Geology topics

Richard J. Camp

Publications and source records attributed to Richard J. Camp.

At least 19 recordsLinked to original sources

USGS2026

Hakalau’s moving castle: How climate change and restoration are shifting an island fortress for forest birds

Hakalau Forest Unit of the Big Island National Wildlife Refuge Complex (hereafter, Hakalau) protects the largest area with the highest endemic forest bird diversity in Hawaiʻi, including four federally listed species. Hakalau’s higher elevation montane forest provides refuge from avian malaria ( Plasmodium relictum ), a primary driver of Hawaiian honeycreeper extinctions. However, recent declines in Hakalau’s birds at lower elevations could indicate that conditions have become suitable for disease vector Culex quinquefasciatus . We evaluated the statuses of Hakalau’s bird populations in the context of recent climatic changes using new survey data from point-transect distance sampling, producing abundance estimates from 1999 to 2024. We stratified our analysis across four elevation ranges (<1500 m, 1500–1700 m, 1700–1900 m, and >1900 m) and assessed trends for each species using state-space models (SSMs). We constrained population trajectories to be biologically realistic by incorporating population dynamic models within the SSMs. We observed highly species-specific abundance trends below 1500 m, predominantly stable to upward trends within 1500–1700 m, stable trends within 1700–1900 m, and upward trends above 1900 m. Declines in Hawaiʻi ʻamakihi ( Chlorodrepanis v. virens ) and endangered ʻakiapōlāʻau ( Hemignathus wilsoni ) abundance coincided with lengthening warm seasonal temperatures indicative of shrinking disease-free habitat below 1700 m. Above 1900 m, however, increases in nearly all species indicate that reforestation has likely restored disease-free habitat since 1999. While most species were stable to increasing overall, surveillance for mosquitoes and disease at lower elevations, documenting changes in habitat, and continuing bird population monitoring can help to gauge their long-term persistence at Hakalau.

Hawaii↗

USGS2026

Improved abundance trajectories with Bayesian population dynamics model: Case study with a Hawaiian honeycreeper

Many wildlife monitoring programmes collect annual data on population abundance. The resulting abundance estimates fluctuate over time partly because of true population change and partly because of observation error. These two components of variation can be separated by fitting the estimates to a population dynamics model within a Bayesian state-space modelling framework. By constraining the population trajectory to be biologically realistic, more precise estimates can be obtained. Independent biological knowledge can be incorporated through choice of model structure and by specifying informative prior distributions on demographic parameters. We illustrate the approach using a 31-year point transect study of the Hawai’i ’ākepa ( Loxops coccineus ). We fitted five models, each making different assumptions about how population change, recruitment and/or adult survival varied over time. Overall, the ’ākepa geometric mean growth rate was 1.02, indicating an increasing population over the 31-year time series, although there were periods of slow decline potentially associated with low recruitment and more rapid recovery associated with pulses of high recruitment. Abundance estimates derived from the population models were substantially more precise than the ‘raw’ point transect estimates: 95% credible interval (CrI) was on average 51.7% (s.d. = 14.1%) narrower.

Hawaii↗

USGS2026

Population densities and trends of landbirds in the National Park of American Samoa

