Data Zone
Justification of Red List category
This species has a very restricted range on the islands of Nihoa and Laysan, where habitat degradation is ongoing. These populations are at risk of extinction through exposure to stochastic factors such as severe climatic events or the introduction of invasive species. It is therefore assessed as Endangered.
Population justification
The naturally occurring population on Nihoa was estimated at 489 ± 70 (SD) in 2010, 699 ± 78 in 2011, 610 ± 210 in 2012, 468 ± 166 in 2013, and 893 ± 303 in 2014 (Gorresen et al. 2016). The most recent estimate of the population on Laysan was 164 in September 2014 (Dalton et al. 2014, Freifeld et al. 2016). Given that the Nihoa subpopulation is thought to fluctuate widely, the population is tentatively placed in the band 250-999 mature individuals. This estimate equates to 375-1,499 individuals in total, rounded here to 350-1,500 individuals.
Trend justification
Although there is a lack of precision over the monitoring methods, the existing data suggest that Millerbird numbers on Nihoa have experienced pronounced fluctuations and have likely ranged between fewer than 50 and more than 800 individuals (VanderWerf et al. 2011). Although it is still thought to undergo substantial fluctuations, the species may not currently be as at risk as in the late 1980s (Gorresen et al. 2016). Numbers on Laysan have greatly increased since translocations in 2011 and 2012 (C. Farmer in litt. 2022). In 2016, population trends were considered inconclusive on Nihoa due to high within-year variance (Gorresen et al. 2016). Although an updated population estimate is not currently available (C. Farmer in litt. 2022), raw detections per effort on Nihoa in 2010-2021 are not indicative of a decline overall (Plentovich et al. 2021). However, detections were reportedly very low in 2022 (R. Rounds in litt. 2022). This may be due to several different reasons including the time of year, however severe drought driven by climate change may be a contributing factor (R. Rounds in litt. 2022, S. Plentovich in litt. 2022). It is currently unclear whether this is a natural fluctuation or if dry conditions from 2022 are indicative of longer-term trends (J. Vetter in litt. 2022), and as such, the population trend is precautionarily considered to be unknown overall.
Acrocephalus familiaris is endemic to the islands of Nihoa and Laysan in the north-western Hawaiian Islands, U.S.A. After becoming extinct on Laysan between 1916 and 1923, translocations were undertaken in September 2011 and August 2012, when a total of 50 Millerbirds were moved from Nihoa to Laysan, a distance of 1,037 km (Freifeld et al. 2016).
It is found in dense cover near the ground, particularly around the shrubs Sida fallax and Solanum nelsonii on Nihoa (VanderWerf et al. 2011, MacDonald 2012), and Scaevola taccada and the grass Eragrostis variabilis on Laysan (Kohley and Rutt 2012, Wilcox et al. 2013, Dalton et al. 2014). The Nihoa Millerbird is relatively evenly distributed across Nihoa, but is slightly more prevalent on upslope (north-ward) (Gorresen et al. 2016). Surveys also indicate a near complete use of available habitat during 2010 to 2014 (Gorresen et al. 2016). On Nihoa the main foods include small beetles, spiders, roaches and larvae (Morin et al. 1997, MacDonald 2012). The original, naturally occurring Laysan population was thought to have fed primarily on moths (Henshaw 1902), however, the species is thought to be a catholic feeder, feeding on the most readily available arthropod prey items (MacDonald 2012). Pairs show year-to-year fidelity in specific territories, with nesting apparently correlated with precipitation and most breeding taking place in the winter months (peaking January-March), although the breeding period may be extended in years of high summer rainfall (MacDonald 2012). The translocated population on Laysan breeds from at least February through September, with a peak in spring-summer months (Wilcox et al. 2013). Nests are located in dense shrubs or bunchgrass, and two eggs are generally laid (Morin et al. 1997, Wilcox et al. 2013).
