Bird Conservation International (2014) 24:252–261. © BirdLife International, 2013 doi:10.1017/S0959270913000373 Population status and trend of the Critically Endangered Montserrat Oriole STEFFEN OPPEL , ANDREW CASSINI , CALVIN FENTON , JAMES DALEY and GERARD GRAY Summary Many island endemics are of great conservation concern due to small range and population sizes. The Montserrat Oriole Icterus oberi is a forest passerine endemic to the Caribbean island of Montserrat, where recent volcanic activity has destroyed a large proportion of suitable forest habitat. From 1997 to 2000 the Montserrat Oriole population declined dramatically even in the remaining forest habitat, leading to its classification as ‘Critically Endangered’. We present trend estimates of the Montserrat Oriole population from 2000 to 2013, and estimate the world population size in 2012 based on repeat point counts and beta-binomial mixture models. Montserrat Orioles recovered between 2003 and 2005, and we found no evidence for a continuing population decline. However, there was large uncertainty around trend estimates, and the power to detect a shallow negative trend was very low. Based on the comparison of count data at 42 points, the Montserrat Oriole population in 2013 was less than half as large as in 1998. To improve future trend estimates a new monitoring design was introduced in 2011, and applied to all subpopulations of the species in 2012. The world population in 2012 was estimated to hold between 307 (95% credible interval 212–503) and 690 birds (476–1131) birds in the two forest fragments on Montserrat, depending on whether the sampling area around each point count was assumed to encompass 100 m or 150 m. Based on these estimates, the Montserrat Oriole currently does not meet the IUCN criteria for ‘Critically Endangered’, and we recommend a revision of the species’ conservation status. Introduction Single island endemic species are of great conservation concern due to their limited range and generally small populations that render them vulnerable to extinction by environmental stochasticity. The islands in the Caribbean are home to nine oriole species of the genus Icterus , of which four are endemic to single islands (Garrido et al. 2005 , Price and Hayes 2009 ). Two of the Caribbean oriole species are classified as ‘Critically Endangered’, the Bahama Oriole Icterus northropi (Price et al. 2011 ), and the Montserrat Oriole I. oberi (Arendt et al . 1999 , Hilton et al . 2003 ). The Montserrat Oriole is endemic to the volcanic island of Montserrat in the Lesser Antilles, and is considered Critically Endangered due to a rapid population decline between 1997 and 2000 (Arendt et al. 1999 , Bowden et al. 2001 , Hilton et al. 2003 ). This population decline was most likely due to an ongoing volcanic eruption, which started in July 1995 and continued into 2013. During the first two years of this volcanic activity almost 60% of the Montserrat Oriole’s original habitat in semi-deciduous and evergreen moist forests were destroyed (Arendt et al. 1999 , Hilton et al. 2005 ). Since 1997, the Montserrat Oriole has been restricted to two small forest remnants on Montserrat, the Centre Hills (11 km 2 ) and a much smaller remnant (2.7 km 2 ) in the South Soufrière Hills (Allcorn et al. 2012 ). Following the initial volcanic destruction of habitat in 1996 and 1997, the population in the Centre Hills forest declined further until 2003, presumably Status of the Montserrat Oriole 253 because of low adult survival and poor reproductive output that may have been caused by volcanic ash fall, drought, and nest predation (Hilton et al. 2003 , Marske et al. 2007 , Allcorn et al. 2012 , Oppel et al. 2013 ). However, an apparent recovery occurred between 2003 and 2005 (Dalsgaard et al. 2007 ), but no information is available on recent trends and the current size of the Montserrat Oriole population. In February 2010, the Soufriere Hills Volcano displayed the most vigorous activity since 1995, leading to massive ash fall on the island (Scientific Advisory Committee 2010 ). Subsequently, Montserrat Orioles were observed at lower elevations than ever before, and there was concern about the effects of this most recent episode of volcanic activity on the Montserrat Oriole population. In this study, we examine the population trend of Montserrat Orioles between 2000 and 2013 using a state-space model to account for random observation error. We further introduce a revised monitoring approach that is designed to estimate abundance while accounting for imperfect detection. Lastly, we use detailed observations of colour-ringed individuals to assess average movement distances of territorial birds, and use this information to extrapolate the size of the global population of Montserrat Orioles. Methods Study area Montserrat (16°45 ′ N, 62°12 ′ W; 104 km 2 ) lies at the northern end of the Lesser Antilles in the eastern Caribbean Sea. The Centre Hills are the largest forest area remaining on the island, a contiguous block of mostly secondary forest covering the steep-sided valleys