The National Park of American Samoa (NPSA), which protects some of the most intact tropical rainforest in the South Pacific, is exposed to recurring, intense tropical cyclones, including Tropical Cyclone Gita in 2018. In combination with other anthropogenic pressures, cyclones represent a potential source of disturbance to landbird populations, particularly native frugivorous and seed-dispersing species. Long-term monitoring provides a means to evaluate population trajectories following disturbance events and to identify potential management gaps. Landbird surveys were conducted in the Tutuila and Taʻū units of NPSA in 2011, 2018, and 2023 using point-transect distance sampling, with vegetation composition and structure quantified at each sampling station. Species-specific detection functions were fitted to count data to estimate densities by unit, with variance estimated using bootstrap procedures. Comparisons of 2018 and 2023 density estimates using two-sample z-tests indicated that changes in most landbird populations were inconclusive. Since 2018, increases in density were observed for the Pacific Imperial Pigeon ( Ducula pacifica ) and Crimson-crowned Fruit Dove ( Ptilinopus porphyraceus ) on Tutuila, while decreases were observed for the Pacific Kingfisher ( Todiramphus sacer ) on Tutuila, and for the Polynesian Wattled Honeyeater ( Foulehaio carunculatus ), Polynesian Starling ( Aplonis tabuensis ), and Samoan Starling ( Aplonis atrifusca ) on Taʻū. The Many-colored Fruit Dove ( Ptilinopus perousii ) remained rare in both units, and the Manuʻa Shrikebill (Clytorhynchus vitiensis powelli) was narrowly distributed on Taʻū, with insufficient detections of either species to estimate density. The cryptic Spotless Crake ( Zapornia tabuensis ) was detected only in 2018. Changes in tree canopy cover and height did not exhibit patterns clearly associated with a major disturbance event and likely reflected differences in sampling frames among surveys. Invasive non-native plants, most notably Miconia crenata , were widespread in both units. Although many landbird populations in NPSA remain abundant, inference from current monitoring is constrained by limited information on vital rates and species-specific responses to invasive species, disease, habitat change, and extreme weather. As climate change is projected to increase tropical cyclone intensity in the South Pacific, sustained monitoring and targeted management will be important for understanding and conserving landbird populations in American Samoa.

National Park of American Samoa, Ta'u, Tutuila↗

USGS2025

Getting ahead of the crises: Developing an avian malaria disease management plan for Hawaiian forest birds

Avian malaria is an existential threat to a majority of native Hawaiian forest birds. Climate change is facilitating the spread of malaria to historically disease-free areas, and despite the risk of native Hawaiian forest bird extinctions from malaria outbreaks, no comprehensive disease management plans exist for forest bird conservation areas. Hakalau Forest National Wildlife Refuge, Hawai‘i, supports a thriving bird community in a historically disease-free area that is now vulnerable to malaria incursion. Drawing on the expertise of land managers and research scientists, we developed an approach that could be used to proactively address the risk of expanding malaria into the Refuge. The plan lays out a multi-level approach that includes options for monitoring and management actions depending on defined threat levels: Vigilant, High Alert, Disease Outbreak, and Crisis levels. Initial Vigilant and High Alert levels monitor bird populations, climate conditions, and mosquito occurrence for signs of possible disease outbreaks, with higher levels shifting toward more direct management responses. While specific actions will change as new tools become available, the proactive approach can help Refuge managers better respond to changing malaria levels in the future and provide a model for managing disease here in Hawai‘i and elsewhere.

Hawaii↗

USGS2025

The continued decline of the Palila (Loxioides bailleui) on Mauna Kea, Island of Hawaiʻi

Palila ( Loxioides bailleui ) are critically endangered Hawaiian honeycreepers specializing on māmane ( Sophora chrysophylla ) seeds and restricted to Mauna Kea volcano on the Island of Hawaiʻi. Recently, the population was estimated to decline by 89% between 1998 and 2021, despite decades of ungulate removal, fence construction, māmane regeneration, fire suppression, and predator control. To inform managers with the most recent update on the status and trends of the Palila population, we analyzed annual bird survey data collected using point-transect distance sampling since 1998, including new annual survey data from 2022, 2023, and 2024. Prior to analysis, we predicted the population trajectory would change between 2021 and 2024 because of continued management actions promoting habitat recovery. We used distance sampling, log-linear regression, and state-space modeling to produce the new estimates and analyze trends across the time series. The 2022 population estimate was 367 to 742 birds (95% confidence interval; point estimate: 545), the lowest in recorded history. The 2023 and 2024 estimates of 374 to 842 birds (point estimate: 596) and 412 to 970 birds (point estimate: 666) were the second and third lowest in our time series, respectively. Our estimates for years before 2022 show population fluctuations between 4000 to 6800 birds from 1998 to 2005, then a steep decline through 2010. For the next decade, abundance fluctuated around 1000 birds, before declining again in 2021 to less than 700 birds. From 1998 to 2024, the population declined by more than 90%, or 205 birds per year, with 100% statistical support for an overall downward trend, despite significant management efforts and research. The greatest threats facing the Palila, if familiar, are not being eliminated swiftly enough to promote their recovery. The currently small and range-limited population is vulnerable to future climate-related events such as drought and fire. Continued monitoring can help to assess the response of Palila to adaptive management actions and changing environmental conditions.

