Introduction
The island of Aguiguan, also known as Goat Island or Aguijan, is part of the Mariana archipelago, a chain of 15 islands located in the western Pacific (Figure 1). The archipelago was designated as an Endemic Bird Area by BirdLife International (Stattersfield et al. Reference Stattersfield, Crosby, Long and Wege1998) and is home to 31 resident land and wetland birds, 13 resident seabirds, and more than 100 migrant or vagrant birds (Reichel and Glass Reference Reichel and Glass1991, Wiles Reference Wiles2005). Of the 31 land and wetland birds, 10 are endemic to the Mariana archipelago, including one endemic genus, Cleptornis, which is represented by one species, the Golden White-eye Cleptornis marchei (Jenkins Reference Jenkins1983, Engbring et al. Reference Engbring, Ramsey and Wildman1986).
Figure 1. Island of Aguiguan showing the survey transects. Transects 1-4 were counted during 1982, 1995, 2000, 2002, and 2008 surveys, whereas transect 5 was established and counted during the 2008 survey. Map includes simulated shadows to highlight topography.
Currently four endemic species are found on Aguiguan, the Golden White-eye, Bridled White-eye Zosterops conspicillatus, Mariana Fruit-dove Ptilinopus roseicapilla and Mariana Swiftlet Collocalia bartschi. The Nightingale Reed-warbler Acrocephalus luscinius, which is listed by the IUCN as ‘Critically Endangered’ (BirdLife International 2014), once occurred on the island but is believed to be extirpated (Esselstyn et al. Reference Esselstyn, Cruz, Williams and Hawley2003, Marshall et al. Reference Marshall, Amidon, Radley, Martin and Camp2008). The Golden White-eye is listed as ‘Critically Endangered’ by IUCN (BirdLife International 2014) and currently only occurs on Aguiguan and Saipan, but is known from the recent fossil record to have formerly occurred on Tinian and Rota (Figure 1; Steadman Reference Steadman1999). The population on Saipan has experienced a significant population decline since 1982, presumably due to habitat loss (Camp et al. Reference Camp, Pratt, Marshall, Amidon and Williams2009). The Bridled White-eye, Mariana Fruit-Dove, and Mariana Swiftlet are all listed as ‘Endangered’ by IUCN (BirdLife International 2014). The Bridled White-eye is also found on Tinian and Saipan; a subspecies was extirpated from Guam in the 1980s presumably due to predation by the introduced brown tree snake Boiga irregularis (Savidge Reference Savidge1987, Wiles et al. Reference Wiles, Bart, Beck and Aguon2003). The Mariana Fruit-Dove is also found on the neighbouring islands of Rota, Tinian, and Saipan but was extirpated from Guam in the 1980s, also likely due to the brown tree snake (Savidge Reference Savidge1987, Wiles et al. Reference Wiles, Bart, Beck and Aguon2003). The Mariana Fruit-dove has also recently colonised the island of Sarigan, a volcanic island to the north of Saipan (RJC and PR pers. obs.). The Mariana Swiftlet is currently found on Saipan and Guam and once occurred on the islands of Rota and Tinian (Steadman Reference Steadman1999, Cruz et al. Reference Cruz, Kremer, Martin, Williams and Camacho2008).
Aguiguan also supports populations of six additional native terrestrial birds, the Micronesian Megapode Megapodius laperous, Yellow Bittern Ixobrychus sinsensis, White-throated Ground-Dove Gallicolumba xanthonura, Collared Kingfisher Todiramphus chloris, Micronesian Mzyomela Myzomela rubratra, and Micronesian Starling Aplonis opaca. The Micronesian Megapode is listed as ‘Endangered’ by IUCN (BirdLife International 2014) and it occurs on 12 islands in the Mariana archipelago and Palau (Baker Reference Baker1951, Amidon et al. Reference Amidon, Marshall and Kessler2010). The White-throated Ground-dove is listed as ‘Near Threatened’ by IUCN while the remaining four species are listed as ‘Least Concern’ due to their wide ranges and large populations (BirdLife International 2014). Currently there is only one non-native bird on the island, the Island Collared-dove Streptopelia bitorquata.
