Loxioides Bailleui)
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THE AUK A QUARTERLY JOURNAL OF ORNITHOLOGY VOL. 101 OCTOBER 1984 NO. 4 ANNUAL VARIATION IN THE DISTRIBUTION, ABUNDANCE, AND HABITAT RESPONSE OF THE PALILA (LOXIOIDES BAILLEUI) J. MICHAEL SCOTT,• STEPHENMOUNTAINSPRING, 1 CHARLESVAN RIPER,III, 2 CAMERONB. KEPLER,3 JAMESD. JACOBI,1 TIMOTHY A. BURR,4 AND JON G. GIFFINs •U.S.Fish and WildlifeService, Patuxent Wildlife Research Center, Mauna LoaField Station, Hawaii National Park, Hawaii 96718 USA; 2CooperativeNational Park ResourcesStudy Unit, Departmentof Zoologyand Ecology, Universityof California,Davis, California 95616 USA; 3U.S.Fish and WildlifeService, Patuxent Wildlife Research Center, 248 KaweoPlace, Kula, Hawaii 96790 USA; 4HawaiiDepartment of Landand Natural Resources, Division of Forestryand Wildlife, Honolulu, Hawaii 96813 USA; and SHawaiiDepartment of Landand Natural Resources,Division of Forestryand Wildlife, Kamuela, Hawaii 96743 USA ABSTRACT.--Westudied the distribution, population size, and habitat responseof the Palila (Loxioidesbailleui) during the 1980-1984 nonbreeding seasonsto infer factorsthat limit the population and to develop management strategies.Distribution was fairly constant from year to year. Palila were confined to the subalpinewoodland on Mauna Kea on the island of Hawaii, occurredbetween 2,000 and 2,850 m elevation, and reachedhighest densitieson the southwestslopes. The populationshowed large annual fluctuations,from 6,400 birds in 1981 to 2,000 in 1984. The width of woodland was the most important variable in determin- ing habitat response.Palila were more common in areaswith greater crown cover, taller trees,and a higher proportion of native plants in the understory.Annual variation in Palila density within a habitat reflectedvariation in levels of their staplefood, mamanepods. The main limiting factorsof the population appearedto be the availability of good habitat and levels of their staple food. Palila had strongly depresseddensities in the Pohakuloa flats area.This low density could not be explainedby grosshabitat featuresor food levels. Site tenacity,thermal stress,disturbance, and diseasewere hypothesizedexplanations. Our study indicatedthat the mosteffective management strategies would be the removalof fetal un- gulatesand certain noxiousplants from Palila habitat and the extensionof the woodland zone to areasnow intensivelygrazed. Received 20 May 1983,accepted 9 March 1984. DISTRIBUTION,population size, and habitat demonstrate the potential value of temporal responseof speciesthat may require manage- analysesof habitat responsein evaluationsof ment to ensuretheir survivalare ecologicalpa- factors that limit bird numbers. rameters of special interest. Annual variation In this study we examine annual variations in these parametersmay offer insight into the in distribution, population size, and habitat re- factorsthat limit the present population, yet sponse acrossthe entire range of the Palila relatively few workers have attempted to eval- (Loxioidesbailleui), a finch-billed Hawaiian hon- uate temporal variation in habitat response. eycreeperfound only in the subalpinewood- Studiesby Raitt and Pimm (1976), Rotenberry lands of Mauna Kea on the island of Hawaii and Wiens (1980a, b), and Rice et al. (1981) (Fig. 1) and one of 30 endangeredor threat- 647 The Auk 101: 647-664. October 1984 648 SCOTTET AL. [Auk, Vol. 101 GENERAL VEGETATION TYPES MAUNA KEA FOREST RESERVE BOUNDARY CONTOURS IN ".J'Z'METERS c/o Ac s/rs Ac 1900 s/rs So c/o So c/o My- So s/vsMY-So O:(ns) D:(xg) Pasture 0 I 2KI Burned Area Fig. 1. The dry nativeforest on MaunaKea, Hawaii. General vegetation types: c/o Ac = closedto open canopykoa (Acacia koa); s/vs Ac = scatteredto veryscattered koa; s/vs So = scatteredto veryscattered ma- mane forest;c/o So = closedto open canopymamane; c/o My-So = closedto open canopymamane-naio; s/vs My-So= scatteredto very scatteredmamane-naio; D:(ns) = dry nativeshrub; D:(xg) = dry exoticgrass pasture;Burned Area = site of 1979fire. ened birds in Hawaii (U.S. Fish and Wildlife nonbreedingseason, when population levels Service1983). The populationsize of the Palila were lowest in the annual cycle,food was most was estimated at 1,600 birds in the 1975 non- likely to be limiting, and bird distributionwas breedingseason (van Riper et al. 1978). most likely to reflect local variation in food The behavior and ecologyof the Palila were levels. intensively studied by Berger (1970) and van The objectivesof this studywere to (1) ex- Riper (1978,1980a). Palila are adaptedfor feed- amine annual variation in distribution, popu- ing on green seed pods of the mamanetree lation size, and habitat response during the (Sophorachrysophylla), although mamane flow- nonbreeding seasonacross the range of the ers, naio (Myoporumsandwicense) fruit, and in- Palila; (2) evaluate some of the factorsthat may sectsare alsoeaten, particularly when podsare limit presentdistribution and numbers;and (3) scarce.The breedingseason extends from April development managementstrategies to ensure to September and coincides with maximum the survival of the species. productionof mamanepods. During the non- STUDY SITES AND METHODS breeding season,local and somelong-distance movement occursin responseto food levels. Study sites.