Hawaii↗

USGS2025

Quantifying landscape-level biodiversity change in an island ecosystem: A 50-year assessment of shifts in the Hawaiian avian community

Hawaii has experienced profound declines in native avifauna alongside the introduction of numerous bird species. While site-specific population studies are common, landscape-level analyses of avian population dynamics are rare, particularly in island ecosystems. To address this gap, we used a density surface model to create a spatio-temporal projection of population densities and distributions across the Island of Hawai‘i, spanning nearly five decades (1976–2023). We incorporated environmental covariates of habitat, precipitation, and elevation, to further refine our projections. Our analysis encompassed nine native and six non-native bird species, inhabiting a range of ecological niches. We found five out of nine native species have declined in density and range size while four were stable. For non-native species, two were stable, one was decreasing, and three were increasing in density and range size. Our landscape projections can inform management by suggesting areas critical for habitat preservation and land acquisition for conservation, identifying where range fragmentation is occurring, and pinpointing locations of multi-species declines that are likely driven by a common cause. Our study demonstrates how long-term, landscape-level monitoring and analyses can advance understanding and addressing biodiversity loss, particularly in vulnerable tropical island ecosystems.

Hawaii↗

USGS2025

Disease-driven collapse of the native Kauaʻi avifauna and the rise of introduced bird species

Hawaii hosts one of Earth’s most unique and threatened avifaunas. Upslope migration of mosquito-vectored avian malaria on Kauaʻi (maximum elevation 1,598 m) has likely caused its rapid loss of avifaunal diversity; only 8 of 13 historic forest bird species remain. We update the status and trends of Kauaʻi forest bird populations since the original (1981) surveys using the latest (2023) survey data and distance sampling. We fit detection functions to species-specific count data and stratified estimates across the Interior (since 1981) and Exterior (since 2000) survey areas, and between low (900–1,100 m), medium (1,100–1,300 m) and high (> 1,300 m) elevation bands (since 2000). Log-linear trends of ʻakekeʻe ( Loxops caeruleirostris ), ʻanianiau ( Magumma parva ), ʻiʻiwi ( Drepanis coccinea ), and Kauaʻi ʻamakihi ( Chlorodrepanis stejnegeri ) steeply declined across the timeseries, with extinction of ʻakekeʻe and ʻiʻiwi expected before 2050. Undetected in 2023, ʻakikiki ( Oreomystis bairdi ) were excluded from analysis. ʻApapane ( Himatione sanguinea ), Kauaʻi ʻelepaio ( Chasiempis sclateri ), Chinese hwamei ( Garrulax canorus ), and white-rumped shama ( Copsychus malabaricus ) were stable overall. Northern cardinal ( Cardinalis cardinalis ) steadily declined, whereas Japanese bush warbler ( Horornis diphone ) and warbling white-eye ( Zosterops japonicus ) exponentially increased. Taxonomic and functional diversity did not vary greatly across our timeseries, while the proportion of introduced species in the Exterior increased from 34 to 59%. However, introduced species do not replace the losses of ecological functions from native species, whose populations are likely declining from avian malaria. Future monitoring can be used to evaluate forest bird population responses to mosquito suppression using the Incompatible Insect Technique.

Hawaii↗

USGS2025

Ten more years of the golden pheasant (Chrysolophus pictus) on Maui, Hawaiian Islands

