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The Condor 112(2):196–212  The Cooper Ornithological Society 2010

Population Trends of Forest Birds at Hakalau Forest National Wildlife Refuge, Hawai‘i R ichard J. Camp1,4, Thane K. P ratt2,5, P. M arcos G orresen1, John J. Jeffrey3,6, and Bethany L. Woodworth 2,7 1

Hawai‘i Cooperative Studies Unit, University of Hawai‘i at Hilo, Pacific Aquaculture and Coastal Resources Center, P. O. Box 44, Hawai‘i National Park, HI 96718 2 U.S. Geological Survey, Pacific Island Ecosystems Research Center, P. O. Box 44, Hawai‘i National Park, HI 96718 3 U.S. Fish and Wildlife Service, Hakalau Forest National Wildlife Refuge, 60 Nowelo St., Suite 100, Hilo, HI 96720

Abstract.  The Hakalau Forest National Wildlife Refuge was established to protect native Hawaiian forest birds, particularly endangered species. Management for forest restoration on the refuge has consisted mainly of removing feral ungulates, controlling invasive alien plants, and reforesting former pastures. To assess effects of this habitat improvement for forest birds, we estimated density annually by distance sampling and examined population trends for native and alien passerines over the 21 years since the refuge was established. We examined longterm trends and recent short-term trajectories in three study areas: (1) reforested pastureland, (2) heavily grazed open forest that was recovering, and (3) lightly grazed closed forest that was relatively intact. Three species of native birds and two species of alien birds had colonized the reforested pasture and were increasing. In the open forest, densities of all eight native species were either stable or increasing. Long-term trends for alien birds were also generally stable or increasing. Worryingly, however, during the most recent 9 years, in the open forest trajectories of native species were decreasing or inconclusive, but in the reforested pasture they generally increased. The closed forest was surveyed in only the most recent 9 years, and trajectories of native species there were mixed. Overall, long-term population trends in Hakalau are stable or increasing, contrasting with declines in most other areas of Hawai‘i over the same period. However, more recent mixed results may indicate emergent problems for this important bird area.

Key words:  bird counts, Hakalau Forest National Wildlife Refuge, Hawai‘i, Hawaiian forest birds, point-transect sampling, population trends.

Tendencias Poblacionales de Aves de Bosque en Hakalau Forest National Wildlife Refuge, Hawái Resumen.  El Hakalau Forest National Wildlife Refuge se estableció para proteger a las aves de bosque nativas de Hawái, particularmente a las especies amenazadas. El manejo para restaurar el bosque en este refugio consistió principalmente en remover ungulados asilvestrados, controlar plantas exóticas invasoras y reforestar áreas que antes estaban como pastizales. Para evaluar los efectos de este mejoramiento del hábitat para las aves de bosque, estimamos su densidad anualmente mediante muestreos de distancia y examinamos las tendencias poblacionales de paserinos nativos y exóticos a lo largo de los 21 años transcurridos desde el establecimiento del refugio. Evaluamos las tendencias de largo plazo y las trayectorias recientes de corto plazo en tres áreas de estudio: (1) pastizales reforestados, (2) bosques abiertos fuertemente ramoneados en recuperación y (3) bosques cerrados con poco ramoneo que se encontraban relativamente intactos. Tres especies de aves nativas y dos de aves exóticas habían colonizado el pastizal reforestado y estaban aumentando. En el bosque abierto, las densidades de las ocho especies nativas estuvieron estables o en aumento. En general, las tendencias de largo plazo de las aves exóticas también mostraron estabilidad o incremento de las poblaciones. Sin embargo, de modo preocupante, durante los nueve años más recientes las trayectorias de las especies nativas en el bosque abierto fueron decrecientes o inciertas, pero en el pastizal reforestado en general tendieron a aumentar. El bosque cerrado fue censado sólo en los nueve años más recientes y allí las trayectorias de las especies nativas fueron variadas. En general, las poblaciones tienden a estar estables o a aumentar en el largo plazo en Hakalau, lo que contrasta con las disminuciones observadas en la ma­ yoría de las demás áreas de Hawái en el mismo período. Sin embargo, los resultados mixtos más recientes podrían indicar la aparición de problemas en esta área de importancia para las aves.