Bird populations throughout the region, including Aguiguan, were first quantitatively assessed in 1982 by Engbring et al. (Reference Engbring, Ramsey and Wildman1986). Subsequent surveys have been conducted on all the islands and recent estimates of population trends and abundance have been completed for some of these islands (see Amar et al. Reference Amar, Amidon, Arroyo, Esselstyn and Marshall2008, Camp et al. Reference Camp, Pratt, Marshall, Amidon and Williams2009, Reference Camp, Amidon, Marshall and Pratt2012). Here we present, for the first time, results from 1995 and 2008 forest bird surveys of Aguiguan along with an assessment of population trends since 1982. Population trends are compared to results from recent surveys on the neighbouring islands of Rota, Tinian, and Saipan and the conservation implications are discussed.
Methods
Survey area
Aguiguan is a small, uninhabited island (7.09 km2; 14o51`N, 145o33`E) located 8 km south-west of the island of Tinian (Figure 1). It is composed of several concentric plateaus bounded by steep limestone scarps; the uppermost plateau averages about 150 m in elevation. The island rises abruptly from the ocean with sheer cliffs, and there are no beaches, bays, or harbours. The climate in the region is tropical with average monthly temperatures ranging from 24°C to 27°C and annual precipitation of 2,000 to 2,500 mm (Mueller-Dombois and Fosberg Reference Mueller-Dombois and Fosberg1998). About half of the island was cleared for agriculture, primarily sugarcane, during the 1930s and 1940s and was subsequently abandoned after the Second World War (Butler Reference Butler1994). Those fallow fields changed to Pennisetum purpureum in 1950 (Fosberg Reference Fosberg1960), to a mix of Lantana camara, Eupatorium spp. and grasses in 1990 (Butler Reference Butler1994), to L. camara and Eupatorium spp. being co-dominant in many fields in 1995 (G. Wiles pers. comm.) and to mainly L. camara in 2008 (Amidon Reference Amidon2009). In addition, some field areas were converted to secondary forests dominated by Leucaena leucocephala and Acacia confusa between the 1950s and 1990s (Amidon Reference Amidon2009). Currently, approximately 49% of the island is covered by native forest (344 ha) while the remainder is secondary forest (141 ha), coastal scrub (28 ha), bare ground (34 ha), and open fields of primarily L. camara (160 ha; Amidon Reference Amidon2009).
Aguiguan is uninhabited but supports a population of approximately 1,400 feral goats Capra hircus which were apparently introduced between 1820 and 1860 (Butler Reference Butler1994, Esselstyn et al. Reference Esselstyn, Cruz, Williams and Hawley2003). The Polynesian rat Rattus exulans occurs on the island (Yackel Adams et al. Reference Yackel Adams, Standford, Wiewel and Rodda2010) but no populations of feral cats Felis catus or dogs Canis lupus familiaris are reported.
Abundance and population trends
An island-wide survey consisting of 66 stations on four transects (random-systematic placement) was conducted by the U.S. Fish and Wildlife Service (USFWS) on 2 and 3 June 1982 (Engbring et al. Reference Engbring, Ramsey and Wildman1986; Figure 1). These four transects and 66 stations were resurveyed by Craig et al. (Reference Craig, Chandran, Ellis and Craig1993) in 1992 (21–22 May), USFWS in 1995 (31 May–3 June; survey by A. Marshall) and 2008 (25–27 June; surveys by S. Kremer and A. Marshall), and the Commonwealth of the Northern Mariana Islands Division of Fish and Wildlife (DFW) in 2000 (31 March–8 April; Cruz et al. Reference Cruz, Arriola, Johnson and Beauprez2000) and 2002 (14–21 March; Esselstyn et al. Reference Esselstyn, Cruz, Williams and Hawley2003). An additional transect of 14 stations was sampled during the 2008 survey for a total of 80 stations. This transect was added to increase the numbers of bird detections and to sample the topmost plateau; placement of this transect on the plateau was random. Complete datasets were available for all surveys expect for the 1992 survey by Craig et al. (Reference Craig, Chandran, Ellis and Craig1993), which was not included in this analysis.