Since the introduction of the Golden Pheasant ( Chrysolophus pictus ) to Haleakalā volcano, Maui, three decades ago, subsequent reports have hinted at an expansion of this nascent population. We draw from a variety of data sources to learn about this pheasant's present status on Maui. First, forest bird surveys conducted every five years revealed that the frequency of Golden Pheasant detections has greatly increased, and the bird has both maintained its former distribution and expanded eastward into Haleakalā National Park (NP). Second, reports to eBird from The Nature Conservancy's Waikamoi Preserve, where Golden Pheasants first appeared on Maui, demonstrate that the frequency of observations has increased and is strongly seasonal, predominantly in the spring. Third, autonomous recording units monitoring endangered forest birds recorded pheasants too, adding new locations. Finally, trail cameras set to monitor mammals picked up pheasants as well, showing males of two color morphs: original “wild-type” and “dark-throated.” Trail cameras also documented a small juvenile at Waikamoi Preserve and both females and males in Haleakalā NP. By “connecting the dots” of mapped occurrences, we traced the pheasant's progression through a narrow band of subalpine cloud forest with open understory, extending from Waikamoi Preserve eastward to upper Kīpahulu Valley, a distance of 14 km. In summary, this body of evidence supports the claim that the Golden Pheasant has established a self-sustaining population on Maui, and we propose that the species' success there may be attributed to the minimal influence of predators and the absence of competing gallinaceous birds in its preferred habitat.

Hawaii↗

USGS2025

Automated methods for processing camera trap video data for distance sampling

Context Population monitoring is an essential need for tracking biodiversity and judging efficacy of conservation management actions, both globally and in the Pacific. However, population monitoring efforts are often temporally inconsistent and limited to small scales. Motion-activated cameras (‘camera traps’) offer a way to cost-effectively monitor populations, but they also generate large amounts of data that are time intensive to process. Aims To develop an automated pipeline for processing videos of ungulates (Philippine deer, Rusa marianna ; and pigs, Sus scrofa ) on Andersen Air Force Base in Guam. Methods We processed camera videos with a machine learning model for object detection and classification. To estimate density using distance sampling methods, we used a separate machine learning model to estimate the distance of target animals from the camera. We compared density estimates generated using manual versus automated methods and assessed accuracy and processing time saved. Key results The object detection and classification model achieved an overall accuracy >80% and F1 score ≥0.9 and saved 36.9 h of processing time. The automated distance estimation was fairly accurate, with a 1.1 m (±1.4 m) difference from manual distance estimates, and saved 16.8 h of processing time. Density estimates did not differ substantially between manual and automated distance estimation. Conclusions Machine learning models accurately processed camera videos, allowing efficient estimates of density from camera data. Implications Further adoption of motion-activated cameras coupled with automated processing could lead to continuous, large-scale monitoring of populations, helping to understand and address changes in biodiversity.

Guam↗

USGS2025

Status and trends of forest bird populations at Hakalau Forest National Wildlife Refuge, 1987–2024

Since 1985, the Hakalau Forest Unit of the Big Island National Wildlife Refuge Complex (hereafter, Hakalau) has protected the largest endemic forest bird diversity in the State of Hawaii. This includes three endangered and one threatened species and their habitats. Hakalau’s vast area (155 km2), mostly high elevation (>1500 m) montane forest, provides refuge from avian malaria (Plasmodium relictum) vectored by introduced southern house mosquitoes (Culex quinquefasciatus). However, increases in the seasonal temperatures optimal for disease carrying mosquitoes associated with climate change have coincided with recent downward trends of native species in the closed forest area of the refuge (1450–1750 m), where disease is most likely to occur. Therefore, to inform refuge management with the most updated information on these populations and their trends, we analyzed forest bird survey data collected using point transect distance sampling with new survey data from 2021–2024. We stratified our analysis across management units, including the open forest and pasture since 1987, and the closed forest since 1999, and across four elevation ranges (<1500, 1500–1700, 1700–1900, and >1900 m) since 1999. We used distance sampling to estimate species- and strata-specific abundances and applied log-linear regression to detect trends across the timeseries. We found a continuation of previous trends, wherein most native forest birds declined in the closed forest strata (mostly below 1700 m) and increased in the highest elevation and pasture strata. Patterns were highly species-specific for the three lower elevations and open forest strata. ‘Apapane (Himatione sanguinea) and warbling white-eye (Zosterops japonicus), the most abundant native and introduced bird species, respectively, increased in nearly all strata. ‘I‘iwi (Drepanis coccinea) and Hawai‘i ‘amakihi (Chlorodrepanis virens virens), also common, decreased in closed forest, were stable in open forest, and increased in pasture. Both species were generally stable or increasing across all elevation bands, except Hawaiʻi ʻamakihi decreased in the below 1500 m and the 1500–1700 m elevation bands. All three of the federally endangered forest bird species declined in closed forest. Hawai‘i ‘ākepa (Loxops coccineus) also declined in the open forest and 1700–1900 m band and was the only species to decline overall. ‘Akiapōlā‘au (Hemignathus wilsoni) declined in closed forest but increased overall. ‘Alawī (Loxops mana, also known as Hawai‘i creeper) also decreased in the closed forest, but was stable to increasing in most strata and remained stable overall. Hawaiʻi ʻelepaio (Chasiempis sandwichensis) declined in closed forest, was stable in open forest and most elevation strata, and increased in pasture. ʻŌmaʻo (Myadestes obscurus) was stable in open and closed forest and at middle elevations and increased in the pasture. Introduced red-billedleiothrix (Leiothrix lutea) declined in closed forest, was stable in the open forest and at most elevations, and increased in the pasture. Coinciding with these changes, seasonal conditions for vector occurrence have continued to lengthen in the lower elevation strata through 2024, while Hakalau’s outplanting efforts continued to increase forest cover in the pasture, suggesting that disease-free habitat may have decreased in the closed forest and increased in the pasture. The declines in the closed forest and mixture of trends in the open forest and middle elevations bands suggest emigration into the higher elevation strata from lower elevation strata, and that possible threats have suppressed forest birds even at elevations >1500 m.