--The entire range of the Palila lies We studied the speciestoward the end of the within the subalpine mamane and mamane-naio October1984] PalilaPopulation and Habitat 649 MAUNA KEA STUDY AREA 1700 1900 ........ LIMIT, MAMANE-NAIO FOREST .... ' FOREST RESERVE BOUNDARY v STUDY AREA LIMITS 0 2KM 2•TRANSECT ROUTES • CONTOURS IN METERS 1900 1700 Fig. 2. Locationof study area and transectson Mauna Kea, Hawaii. woodlandon Mauna Kea (Fig. 1). Our samplinguni- absence and to detect recolonization. The stations verse was the present range of Pallia, a 139 km 2 area were the sample points for monitoring plant phe- (Fig. 2). We also sampledthe north slopesof Mauna nology,counting birds, and verifying the vegetation Kea, where Pallia no longer occur(transects 116 and map. 117 in area 3), in order to document repopulation of Annual surveys were conducted in the nonbreed- the area. Data from this area were not included in ing seasonduring 7-21 February 1980, 20-29 January our analysis. 1981,22 February-4March 1982,25 February-4March The woodlandon Mauna Kea rangesfrom 1,800to 1983, and 6-9 February 1984. We used the variable 3,000 m elevation.The trees are generally short (3- circular-plotmethod with a 6-min samplingperiod 10 m), and canopy cover varies from very scattered (Reynolds et al. 1980). This period was determined (< 5%) to partly closed(60%). Rainfall averages35-75 to be long enough to hear or see Pallia near the sta- cm annually. tion, yet short enough to minimize the chancesof Mamane occurs around the entire mountain, but countingthe samebird twice or of birds moving into naio is mainly restrictedto the southwestslopes. A the count area (Scott and Ramsey 1981). During this detaileddescription of the vegetationis given in Hartt period, observersrecorded each Pallia heard or seen and Neal (1940). exceptfor birds flying high overhead(Reynolds et Samplingdesign.--Information from the 1975 sur- al. 1980). The countswere conductedfrom approxi- vey (van Riper et al. 1978) was used to stratify the mately 0730 to 1300,the period of greatestPallia ac- subalpinewoodland according to Pallia densityinto tivity (van Riper 1978).Observers were assignedto three areas:a high-density area (area 1), a low-den- transectsby random draw within a sampling area, sity area (area 2), and a supplemental area (area 3), and no countswere conductedwhen precipitation or where Pallia once occurredhistorically (Fig. 2). winds greaterthan 4 on the BeaufortScale (>29 km/ Transectswere randomly placed a minimum of 500 h) occurred.Transects 102 and 103, the area of high- m apart and approximately perpendicular to eleva- est density, were each sampled twice and a mean tional contoursin areas1 and 2. Stationswere placed density was computedfor those stations. 150 m apart from approximately 75 m below treeline The 20 observers that participated in the survey to the lower woodland boundary.In area 3 we placed were trained in identifying Pallia and estimatingde- 24 stationsalong two transectsrandomly located in tection distances(Kepler and Scott 1981). All but two the bestremaining woodland only to verify the bird's observersmet hearing standardssuggested by Emlen 650 SCOTTET AL. [Auk,Vol. 101 and DeJong(1981), and one of these did not partici- Canopy height (m); pate after 1980. Mamane biomass--mamane cover (%) times mamane Densityestimates.--Using the methods of Ramsey height (m); and Scott (1981), we calculateddensity estimatesfor Naio biomass--naio cover (%) times naio height (m); Palila. Using values pooled over 4 yr, we derived Total tree biomass--the sum of mamane and naio correction factors for each observer and vegetation biomass; type as samplesizes allowed. The effective area sur- Shrub cover (%); veyed at a stationvaried from 0.69 to 1.48 ha. Grasscover (%)--cover of grassesand herbs; Populationswere estimatedfor eacharea from the Mamane flower biomas--the averageprecentage of Pallia densities at the individual stations. To estimate mamane branches in flower on the 10 trees sam- the total population in the Palila universe, we pied at station multiplied by mamanebiomass; weighted the populations and variancesin areas 1 Mamame fruit biomass--same as for mamane flower; and 2 by the size of the area and pooled the results. Naio fruit biomass--same as for mamane flower; Densitieswere plotted by