All surveys followed standard point-transect methods (see Buckland et al. Reference Buckland, Anderson, Burnham, Laake, Borchers and Thomas2001 for details). The USFWS surveys consisted of 8-minute counts, where horizontal distances to all birds heard and/or seen were estimated and recorded. The DFW counts lasted only five minutes. Sampling conditions recorded included cloud cover, rain, wind, noise level, detectability, and habitat type, all of which were later used as covariates in density calculations (see below). Counts commenced at sunrise, continued for up to four hours, and were conducted only under prescribed conditions. Stations were surveyed by two observers in 1982 and one observer in all subsequent surveys. Data from both counters were used to estimate 1982 densities, and the sampling effort was adjusted appropriately.
The raw data from the surveys were transcribed and compared to the electronic version on a line-by-line basis to correct transcription errors. All errors were corrected and rechecked for accuracy. After line-by-line proofing was completed, individual datasets were spot-checked for error rates. Ten percent of the survey-specific records were randomly selected and proofed to the raw data. If the error rate exceeded 1%, the whole dataset was proofed again using line-item procedures and a new set of random records checked via the spot-checking methods.
Population status was calculated as density (birds/ha) and abundance (mean of the density estimates weighted by habitat area). Density was calculated using the program DISTANCE, version 5.0, release 2 (Thomas et al. Reference Thomas, Laake, Strindberg, Marques, Buckland, Borchers, Anderson, Burnham, Hedley, Pollard, Bishop and Marques2006) from species-specific global detection functions, where data were post-stratified by survey. Data were right-truncated to facilitate model fitting (Buckland et al. Reference Buckland, Anderson, Burnham, Laake, Borchers and Thomas2001). Candidate models included half-normal and hazard-rate detection functions with expansion series of order two (Buckland et al. Reference Buckland, Anderson, Burnham, Laake, Borchers and Thomas2001). Sampling covariates were modelled in the multiple-covariate engine of DISTANCE (Thomas et al. Reference Thomas, Laake, Strindberg, Marques, Buckland, Borchers, Anderson, Burnham, Hedley, Pollard, Bishop and Marques2006, Marques et al. Reference Marques, Thomas, Fancy and Buckland2007). The model with the lowest second-order Akaike’s information criterion corrected for small sample size (AICc) was used to select the detection function that best fitted the shape of the survey data (Burnham and Anderson Reference Burnham and Anderson2002).The USFWS data were pooled, separately from the DFW surveys, to ascertain the best model of a species-specific global detection function for the 8-min counts (Burnham Reference Burnham1981, Buckland et al. Reference Buckland, Anderson, Burnham, Laake, Borchers and Thomas2001, Buckland Reference Buckland2006). Due to the limited detections for the 5-minute DFW counts, DFW survey data collected during 5-minute counts on the neighbouring islands were also used to develop the DFW count global detection function. Covariates (sampling conditions, habitat types, island, and survey year) were used to generate the global detection function when the best approximating model was improved by four or more AICc units. Detection function parameters used to derive population densities for each species on Aguiguan can be found by dataset in Appendix S1 in the online Supplementary Materials. Variances and confidence intervals were derived by 2.5 and 97.5 quartiles from bootstrap based methods (Buckland et al. Reference Buckland, Anderson, Burnham, Laake, Borchers and Thomas2001, Thomas et al. Reference Thomas, Buckland, Rexstad, Laake, Strindberg, Hadley, Bishop, Marques and Burnham2010). Survey-specific, density-by-station values were generated for population trend analyses (see Population trends below) from the global detection function. Area of habitat types came from Engbring et al. (Reference Engbring, Ramsey and Wildman1986) and Amidon (Reference Amidon2009). Because the area of habitat types was not available for the 1995, 2000, and 2002 surveys, abundance was calculated only for the 2008 survey.