Hawaii↗

USGS2025

Fine-grained temporal population monitoring of a declining, critically endangered Hawaiian honeycreeper

Annual point counts are commonly used to monitor birds to track population densities across space and time. Palila ( Loxioides bailleui ) are surveyed annually in the first quarter, but we recently instituted quarterly sampling that offers a unique opportunity to improve estimator precision. We conducted point-transect distance sampling point counts during the first quarter of 2020 through 2024, and the second through fourth quarters in 2022 and 2023, and the second quarter in 2024. The reduced sampling intensity during the quarterly counts, however, requires model-based methods to estimate abundance to the entire sampling frame. We modeled spatial and temporal correlation using a soap film smoother within a generalized additive modeling framework, a density surface model, fitted to palila counts each quarter for the five-year timeseries to track changes in population abundances. Our results indicate that palila maintained a high-density hotspot throughout the five-year timeseries; however, the extent of the hotspot declined substantially over the timeseries while densities within the hotspot declined from about 3 birds/ha in 2020 to about 1 bird/ha in 2024, which resulted in a 66% decline in palila abundances over 5 years. Density surface model estimates give on average a confidence interval width that was 74.7% shorter than the associated distance sampling confidence interval widths. Our results indicate that palila may benefit most if management actions were applied within the remaining hotspot. Additionally, this temporally fine-grained sampling provides information on seasonal movement patterns and resource tracking, and population response to management and conservation actions. Our spatially explicit, model-based approach is applicable to a wide range of monitoring programs, particularly those with inconsistent, opportunistic spatial coverage.

Hawaii↗

USGS2025

Pacific island landbird monitoring report, Kalaupapa National Historical Park, 2021

In 2021, landbird surveys were conducted at Kalaupapa National Historical Park on the island of Molokaʻi to assess changes in species composition, distribution, and population densities since 2005. Point-transect distance sampling surveys were conducted on six transects at 50 landbird monitoring stations within an 1,834-hectare area. A total of nine landbird species were detected, with the ʻApapane ( Himatione sanguinea ) the only endemic Hawaiian species detected. Sufficient detections of six species allowed for population density and abundance estimates, which were compared to 2005 estimates using a two-sample z-test. The abundance of ʻApapane declined by 57% to 2,476 ± 729 (SE) birds in 2021 compared to 2005. Population densities of the House Finch ( Haemorhous mexicanus ), Japanese Bush Warbler ( Horornis diphone ), Warbling White-eye ( Zosterops japonicus ), and White-rumped Shama ( Copsychus malabaricus ) were also lower in 2021 than in 2005, but comparisons were inconclusive. Since 2005, the Red-billed Leiothrix ( Leiothrix lutea ) irrupted within the survey area to an estimated abundance of 11,088 ± 1,208 birds. The Warbling White-eye was the most abundant species, with an estimated 101,724 ± 11,692 birds. Surveyors failed to detect the Hawaiʻi ʻAmakihi ( Chlorodrepanis virens ), which has become increasingly rare on Molokaʻi. The federally threatened ʻIʻiwi ( Drepanis coccinea ) was last seen on Molokaʻi in 2010, further raising concerns about its potential extirpation. The Olomaʻo ( Myadestes lanaiensis ), an endemic thrush, has not been detected since 1980 and is likely extinct. These findings show the ongoing shift toward non-native bird communities and highlight the challenges of conserving native forest birds amidst widespread avian malaria transmission, introduced predators, and extensive habitat degradation.