Manuscript received 19 December 2008; accepted 21 October 2009. 4 E-mail: [email protected] 5 Current address: P. O. Box 420, Volcano, HI 96785. 6 Current address: P. O. Box 40, Pepe’ekeo, HI 96783. 7 Current address: Department of Environmental Studies, University of New England, 11 Hills Beach Road, Biddeford, ME 04005. The Condor, Vol. 112, Number 2, pages 196–212. ISSN 0010-5422, electronic ISSN 1938-5422.  2010 by The Cooper Ornithological Society. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/ reprintInfo.asp. DOI: 10.1525/cond.2010.080113

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INTRODUCTION It is widely recognized that native Hawaiian birds are greatly imperiled (BirdLife International 2000, Scott et al. 2001, Pratt et al. 2009). Naturalists from the 1890s through 1950s witnessed declining populations, contracting species’ ranges, and a great many extinctions (Banko and Banko 2009). Even in the modern period of conservation activity in the Hawaiian Islands, population trends for most species have been downward, with perhaps as many as 10 extinctions from 1977 to 2004 (Scott and Kepler 1985, Gorresen et al. 2009). Despite some successes in Hawaiian seabird and waterbird conservation (BirdLife International 2000), reversing declines in forest birds has been particularly challenging owing to continued deterioration of habitat and the harmful effects of invasive alien species (Price et al. 2009). Comprehensive surveys from 1976 to 1981 by Scott et al. (1986)—the Hawai‘i Forest Bird Survey or HFBS—provided the first quantitative estimates of Hawaiian forest bird populations. These authors established that the majority of native land birds survived mainly at high elevations above the limits of avian diseases, the mosquito vectors that transmit them, and other factors resulting in habitat deterioration and depletion of food resources. Recovery planning has depended on

restoration of montane forests as its principal strategy (U.S. Fish and Wildlife Service 2006). The approaches to habitat restoration include mainly removing domestic livestock and feral ungulates, controlling invasive alien plants, and reforestation. In nearly all cases, however, habitat-improvement projects are relatively new—mainly since the 1980s—and their effects on forest bird populations have not been previously studied. Scott et al. (1986) also identified geographic gaps in forest bird protection, finding that the largest populations of forest birds and prime habitat on Hawai‘i Island fell outside the bestprotected areas (U.S. Fish and Wildlife Service 1982). Ranking high among the top-priority sites were the high-elevation rainforests on the eastern slopes of the volcano Mauna Kea. Here, the first national wildlife refuge to protect and restore Hawaiian forest birds was established in 1985—the Hakalau Forest National Wildlife Refuge (hereafter, Hakalau; Fig. 1). Hakalau is an important test case for habitat restoration as a tool for restoring populations of Hawaiian forest birds because of its relatively early establishment, history of active habitat management, large area, and the long-term monitoring of its bird populations. As is typical over much of the best forest bird habitat on Hawai‘i Island, the upper-elevation forest of

FIGURE 1.  The Hakalau Forest Unit of the Hakalau Forest NWR (heavy outline) relative to general land-cover types (A: dark = forest, light = pasture) from a 2002 Landsat Enhanced Thematic Mapper image. Survey stations (B) span the reforested pasture, open-forest, and closed-forest study areas, each with different histories of survey and resource management. Transects monitored by the refuge (our study) lie within the shaded area, whereas transects sampled by the Hawai‘i Forest Bird Survey of 1977 cross both shaded and unshaded areas.

