CALIFORNIA BIRDS Volume 3, Number 1, 1972 DO BIRDS FLOCK IN HAWAII, A LAND WITHOUT PREDATORS? Edwin O. Willis Birds of a feathersometimes flock together,as anyoneknows who has ever seen blackbirdsor sandpipersin fall. More surprisingly,in many parts of the world birdsof different feathersflock together.H. W. Bates (1863), in his classic"Naturalist on the River Amazons," reportshow the empty forest seemedto come alivewith dozensof kinds of birds whenever one of the diverse Amazonian "mixed flocks" passedby. Lessspectacular but similarmixed flocksof Plain Titmice, Hutton's Vireos, and other birds can be seenin California oak woodland. Those who watch for mixed flocks will be surprised how common they are, from migrant warblers among Common Bushtitsin chaparralto CactusWrens among Black-throated Sparrows on the desert. Some mixed flocks gather for obviousreasons, such as the gulls one seesat garbagedumps or the flocksof birds that follow army ants for flushed insectsin tropical America. The birds in most complex flocks,however, seem to congregateand follow each other without obvious reasons.Probably no biologist can say what a warbler gainsby followingBushtits about, althoughmany biologists nowadaysare studying mixed flocks. Reasonssuggested for mixed flocks mostly fall in two categories.Perhaps the birds lead eachother to food, or avoid competingfor food, by watchingeach other. Perhapsthe birdsavoid predation by associatingwith eachother. One bird I studied in Panama, the small Spotted Antbird (Hylophylax naevioides),definitely spent lesstime callingor fleeing nervouslywhen it associatedwith birds of other species(Willis, in press).This gaveit the opportunityto stareat the groundto forage. Calif. Birds 3:1-8, 1972 1 FLOCKING OF BIRDS IN HAWAII Presumably,if there were no predatorsthere would be no reasonfor such birds as Spotted Antbirds to join other species;they could forageas they pleasedwithout keepingnear other alert birds.Mixed flocks bound togetherby warinesswould be absent;flocks would be associatedwith feedingopportunities only. Birds in an area without predatorscould form a natural ex- periment.If flocks like thoseof the Vireo and the Titmouseexisted in such an area, it would be likely that thesemixed flocks do form for food reasonsalone. Birds and other organismsin placeswithout predatorslose their defensivemechanisms rapidly. Island birds are notablyfearless, and plantson Hawaiiseldom have spines or poisons (Carlquist,1970). Warner(1968) found Hawaiianhoneycreepers do not sleep with their heads in their back feathers,and are therefore easy prey for introducedmosquitoes. The problemis, it is rare to find a placewithout predators.It is especiallydifficult to find sucha placewith many speciesof birds,so that one can get mixed flocks.The naturalexperiment, however, has been performedon certainoceanic archipelagos that have few or no predators.As Darwin found during the voyageof the Beagle,the isolatedarchipelago of the Galaparoshas many speciesof finchesbut hardly any native predators.However, there are many somewhat predatorylizards on the Galaparos. Evolutionon the HawaiianIslands has produceda groupof land birds as diverse as the Darwin's Finches, in a land almost without predators.There are open-countryHawaiian Hawks (Buteo solitarius} on the island of Hawaii and Short-earedOwls (Asio ]lammeus} on all islands, but no predatory native mammalsor reptiles. Unless the hawk and owl fed on insectsand nestlings,they may have come to Hawaii as new immigrantsafter the Polynesiansintroduced rats; they seldom eat adult birds. If there were mixed flocks of the diverse nativeHawaiian birds, they surelycould not be causedby predation except perhapson the islandof Hawaii itself. The Drepanididae,or Hawaiianhoneycreepers, are a fascinatingly diversegroup of birds. Havingevolved from one or two originalkinds of birds, so far back that nobody now knows whether they were tropical honeycreepersor Northern finches, the many types of drepanididsnow possessthe widest variety of types of bills in any bird family. There are smallwarblerlike ones with straightbills (the Anianiau, Loxops parva} or slightlyalecurved ones (the Amakihi, Loxops virens}, others with vireolike bills (the Creeper,Loxops maculata, of the forest undergrowth,and the Akepa, Loxops 2 FLOCKING OF BIRDS IN HAWAII coccinea,of the leafy tree crowns)and f'mallystrange rare creatures with downcurvedbills over 2 inches in length (the Akaialoa, Hemignathusprocerus]. One, the Akiapolaau[Hemignathus wilsoni], has a short, straightlower mandiblethat it usesto flake bark off trunks with woodpeckerlikehammering, while its long downcurved upper mandibleprobes for insectsin crevices.The related Nukupuu [Hemignathuslucidus] differs mainly in having the short lower