Hawaii↗

USGS2025

Using distance sampling with camera traps to estimate densities of ungulates on tropical oceanic islands

Reliable population estimates are one of the most elementary needs for the management of wildlife, particularly for introduced ungulates on oceanic islands. We aimed to produce accurate and precise density estimates of Philippine deer ( Rusa marianna ) and wild pigs ( Sus scrofa ) on Guam using motion-triggered cameras combined with distance sampling to estimate densities from observations of unmarked animals while accounting for imperfect detection. We used an automated digital data processing pipeline for species recognition and to estimate the distance to detected species. Our density estimates were slightly lower than published estimates, consistent with management to reduce populations. We estimated the number of camera traps needed to obtain a 0.1 coefficient of variation was substantial, requiring > ten-fold increase in camera traps, while estimates with precision of 0.2 or 0.3 were more achievable, requiring doubling to quadrupling the number of camera traps. We provide best practices for establishing and conducting distance sampling with camera trap surveys for density estimation based on lessons learned during this study. Future studies should consider distance sampling with camera traps to efficiently survey and monitor unmarked animals, particularly medium-sized ungulates, in tropical, oceanic island ecosystems.

Journal of Tropical Ecology↗

USGS2025

Population trends of native and non-native forest birds on the island of Molokai, Hawaii

Hawaiʻi’s avifauna has undergone profound changes over the past 1,000 years, with many endemic bird species driven to extinction and non-native introductions reshaping forest ecosystems. On the island of Molokaʻi, habitat degradation and the introduction of mosquito-borne disease, mainly avian malaria ( Plasmodium relictum ), have caused extinctions and severe declines in native forest bird populations. To assess these changes, point-transect distance sampling surveys were initiated in 1979 and 1980 and repeated six times, most recently in 2021, covering a 3,527 ha area. This study analyzes species composition, population densities, abundances, and long-term trends for species with sufficient detections using log-linear regression. Since 1979–1980, a total of 17 non-native and 3 native species were detected across all six surveys. The non-native Warbling White-eye ( Zosterops japonicus ) had the highest density and abundance in every survey, while abundance of Red-billed Leiothrix ( Leiothrix lutea ) significantly trended upward. The Japanese Bush Warbler ( Horornis diphone ) irrupted in the survey area, increasing in abundance by more than 99% since 1979–1980. Among native species, ʻApapane ( Himatione sanguinea ) populations increased by 33%, while Hawaiʻi ʻAmakihi ( Chlorodrepanis virens ) declined by 78% from their peak in 1995. Federally threatened ʻIʻiwi ( Drepanis coccinea ), highly susceptible to avian malaria, have not been recorded on Molokaʻi since 2010, and the endemic Olomaʻo ( Myadestes lanaiensis ) has not been detected since 1980 and is possibly extinct. Our results underscore the ongoing conservation crisis for Molokaʻi’s forest birds and the continued vulnerability of native birds to disease and habitat loss.

