.-We recorded Bald Eagle (Haliaeetus leucocephalus) incursions at eight Great Blue Heron (Anita
<br />herodias) colonies in the Pacific Northwest in 1988. The incursion rate was one per 10.3 hours of observation.
<br />Herons showed three acoustic responses to eagle intrusions: sudden silence, roaring, and no change. Two
<br />incursions resulted in predation of heron chicks. We suspect that eagle predation is more frequent than what
<br />we observed and could adversely affect heron productivity at some colonies. Received 7 March 1989, accepted
<br />/J April 1989. . , , .
<br />'- Key Words.-Great Blue Heron, Bald Eagle, Puget Sound, GeorgIa Stralt, PaCific Northwest, Washington,
<br />:,~. British Columbia, predation, coloniality.
<br />
<br />
<br />, Great Blue Herons (Ardea herodias) are
<br />"found throughout the Pacific Northwest
<br /><l>(Wahl and Speich 1984, Butler and
<br />~1'Campbell 1987), In British Columbia, 27
<br />~"heron colonies account for approximately
<br />f$:41QOO breeding pairs in the Strait of Geor-
<br />iJiigia(Butler 1989), and a maximum of 1000
<br />mpairs are found in the Puget Sound basin
<br />f";(!'igrman, unpublished data). Over the last
<br />"loiyears, the number of breeding Bald
<br />.~gles (Haliaeetus lew:ocephalus) has in-
<br />tf~~sed by 34% in Puget Sound (McAllis-
<br />~r'.(t ai, 1986) and by 30% in the Gulf
<br />~l~nds of the Strait of Georgia (Vermeer
<br />i~ar}989), Prey items found under eagle
<br />,.!~sts. indicate that herons are part of an
<br />' . gl~'s diet (Imler and Kalmbach 1955,
<br />.~r!'leer et al. 1989), However, heron re-
<br />, ;::;iD~jound under eagle nests may not in.
<br />~~.\e,.predation because eagles are known
<br />,v,ellgers (Stalrnaster 1987).
<br />;l}V.\he Pacific Northwest, Bald Eagles
<br />!p~~l1).es attack and eat juvenile and
<br />d~I'(Ilayer 1979, Forbes 1987) and nestl-
<br />,g~Eeat Rille Herons (Kelsall and
<br />I P~~1'1980, Simpson 1984). In this note
<br />, ~ribe the frequency of Bald Eagle
<br />ns and occasional predation of
<br />;, erons at eight Great Blue Heron
<br />
<br />~,Preat Blue Heron colonies ac-
<br />g;,f"r 712 pairs were studied in the
<br />./Ihwest in 1988 (Table I). Incur-
<br />e..~.,defined as the presence of
<br />"of,:above the canopy containing
<br />~f,.AcoustlC responses of herons
<br />
<br />Colonial Waterbirds 12(2): 215.217, 1989
<br />
<br />to each eagle incursion were classified into
<br />three categories: silence (sudden total ces-
<br />sation of vocal activity), roaring (syn_
<br />chronized vocal uproar), and no change in
<br />colony activity. Adult herons were sexed
<br />on the basis of males beings slightly larger
<br />than females (Simpson 1984). Heron
<br />chicks were aged from their presumed
<br />hatching date. Hatching date was deter_
<br />mined from either (I) changes in adult be-
<br />havior when the chicks hatched, (2)
<br />eggshell collections in colonies (adult be-
<br />rons removed eggshells as soon as chicks
<br />hatch), and (3) vocalizations from newly_
<br />hatched chicks (Brandman 1976).
<br />Eagles were observed 76 times in or
<br />above seven of the eight colonies. Fifty-six
<br />incursions into the colonies were recorded
<br />in 578 hours of observations (Table I), or
<br />one incursion per 10.3 h. Two incursions
<br />resulted in chick predation. Eagles elicited
<br />silence, roaring and no response in respec.
<br />tively 10%,47,5%, and 42.5% of the incur_
<br />sions. Heron acoustic responses did not
<br />appear to follow any patterns. Silence and
<br />roaring were separated by time interVals
<br />of normal colony activity,
<br />We observed two predation events,
<br />The first took place on Sidney Island,
<br />British Columbia On 10 June 1988. At
<br />05 I4 h PST, an adult eagle flew through
<br />the colony, eliciting roaring. At 0540 h
<br />loud heron vocalizations were heard at the
<br />periphery of the colony when an eagle was
<br />observed on the rim of a heron nest lo-
<br />cated 20 m up in a Red Alder (Alnus rubra),
<br />
<br />215
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