of a dormant volcanic cone rising to a maximum elevation of 737 m above sea level (Hilton et al. 2003 , Allcorn et al. 2012 ). A smaller forest remnant exists in the South Soufriere Hills, to the south of the active volcano. This forest area lies in a volcanic exclusion zone where volcanic hazards prevent regular public access, but the forest is separated from the active volcano by a deep valley and thus not vulnerable to recurring pyroclastic flows. The natural vegetation in both forests is tropical moist broadleaf forest above 200–300 m, with small areas of elfin forest on exposed ridges at high elevations. Canopy heights of 15–25 m are typical, and the understorey is frequently dominated by Heliconia plants. Bird monitoring Montserrat Orioles were monitored using point counts between 2000 and 2013 by the Montserrat Department of Environment. Because bird detection is challenging in tropical forests, a distance sampling approach was employed in the surveys before 2012 to account for imperfect detection (Hilton et al. 2003 ). However, most forest birds including the Montserrat Oriole are only recorded acoustically, and the distance sampling approach may lead to highly biased population estimates (Hilton et al. 2003 , Alldredge et al. 2007a , 2007b ). Recent analytical developments allow the estimation of detection probability from repeated counts rather than from estimated distances between birds and observer (Royle and Nichols 2003 , Kéry et al. 2005 , Royle et al. 2005 , Kéry 2008 , Chandler and King 2011 ). Thus, a new survey protocol was established in 2011 to overcome the complications associated with estimating the distance to a vocalising bird in dense tropical forest habitat. From 2000 to 2013, bird monitoring was conducted at fixed census stations in the Centre Hills during the breeding season of Montserrat Orioles between late March and mid-July (Allcorn et al. 2012 ). A total of 90 census stations were originally established throughout the Centre Hills along accessible walking routes (Hilton et al. 2003 , Dalsgaard et al. 2007 ), however, some of the census stations were too close together to provide independent observations for Montserrat Orioles, and others were in unsuitable habitat. For the analysis of long-term trends, we chose a subset of 50 permanent census stations that were all at least 200 m apart, did not change between S. Oppel et al. 254 2000 and 2013, and at which Montserrat Orioles were observed at least once during the 13 years of monitoring. We counted birds during 10-min point counts after a 3-min settling down period upon arrival at a census station. All counts were conducted between 05h45 and 14h00 local time, and detailed descriptions of the bird surveys can be found elsewhere (Hilton et al. 2003 , Dalsgaard et al. 2007 ). From 2011 to 2013, we employed a new survey protocol during which each of 70 point count stations were surveyed 2–3 times within a 3-week period. These 70 stations were all > 200 m apart and were a combination of existing points from previous surveys and newly established points spread throughout the entire Centre Hills forest. The repeat surveys followed the same 10-min point count protocol described above, and were conducted at the beginning of the breeding season (last week of March and April) to ensure that the Montserrat Oriole population was demo- graphically closed during the survey period. In 2012, volcanic hazards were lower than in any of the previous years (Scientific Advisory Committee 2012 ), allowing us to access the South Soufriere Hills. We established 19 census stations that were > 200 m apart and conducted two repeat surveys at these stations in mid April 2012. Estimation of movement distances of territorial orioles In May 2011 and 2012 we individually colour-ringed adult territory-holding orioles and observed their movements during 15 days of intense observations. During these intense observations the focal individual was observed from a distance of 10–20 m, which was subjectively assessed to minimally influence the behaviour of the bird. The focal individual was then followed through the forest for as long as possible, and all movements were logged with a handheld GPS unit (Garmin eTrex HS). These observations yielded a minimum movement radius for territorial birds from a fixed focal point (i.e. the nest) in their territory during the breeding season. Estimation of population trend and population size Previous analyses of Montserrat Oriole population trends used raw count data (Hilton et al. 2003 , Dalsgaard et al. 2007 ) because several of the assumptions underlying the distance sampling approach may be violated during Montserrat Oriole surveys (Hilton et al. 2003 ). We followed these previous approaches and used the raw census data to estimate a population trend for count data from 2000 to 2013. In addition, we compared the raw count data at 42 points in the Centre Hills that were surveyed both in late March 1998 and in early April 2013 with a paired samples Mann-Whitney U -test to assess the change in population size between 1998 and 2013.