Hawaii↗

USGS2025

Forest bird population status on Saipan, a small oceanic island

Tropical oceanic islands are critical biodiversity hotspots where population monitoring can help to determine the status and trends of rare and endangered species. Saipan is the second largest island in the Mariana Islands and contains many endemic and range-restricted bird species. Surveys of forest birds were conducted on Saipan using point-transect distance sampling, starting in 1982. To determine population status for Saipan avifauna, we generated abundance estimates based on surveys conducted in 1982, 1997, 2007, and 2018. We also assessed community composition by exploring if there were changes in assemblage diversity, total forest bird abundance, and ratio of native to non-native populations over time. For the 2018 survey, there were 5791 detections of 18 species during 242 counts, with six of 18 species detected at > 50 % of the counts and with relatively high abundance, while 12 species were detected at < 50 % of the counts and were uncommon or rare. The most abundant species was Bridled White eye (nosa', Zosterops conspicillatus saypani ) at > 40 birds/ha and the least common species was the Saipan Reed Warbler (gå'ga' karisu, Acrocephalus hiwae) at < 1 bird/ha. We found that the community composition is becoming slightly more even over time and the overall community abundance is stable (around 375,000 birds), but non-native species have become more common while native species have remained stable. Results from this study can be used to inform conservation management of forest birds in Saipan and contribute to a broader understanding of forest bird status in the Marianas.

Saipan↗

USGS2025

Breaking down Palila decline: Assessing the role of drought and vegetation health in the population loss of an endangered Hawaiian honeycreeper

The Palila ( Loxioides bailleui ), the last member of the once speciose finch-billed Hawaiian honeycreeper clade (Drepanidinae) in the main Hawaiian Islands, faces critical conservation challenges as an endangered species. Understanding the drivers of its decline is essential for effective management. We used additive decomposition models to examine temporal trends in climatic variables (temperature, precipitation, drought) and Normalized Difference Vegetation Index (NDVI), a vegetation health metric hypothesized to be associated with long-term trends in Palila abundance at landscape (250 m) scales on the Island of Hawai'i. A breakpoint analysis identified 2005–2009 as critical years of Palila decline. Vegetation health metrics at the 250 m scale lined up well both spatially and temporally with trends in Palila declines, with a significant browning from January 2004 to January 2014. Given the strong correlation between vegetation health and drought metrics at the landscape scale (r = 0.75, p < 0.001), NDVI changes appeared driven by drought. To enable the future projection of habitat quality in this area, we explored a stepwise linear regression to explain the variation in MODIS NDVI in recent years. We found that 87 % of the variability in NDVI can be explained by wet season precipitation and vapor pressure deficit from the previous dry season. The model is largely driven by a strong positive correlation between wet season precipitation and NDVI (r = 0.72, adjusted p < 0.001). Areas that maintained a low likelihood of NDVI decline throughout the time series and experienced increases in predicted Palila count represent potential drought microrefugia for the species. This higher elevation microrefugia is likely resilient against decreases in wet season precipitation through supplemental water retention from fog drip. While NDVI rebounded after 2014, Palila have not recovered. Our analysis highlights the importance of trend decomposition for monitoring endangered species with limited rebound potential due to small population dynamics and indicate continued warm, dry conditions may prevent Palila recovery without intervention.

Hawaii↗

USGS2025

2022–2024 Status and trends of the Palila (Loxioides bailleui)

Palila (Loxioides bailleui) are critically endangered Hawaiian honeycreepers specializing on the seedpods of māmane (Sophora chrysophylla) and restricted to Mauna Kea volcano on the Island of Hawaiʻi. A previous analysis of survey data estimated an 89% population decline between 1998 and 2021. Using the most recent annual survey data from 2022, 2023, and 2024, we report updated annual population estimates and trends since 1998. The 2022 population estimate was 367–742 birds (point estimate: 545); the 2023 population estimate was 374–842 birds (point estimate: 596); and the 2024 population estimate was 412–970 birds (point estimate: 666). Our estimates for survey years prior to 2022 were within the confidence intervals of the estimates from the previous analysis. Our models likewise showed a population fluctuating between 4,000 and 6,800 birds from 1998 to 2005 (except for an unusually low estimate in 2000), and then a steep decline through 2010. For the next decade, palila abundance fluctuated between 776 and 1,346 birds, before declining again in 2021 to 679 birds. From 1998 to 2024, the population declined by >90% or 203 birds/year, with very strong statistical evidence of an overall downward trend.