mandibledowncurved. Related birds, mostly the size of large spar- rows, had thick finchlike bills or even parrofiike onesto crackseeds or tear open twigs.Regrettably, most f'mchlikebirds except the Palila [Psittirostrabailleui] are now rare or extinct. Another line, mostly extinct now with the exception of the common red Apapane[Himatione sanguinea]with its short, slightly decured bill and the fairly common red and black Iiwi [Festiaria coccihealwith its long downcurvedbill, adds to the diversityof the native Hawaiian avifauna. In addition, there are a little native flycatcher, the Elepaio {Chasiempsissandwichensis}, two native thrushes(including the Omao, Phaeornisobscura}, and numerous introducedbirds. The avifaunais certainlydiverse enough that mixed flockscould form, althoughthe nativebirds have suffered greatly and many are now rare or extinct. Introducedmosquitoes brought them introduced diseases(Warner, 1968), introducedants killed insects they fed on (Zimmerman,1970), and the original forestswere reducedto a quartertheir originalextent (Carlquist,1970). G. C. Munro, one of the collectorsat the rum of the century,saw the native Hawaiian birds duringtheir horrible decline.In 1944, he recorded his memoriesand field notes of half a century on the islands.On Kauai, he reports,the Creeperformed small flockswith Akepas.The constantchipping of the Creeperattracted other birds. To find certain rare species,such as the Nukupuu,he followedthe chippingof the Creeper. Since there were no native predatorson Kauai other than Short-earedOwls, Munro's report interestedme greatly. Richardsonand Bowles(1964:29) reported Anianiauoften foragetogether with Amakihi. It seemedthat there might indeedbe mixed flockson Hawaii in the absenceof predators. However,one must be cautiousof reportsof mixed flocks.Even birds that ignore each other occasionallycome in contact, and an observerwho comes up at that moment will think the birds are associating.Sometimes birds mobbingan observerwill attract other species.One needsto watch critically to see if birds actually follow each other. 3 FLOCKING OF BIRDS IN HAWAII Thus, when my wife Yoshika and I went to Hawaii, I looked for mixed flocks. I quickly found that someof the other thingsI looked for, such as rare or reportedlyextinct birds, were very hard to find. As Berger(1970) reports,bird studyon Hawaiiis very difficult for a variety of reasons.Many of the nativebirds are so rare or restricted to undisturbedforests that a major expeditionis neededto hunt them down. I tried hiking in to somesuch places, such as a 10-mile mountainclimb to the upperKipahulu Valley on the eastside of the extinct volcano Haleakala on the island of Maui; but I found that I had only three or four hoursof birdingbefore I had to starthiking back. Naturally, I missedthe rare species. However,if one gets to relativelyundisturbed forests above 1000 meters elevation,he finds that the remainingnative birds of Hawaii are often remarkably common and far outnumber the introduced birds in someareas. On our first day on Hawaii,in the forestsof ohia trees and tree-fernsalong KilaueaCrater, it seemedthat every other treetop had its Apapaneor two, every fifth tree a tail-flickinglittle Amakihi. These are the two commonestsurviving honeycreepers, common on all the larger Hawaiian islandsfrom the last scrubby ohiasin the crater of Haleakaladown to the slopesabove Honolulu. 'From Kilauea, we went to cabinsat PohakuloaState Recreation Area in the dry saddlebetween the massivevolcanoes of MaunaKea and Mauna Loa. In the dry scrub and eucalyptus,House Finches, House Sparrowsand CaliforniaQuail outnumberednative birds. The pretty little Red-billedLeiothrix (Leiothrix lutea] were in flocksof up to 10 birds, and I watchedsmall groups of introducedWhite-eyes (Zosteropspalpebrosus] and a few Amakihi follow one suchflock. There were many White-eyesand Amakihis away from this mixed flock, however. From Pohakuloa,I drove up on the slopesof MaunaKea to what provedthe best birding area on the "Big Island"of Hawaii.This was the scrubby, low grassywoodland of mamane and naio trees at Andrew Berger'sstudy area above Puu Ulaau cabin. Here, Elepaio flitted from bushto bush,Amakihi were more commonthan Elepaio, crimsonApapane and Iiwi paradedfrom treetop to floweringtreetop, and Palila whistledfrom a few of the mamanetrees. The scoldingof a few Leiothrix, almost the only introduced birds besides the commonWhite-eyes and grass-dwellingSkylarks, brought in the most excitingbird--a bright yellow Akiapolaauwith its absurdlyshaped bill, to tap on branchesinside a mamanenear me and then to perch near me as I squeakedand drew in curious Amakihi from all 4 FLOCKING OF BIRDS IN HAWAII directions.One feds sad that this forest is graduallydying, mainly from overgrazingby