Changes in bird density among the annual estimates were assessed using repeated measures analysis of variance (ANOVA: PROC MIXED; SAS Institute Inc., Cary, NC). To stabilise error variance, density-by-station values were ln(density+1) transformed. Because the number of repeated measures was too small to fit a covariance model, the variance-covariance structure was assumed to be a compound symmetry, homogeneous-variance model (Littell et al. Reference Littell, Milliken, Stroup and Wolfinger1996). Degrees of freedom were adjusted using the Kenward-Roger adjustment statement, and a Tukey’s adjustment was used to control experiment-wise alpha = 0.05 for multiple-comparison procedures.
Results
Thirteen terrestrial species were detected on the Aguiguan surveys from 1982 to 2008 (Table 1). Sufficient numbers of individuals were detected for eight native and one non-native species to calculate density and abundance estimates (Table 2). Micronesian Megapodes were detected during each of the surveys but at low numbers. Mariana Swiftlets were recorded during the 1982, 1995, and 2008 surveys but point-transect sampling is not an appropriate method for estimating densities for this species (see Discussion below). The Yellow Bittern was only recorded on-count during the 1982 survey but was observed off-count during the 2008 survey. The Nightingale Reed-warbler was only recorded on-count during the 1995 survey but was observed off-count during the 1982 survey. In 2008, the Bridled White-eye was the most abundant species, with over 50,000 birds on the island, and Collared Kingfisher, Island Collared-dove, and White-throated Ground-dove were the least abundant species, each with < 600 individuals (Table 2).
Table 1. Summary of terrestrial birds detected from the five point-transect surveys on Aguiguan. In 1982, 66 stations were sampled on four transects (88 counts; several stations were counted more than once; effort not adjusted for indices below), and the same number of stations were counted in 1995, 2000 and 2002. In 2008, 80 stations were sampled along five transects. The numbers of birds detected (# Detect) and birds per station (BPS) are presented by survey. Speciesnot detected during a survey are noted with “—”. In 2000 and 2002 only select species were recorded, thus other species are noted with “nr” for not reported. Nomenclature generally follows the AOU checklist and Reichel and Glass (1991). Density estimates were produced for birds in bold.
a Conservation status based on IUCN categories (BirdLife International 2014), except Alien, which indicates non-native species: CR – Critically Endangered, EN – Endangered, LC – Least Concern, and NT – Near Threatened.
b Diet types based on Jenkins (1983) and Craig (1996): C – carnivorous, F – frugivorous, H – herbivorous, I – insectivorous, and O – omnivorous .
Table 2. Population density (birds/ha± SE, with 95% CI) and abundance estimates for native and alien land birds on Aguiguan. Abundance estimates (abundance equaled the mean of the density estimates weighted by habitat area with 95% CI) were only calculated in 2008. Species not recorded designated by ‘nr’.
Densities of six native birds—White-throated Ground-dove, Rufous Fantail Rhipidura rufifrons, Golden White-eye, Bridled White-eye, Collared Kingfisher, and Micronesian Starling—and one non-native bird—Island Collared-dove—were significantly greater in 2008 than 1982 (Table 3, Figure 2, Appendix S2). Density estimates for the Rufous Fantail, Golden White-eye, and Micronesian Starling in 1995, 2000, and 2002 showed generally increasing trends for these species. White-throated Ground-dove densities appeared to decline in 2002 but the confidence intervals for that year overlapped with all other estimates indicating the apparent decline was not significant. Bridled White-eye densities showed an increase in 1995 and subsequent decline in 2000 and 2002 to levels that were similar to those recorded in 1982 indicating some fluctuation in trends for this species. Density estimates for the Island Collared-dove were not available for 2000 and 2002. However, density estimates for 1995 were similar to those reported in 1982 (Table 2), indicating that the increase in density may have been recent.