Hawaii↗

USGS2025

A monitoring framework to assess forest bird population response to landscape scale mosquito suppression using the Incompatible Insect Technique

The Birds, Not Mosquitoes Monitoring and Support Science Working Group detailed methods for monitoring the population response of Hawaiian forest birds during implementation of the Incompatible Insect Technique (IIT) on the islands of Maui and Kauaʻi. The group prioritized methods for measuring the influence of mosquito suppression on populations within IIT treatment and control areas and identified focal species for IIT efficacy monitoring in birds. Three primary metrics were established to assess the impact of IIT on vulnerable species: population demography, density, and geographic range. Each metric can be evaluated using multiple methods. This report reviews those methods, with emphasis on approaches supported by pre-IIT baseline data and compatible with a before-after control-impact (BACI) study design for evaluating population responses over time. Focal avian species were selected based on population size estimates, fecundity, and disease susceptibility. We identified ʻākohekohe (Palmeria dolei), ʻiʻiwi (Drepanis coccinea), Maui ʻalauahio (Paroreomyza montana), Hawaiʻi ʻamakihi (Chlorodrepanis virens), Kauaʻi ʻamakihi (Chlorodrepanis stejnegeri), Kauaʻi ʻelepaio (Chasiempis sclateri), and ʻanianiau (Magumma parva) as focal species for monitoring population level response to disease suppression. Populations of kiwikiu (Pseudonestor xanthophrys), ʻakikiki (Oreomystis bairdi), akekeʻe (Loxops caeruleirostris), and the ʻiʻiwi population on Kauaʻi may be too small (e.g., <100 individuals) to effectively monitor, and it is unlikely that sufficient data can be collected from these birds to show IIT efficacy in a relatively short time frame (i.e., 5–10 years). Despite the logistical challenges to IIT implementation, there is potential to maintain disease-free status in individual populations of birds. Indeed, the continued existence of these critically endangered species in the wild within or near IIT treatment areas could be considered an accomplishment of IIT, given the current predictions for their extinction in the wild within 5–10 years. Demographic monitoring methods, including territory mapping, nest monitoring, mist-netting, and mark-recapture studies, provide direct evidence of survivorship and reproductive output. When combined with disease surveillance, these approaches could provide the most robust evidence of increased survivorship and productivity resulting from avian malaria suppression via IIT. However, demographic studies require several years of monitoring to achieve statistically robust BACI comparisons of survivorship and are more difficult to implement relative to other approaches. Given that these field efforts are labor-intensive and heavily reliant on personnel availability and funding, demographic monitoring could be conducted when adequate resources permit. On both Maui and Kauaʻi, passive acoustic monitoring (PAM) was identified as a priority method for monitoring the range, occupancy, and relative abundance of focal species. Autonomous recording units (ARUs) can record bird vocalizations in remote areas for several months. Innovative machine learning techniques permit rapid and semi-autonomous identification of most endemic honeycreepers on each island, maximizing sampling efficiencies and minimizing data processing costs. We predict mosquito suppression could support expansion of focal species into areas where disease transmission is currently excluding these species and expect acoustic monitoring data of focal species to reflect these spatial patterns. Additionally, the relative occupancy and call densities can be monitored temporally and spatially to assess the efficacy of IIT for supporting positive growth in vulnerable bird species. It is not yet clear if PAM is more effective than other methods, such as distance sampling, for detecting trends in the densities of rare species. However, the increased detections resulting from the larger sample size per observation point using ARUs will likely improve accuracy in detecting changes in species’ ranges. Collection of during and after treatment data within the BACI design could help to provide critical information to track avian population response, recovery, and potential range expansion related to IIT efforts. Point-transect distance sampling (point-counts) was prioritized as a method for monitoring population densities of focal species. Extensive historical sampling across focal species’ ranges provides a robust baseline for detecting change. These counts provide updated population densities and can be used to assess the distribution of focal species within IIT treatment areas. However, detecting subtle population changes with traditional distance sampling requires intensive spatial and temporal effort and may be less effective for rare species. To improve resolution, density surface modeling can integrate multiple data sources (e.g., point-counts, PAM, spot-mapping, and resightings) to estimate species-specific densities at finer spatial scales, including within and outside IIT treatment areas. This integrated modeling approach allows for detailed comparisons and may reveal early signs of recovery, including recolonization of formerly occupied sites. A coordinated monitoring strategy can allow managers to evaluate the success of mosquito suppression as a conservation intervention and support adaptive management in the face of emerging challenges.

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