The species exhibits extensive and strikingly low levels of genetic diversity as a result of recent severe bottlenecks (Addison and Diamond 2011). These may have been caused by climatic events, anthropogenic influences (impacts of Nihoa's prehistoric human population, and perhaps of modern introduction of nonnative plants and insects) or other stochastic fluctuations in habitat and consequently in population size. The Millerbird's extinction on Laysan in the early 20th century was ultimately caused by the introduction of rabbits and livestock, which denuded the island of vegetation (severe declines in the species' invertebrate prey and suitable habitat). On Nihoa, the population size is probably regulated primarily by precipitation levels, which affect the abundance of arthropod prey (extended droughts for example, are likely to have a negative impact). Hurricanes may be an increasing threat to this species as a result of climate change (USFWS 2017). Severe weather events may cause direct mortality of Millerbirds; a single severe storm could extinguish an entire subpopulation (USFWS 2010). Climate models indicate that hurricanes in the northwestern Pacific are expected to increase in intensity (5.4%), frequency (2.8%) and duration (1.4%) by 2100 (Emanuel et al. 2008). The Laysan population is likely to become more vulnerable than the Nihoa population based on dynamic wave-driven inundation projections (Gorresen et al. 2016). A recent modelling study of passive sea-level rise (SLR) showed that approximately 4 percent of the total land areas in the Northwestern Hawaiian Islands will be lost with +1.0 m of SLR and 26 percent will be lost with +2.0 m SLR (Reynolds et al. 2012). For Laysan Island this was predicted to be 7 percent at 1.0 m SLR and 24 percent at +2.0 SLR (Reynolds et al. 2012). However, thus far the subpopulations have survived severe weather without apparent long-term effects (C. Farmer in litt. 2022). Drought is likely to be exacerbated by climate change and this may be causing declines (R. Rounds in litt 2022). Since the species has an extremely small range and severely low levels of genetic diversity, it is also particularly vulnerable to extinction through exposure to disease and the introduction of invasive species. Population outbreaks of the non-native grasshoppers (e.g. the Gray Bird Grasshopper Schistocerca nitens) have periodically denuded 90% of the island's vegetated area, especially the shrubs in which Millerbirds nest. Outbreaks such as these may have been responsible for reductions in the bird population to well below 200 individuals in 1994, 1996, and 2005 (Latchininsky 2008). Habitat degradation is also caused by invasive plant species such as sandbur Cenchrus echinatus. Fire is a past and potential threat (Morin et al. 1997) and introduction of detrimental non-native species, especially predators such as rats (Rattus spp.) is a permanent possibility. Yellow crazy ants Anoplolepis gracilipes may pose an additional threat given that the species are present at the Hawaii base yard where packing for trips to the Northwestern Hawaiian Islands occurs (USFWS 2017). Nihoa and Laysan Finches (Telespiza ultima and T. cantans, respectively) may prey upon the eggs of Millerbirds, but since these species evolved together, it is unlikely to present a significant threat (M.A. MacDonald in litt. 2008). Avian diseases (West Nile virus and avian influenza) also pose a risk if introduced to the islands (USFWS 2017).
Conservation and Research Actions Underway
Nihoa is part of the Hawaiian Islands National Wildlife Refuge and Papahānaumokuākea Marine National Monument (H. Freifeld in litt. 2010). Legal access is controlled by a permit system that restricts access to biologists, other researchers, and native Hawaiian cultural practitioners. Strict biosecurity protocols are followed to ensure that legal visitors do not accidentally introduce new species via seeds, eggs or arthropods travelling on clothes and equipment. Visiting scientists make efforts to control alien plants by hand pulling and other methods (e.g. Plentovich et al. 2014). A scoping document for translocations was produced in 2007, identifying Laysan, Kure and Lisianski as the most suitable islands on the basis of island size, elevation and a lack of predators (Morin and Conant 2007). Development of detailed translocation, release, and monitoring methods were finalized in 2011 (Farmer et al. 2011) and two successful translocations to Laysan in 2011 and 2012 resulted in a nascent population there, last estimated at 164 birds (Freifeld et al. 2016). The population will continue to be monitored and the data used to determine population persistence and growth. New, more rigorous population monitoring methods were introduced on Nihoa in recent years (Gorresen et al. 2012, Gorresen et al. 2016); these are expected to yield estimates of total numbers with improved confidence and, if monitoring is done annually, early detection of significant trends.
13 cm. Small, nondescript, thin-billed warbler. Brown above, darkest on crown, white below. Voice Simple song of rapid, metallic notes.
Text account compilers
Vine, J.
Contributors
Conant, S., Freifeld, H., MacDonald, M.A., Morin, M., Farmer, C., Plentovich, S., Rounds, R. & Vetter, J.
Recommended citation
BirdLife International (2024) Species factsheet: Millerbird Acrocephalus familiaris. Downloaded from
https://datazone.birdlife.org/species/factsheet/millerbird-acrocephalus-familiaris on 22/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 22/12/2024.