Table 3. Repeated measures analysis of variance results to assess for differences in Aguiguan bird densities among years. All species showed significant among-year fixed effects at the alpha 0.05 level (see Appendix S2 in online Supplementary Materials). Change was defined as increasing (▲) or stable (⚊), no species showed conclusive evidence of a declining trend.
Figure 2. Density estimates (birds/ha and 95% CI) for nine native and alien birds on Aguiguan from point-transect surveys conducted in 1982, 1995, 2000, 2002, and 2008.
No differences in densities were detected between the 1982 and 2008 surveys for Mariana Fruit-dove and Micronesian Myzomela. Density estimates for the Mariana Fruit-dove from 2000 and 2002 indicate that it experienced a decline those years. Micronesian Myzomela density estimates were highly variable but showed stable trends throughout the entire survey period.
Discussion
Abundance and population trends
Despite increasing or stable population trends on Aguiguan, population trends for native forest birds on the neighboring islands of Saipan, Tinian, and Rota have been mixed (Amar et al. Reference Amar, Amidon, Arroyo, Esselstyn and Marshall2008, Camp et al. Reference Camp, Pratt, Marshall, Amidon and Williams2009, Reference Camp, Amidon, Marshall and Pratt2012). Of special concern is the Critically Endangered Golden White-eye on Saipan, which supports a third of the density estimated on Aguiguan (7 and 24 birds/ha, respectively) and has declined significantly since 1982 (Camp et al. Reference Camp, Pratt, Marshall, Amidon and Williams2009). In addition, Mariana Fruit-dove and Micronesian Mzyomela showed declines on Rota and Tinian while the Rufous Fantail declined on Rota and Saipan (Amar et al. Reference Amar, Amidon, Arroyo, Esselstyn and Marshall2008, Camp et al. Reference Camp, Pratt, Marshall, Amidon and Williams2009, Reference Camp, Amidon, Marshall and Pratt2012).Why these species have declined on the neighbouring islands is unknown, though Amar et al. (Reference Amar, Amidon, Arroyo, Esselstyn and Marshall2008) and Camp et al. (Reference Camp, Pratt, Marshall, Amidon and Williams2009, Reference Camp, Amidon, Marshall and Pratt2012) listed habitat loss, predation, food availability, pesticides, ectoparasites, and disease as potential factors. However, the declines on the neighboring islands may provide some insight into why native bird populations are stable or increasing on Aguiguan.
One feature which distinguishes Aguiguan from the neighbouring islands is that it does not support a population of the Malayan house rat Rattus diardii. Yackel Adams et al. (Reference Yackel Adams, Standford, Wiewel and Rodda2010) reported that only the Polynesian rat occurred on Aguiguan while Wiewel et al. (Reference Wiewel, Adams and Rodda2009) reported high densities of Malayan house rat on Rota, Tinian, and Saipan. The Polynesian rat was an early introduction to the Mariana archipelago (AD 1000–1200; Steadman Reference Steadman1999) and is generally considered to be less detrimental to avian populations than other introduced Rattus species (Atkinson Reference Atkinson and Moors1985). It is possible that the differences in trends and densities between Aguiguan and the neighbouring islands may, in part, be related to rats.
Another possible factor is the conversion of open fields to secondary forest and L. camara thickets on Aguiguan during the survey period. Amidon (Reference Amidon2009) reported that the area of open field habitats declined by approximately 53% from the estimate provided by Engbring et al. (Reference Engbring, Ramsey and Wildman1986) for the 1982 surveys, while secondary forest and L. leucocephala both increased over that period by 14% and 6%, respectively, and native forest remained stable. The area covered by L. camara in 1982 was not reported in Enbgring et al. (Reference Engbring, Ramsey and Wildman1986), though they did report that the species was present on the island. However, notes written on the aerial photographs used by Engbring et al. to estimate land cover indicate that areas dominated by L. camara in 2008 were dominated by grass species in 1982.