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koa (Acacia koa) and ‘o-hi‘a-lehua (Metrosideros polymorpha) in the area that is now the refuge was converted to cattle pasture in the 1800s, leading to replacement of forest by non-native grassland (Scott et al. 1986). An additional environmental stressor to forest health has been the feral pig (Sus scrofa), which damages the forest understory, spreads alien plants, and creates mosquito habitat (LaPointe et al. 2009, Price et al. 2009). The eradication of feral cattle (Bos taurus) and pigs has been a primary management goal at Hakalau (U.S. Fish and Wildlife Service 1996), and to date about 4800 ha of the refuge is now ungulate free or nearly so (Maxfield 1998, Hess et al. 2006). Control of four species of invasive weeds has also been undertaken at Hakalau. Finally and importantly, to accelerate habitat recovery in open pastures, refuge staff and volunteers have planted 2000 ha with seedlings of native trees, mainly koa (Hakalau Forest National Wildlife Refuge [NWR], unpubl. data). Detecting and interpreting trends in bird populations and their response to management are important components of assessing conservation actions at Hakalau. Since 1987 birds have been surveyed annually there in reforested pasture and formerly grazed open forest between 1300 and 2100 m. Surveys in these two areas sample about 35% (5500 ha) of the refuge. An additional 1100 ha in relatively intact closed forest has been surveyed annually since 1999. Such long-term datasets of population densities are useful for describing baseline variability in populations, detecting biologically relevant changes, and revealing population responses to management (Camp et al. 2009b). Thus, the frequent and long-term monitoring of forest birds at Hakalau offers one of the best opportunities to examine the outcome of forest restoration for these birds in Hawai‘i. The purpose of this study is to appraise changes in forest bird populations in light of habitat restoration. Because native bird populations are decreasing at many locations on Hawai‘i Island (Gorresen et al. 2009), we ask the question: Are native bird populations stable or increasing at Hakalau? Although a causal relationship between habitat restoration and changes in the bird populations cannot be identified from these data, it is reasonable to expect that the habitat changes underway at the refuge influence bird trends and could halt or reverse population declines. We examine this expectation by comparing trends in native birds within Hakalau to trends in populations outside the refuge under different practices of forest and ungulate management. METHODS Study area

The 15 390-ha Hakalau Forest NWR (19° 51′ N, 155° 18′ W) on the windward slope of Mauna Kea is the largest protected and actively managed area of mid- to high-elevation rain forest on the island of Hawai‘i and in the state of Hawai‘i (Fig. 1). Mean daily air temperature averages 15° C with an

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annual variation of 1900 m) it consists of open grassland, relict mature koa trees, and recently planted forest (Fig. 1). Nonnative plants may be found in native forest at all elevations, the most injurious species being various pasture grasses, gorse (Ulex europaeus), blackberry (Rubus argutus), banana poka (Passiflora tarminiana), and holly (Ilex aquifolium). For purposes of analysis, we divided the refuge into three study areas that reflect the history of management and monitoring at Hakalau (Fig. 1): (1) the pasture study area at 1650– 2000 m elevation was deforested grassland at the beginning of the study. The area was gradually reforested mainly by planting, so that by the end of the study period it was covered mostly by stands of koa trees as much as 20 years old. This study area has been surveyed intermittently since 1987 with 35 stations (area = 1314 ha). Because fewer than 30 stations were sampled in 1992, 1993, 1994, 1995, and 1997, we did not use survey data from those years in our analyses of the pasture study area. (2) The open-forest study area, elevation 1400–1920 m (area = 3373 ha), was once heavily grazed. Most regeneration has proceeded with recruitment from the forest itself rather than by planting. The vegetation is not uniform, and the northeastern portion has a more complex and denser structure (canopy, understory, and ground cover) than the southwestern portion. This area has been surveyed since 1987, longer than the closed-forest study area, averaging 277 stations. (3) Last, the closed-forest study area, the least modified by grazing, has been surveyed only since 1999 with 197 stations (area = 1998 ha). The ecological significance of this study area to the refuge’s management is that, because of its lower overall elevation (1400–1700 m), declines in native birds due to avian disease might be expected to appear there first (van Riper and Scott 2001). Note that the reforested pasture and open-forest study areas have been managed and monitored for a longer time (1987–present) than has the closedforest study area (1999–present). Bird species

We estimated the density and population trends of eight native and four introduced species. Only a small portion—11 species—of the refuge’s original avifauna survives. The missing component includes as many as 13 historically known species that could once have occurred on the refuge, but these are now either extinct or extirpated from the area that is now the refuge, together with an unknown number of species that disappeared before Western contact in the late 18th century (Banko and Banko 2009). Densities of native birds decline at lower elevations, and several species are essentially absent from the lowest elevations of the refuge. Our survey methods (see below) were effective at sampling the refuge’s eight