The conversion of abandoned agricultural land to secondary forest and L. camara may provide additional foraging resources to omnivorous birds, and may partially explain increases in the densities of Golden White-eye and Bridled White-eyes. Golden White-eyes forage on the fruits and flowers of L. camara, while Bridled White-eyes forage on the fruits of L. camara and the flowers of L. leucocephala (Craig Reference Craig1996). Increases in secondary forest may have also lead to increases in nesting habitat on Aguiguan for Bridled White-eyes, Golden White-eyes, and Rufous Fantails, which all nest in secondary forest on Saipan (Sachtleben Reference Sachtleben2005). Habitat-related increases in White-throated Ground-Dove, Collared Kingfisher, and Micronesian Starling are less certain as kingfishers are carnivorous and it is unknown whether ground-doves and starlings forage on L. camara or L. leucocephala. However, increased white-eye and fantail densities would also increase the potential availability of these species to kingfishers and starlings, which are known to depredate them (Craig Reference Craig1996, Sachtleben Reference Sachtleben2005).
As noted in the Methods, we were unable to obtain the complete data set for the 1992 survey by Craig et al. (Reference Craig, Chandran, Ellis and Craig1993). In addition, the density estimates reported in Craig et al. (Reference Craig, Chandran, Ellis and Craig1993), like the density estimates by Engbring et al. (Reference Engbring, Ramsey and Wildman1986), are not directly compared to the estimates provided in this study due to advances in the analysis of point-transect data since the initial work by Reynolds et al. (Reference Reynolds, Scott and Nussbaum1980). The raw counts of detections reported in Craig et al.(1993) fall within the range of detections reported here indicating, at the very least, that the 1992 detections were comparable to the number of detections from the other surveys. However, more detailed assessments of trends using raw counts are not possible due to variation in detectability across the surveys.
Craig et al. (Reference Craig, Chandran, Ellis and Craig1993) noted that their average detection distances were consistently smaller than those from the 1982 surveys, likely altering their estimated detection functions for the 1982 and 1992 surveys. We assessed the average detection distances for the 1995 and 2008 surveys and observed that the average detection distances for those surveys were smaller than the 1982 survey estimates and similar to those reported in Craig et al. (Reference Craig, Chandran, Ellis and Craig1993). We accounted for this difference by pooling the data from the 1982, 1995, and 2008 surveys to estimate a global detection function for each species and post-stratifying by survey. Therefore, this difference in average detection distance does not affect the results of the analysis. However, the higher average detection distances for the 1982 compared to subsequent surveys is worth exploring as it could imply a problem with the data or a change in conditions on Aguiguan.
The 1982 survey of Agugiuan was completed after the survey of Rota, Tinian, and Saipan (Engbring et al. Reference Engbring, Ramsey and Wildman1986). Therefore, we do not believe that there are any errors in the data related to observer experience or methodological issues. In addition, the 1982, 1995, and 2008 surveys all occurred during the months of May and June so seasonal differences are unlikely. Instead we believe that differences in average detection distances reflect changes in the habitat along the four transects sampled. As noted earlier, forested habitats on Aguiguan have increased since the 1982 survey with the conversion of open field areas to forested habitats. Detection distances in forested habitats are expected to be shorter than estimates in open field due to decreased visibility. In addition, a goat removal program was initiated on Aguiguan in 1989 and 1990 (Rice Reference Rice1993) that led to a substantial reduction in goats, from 800–1,500 in 1987 to around 40 in 1990 (Reichel Reference Reichel1988, Rice Reference Rice1991). Rice (Reference Rice1993) reported increased vegetation growth during and after the goat removal. Esselstyn et al. (Reference Esselstyn, Cruz, Williams and Hawley2003) estimated that the goat population was around 1,400 individuals in 2002 indicating that goat numbers had rebounded after the control programme. However, the increased vegetation growth after the control program may have carried over into the 1992 and later bird surveys and reduced the overall visibility and detection distances along the survey transects. In addition, increased vegetation may attenuate bird vocalisations and reduce detection distances further. These factors may account for the lower average detection distances observed for surveys after 1982.