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extant forest passerines but were not suitable for estimating densities of the ‘Io (Hawaiian Hawk; Buteo solitarius), Pueo (Short-eared Owl; Asio flammeus), and Ne- ne- (Hawaiian Goose; Branta sandvicensis), and data for those species are not presented here. The eight native species sampled are the Hawai‘i ‘Elepaio (Chasiempis s. sandwichensis), a monarch flycatcher (Monarchidae), the ‘Oma‘o (Myadestes obscurus), a thrush (Turdidae), and six Hawaiian honeycreepers (Fringillidae: Drepanidinae), the ‘Akiapo-la-‘au (Hemignathus munroi), Hawai‘i ‘Amakihi (Hemignathus virens), Hawai‘i Creeper (Oreomystis mana), Hawai‘i ‘Akepa (Loxops c. coccineus), ‘I‘iwi (Vestiaria coccinea), and ‘Apapane (Himatione sanguinea). The ‘Akiapo-la-‘au, Hawai‘i Creeper, and Hawai‘i ‘A kepa are listed as endangered by the U.S. Fish and Wildlife Service and the state of Hawai‘i, and the refuge encompasses a core portion of their ranges (U.S. Fish and Wildlife Service 2006, Gorresen et al. 2009). In addition, 16 species of introduced birds now occupy the refuge, but only four of these have invaded the forest and exist in densities that can be readily tracked: the Red-billed Leiothrix (Timaliidae; Leiothrix lutea), Japanese White-eye (Zosteropidae; Zosterops japonicus), Northern Cardinal (Cardinalidae; Cardinalis cardinalis), and House Finch (Fringillidae; Carpodacus mexicanus). Bird sampling

In 1977 the HFBS sampled what is now the refuge along three transects spaced about 3 km apart with 95 stations at 134-m intervals (Scott et al. 1986). Soon after establishment of the refuge, in 1986, a new series of 11 transects was laid out and later expanded to a total of 15 transects, with a range of 196 to 343 stations surveyed across the refuge. Annual bird sampling commenced in 1987. To assure the stations’ independence, distances between them varied from 150 to 250 m (Reynolds et al. 1980, Scott et al. 1986). All surveys have followed the same point-transect sampling procedures initiated by Scott et al. (1986). (Here we use the more current term point-transect sampling rather than the term variable circular plot count used by Scott et al. 1986.) Before the surveys observers were trained in distance estimation and bird vocalizations, thereby minimizing variability among observers and standardizing for local conditions (Kepler and Scott 1981). Observers recorded the detection type (heard, seen, or both) and horizontal distance from the station’s center point to individual birds detected during an 8-min count. Birds only flying over or through the survey area were excluded. Both males and females singing and calling were recorded, although the sex of individuals was not noted. Most of the birds encountered were adults because counts were timed during the breeding season (see below) and most juveniles had not yet fledged. Observers also recorded cloud cover, rain, wind, gust, and time of day at each station. Sampling was typically conducted

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between dawn and 11:00 and halted when rain, wind, or gust exceeded prescribed levels (light rain and wind level 3 on the Beaufort scale). We included weather variables and time of day as covariates in the analyses to account for possible differences in the detection probabilities due to the conditions when the count was conducted, as explained later. The detectability of forest birds varies through the year because of changes in vocal activity associated with breeding (Best 1981) and because birds may move in or out of the study area in response to phenology of food resources (Simon et al. 2002). To minimize biases associated with differences in sampling periods, we restricted density estimates to the breeding season only. The original 1977 HFBS was conducted in July, a month when birds’ breeding has generally finished and many nectarivorous birds have dispersed in search of flowers (Scott et al. 1986; Ralph and Fancy 1994, 1995). Since the HFBS, from 1987 to 2007, annual surveys were conducted mainly during March and April to correspond with the breeding season of most species. Because of this disparity in the months sampled, and the great difference in number and location of stations sampled, we excluded the 1977 HFBS data from our trend analyses. Density estimation

Point-transect methods, a form of distance sampling, are used to correct abundance estimates for individuals that go undetected. This correction is accomplished by modeling a speciesspecific detection function and calculating a probability of detection, which is subsequently used to estimate bird density (Buckland et al. 2001). Robust estimates rely upon the critical assumptions that all birds are detected with certainty at the station’s center point, birds are detected prior to any responsive movement, and distances are measured without error. Buckland et al. (2001, 2004) described distance-sampling procedures and analyses in detail. We estimated density only for species for which we had detections sufficient to characterize detectability adequately (Appendix 1; Buckland et al. 2001:241). We did not estimate species-specific densities for strata in which