We did not assess population sizes and trends for three species of native birds—Micronesian Megapode, Yellow Bittern, and Nightingale Reed-warbler—because of the small numbers of detections for each species, which reflect their small population sizes. Density estimation relies on detecting a total of 80–100 individuals (Buckland et al. Reference Buckland, Anderson, Burnham, Laake, Borchers and Thomas2001), which was not achieved for any of these species despite pooling detections from different surveys. Aguiguan supports a small Micronesian Megapode population (Esselstyn et al. Reference Esselstyn, Cruz, Williams and Hawley2003), with similar numbers of birds detected during each of the surveys, including the 1992 Craig et al. (Reference Craig, Chandran, Ellis and Craig1993) survey (range 12–19 birds). Amidon et al. (Reference Amidon, Marshall and Kessler2010) used play-back calls to increase Micronesian Megapode detections and estimated the Aguiguan population to be about 100 birds (61–206, 95% CI). Yellow Bittern occur in very low numbers on Aguiguan (Engbring et al. Reference Engbring, Ramsey and Wildman1986). No birds were detected during the 2008 survey, although one was seen along a transect outside of the count period and one bittern was detected during the count period on the 1982 survey. During the 1982 survey on Aguiguan, four Nightingale Reed-warbler incidental sightings were recorded, but none occurred during the 8-min counts (Engbring et al. Reference Engbring, Ramsey and Wildman1986). In 1995, three reed-warblers were detected at two stations during the counts. The reed-warbler has not been observed on Aguiguan since the 1995 survey and is likely extirpated from the island (Esselstyn et al. Reference Esselstyn, Cruz, Williams and Hawley2003, U.S. Fish and Wildlife Service 1998). Marshall et al. (Reference Marshall, Amidon, Radley, Martin and Camp2008) estimated that with the survey effort to date there was a 3% chance that a population of at most a few individuals persists on Aguiguan.
Point-transect methods are also not appropriate for assessing densities of the Mariana Swiftlet (Camp et al. Reference Camp, Pratt, Marshall, Amidon and Williams2009) because of the swiftlet’s erratic flight behaviour, the difficulty in estimating horizontal distances to birds detected in flight, and violation of the no movement assumption of distance sampling (Buckland et al. Reference Buckland, Anderson, Burnham, Laake, Borchers and Thomas2001). The number of swiftlets detected in 1995 and 2008 was lower than that recorded for 1982 (Table 1). However, Cruz et al. (Reference Cruz, Kremer, Martin, Williams and Camacho2008) reported that the Aguiguan swiftlet population was likely stable, based on counts at roosting caves between 1985 and 2002.
Conservation implications
With declining population trends for many of the endemic terrestrial birds on the neighbouring islands, increasing trends and high densities on Aguiguan are good news for native bird conservation in the region. Currently DFW, USFWS, and the members of the Association of Zoos and Aquariums are establishing “insurance” populations of endemic species on different islands in the region and in captivity to help ensure the long-term conservation of these species (Marianas Avian Conservation Working Group 2013). Aguiguan birds could assist these efforts by serving as source populations for translocations or restoration efforts on neighbouring islands. For example, Golden White-eyes on Aguiguan may be used as source population for translocations to other islands in their known range.
Though the outlook for birds on Aguiguan is good, maintenance of the island’s forest and efforts to ensure additional alien species are not introduced will benefit Aguiguan’s unique avian assemblage. Ecotourism has been proposed for Aguiguan (Eugenio Reference Eugenio2013), which could increase traffic to the island and potentially promote the spread of invasive species to it. The presence of a large feral goat population and the invasive weed, L. camara, on Aguiguan may also pose a long-term threat to the remaining native forest on the island. We saw little evidence of tree recruitment in any of the native forest on Aguiguan in 2008, likely due to goat browsing, and similar observations were noted during previous surveys (Engbring et al. Reference Engbring, Ramsey and Wildman1986, Cruz et al. Reference Cruz, Arriola, Johnson and Beauprez2000, Esselstyn et al. Reference Esselstyn, Cruz, Williams and Hawley2003). Lantana camara has become established within the native forest on Aguiguan in openings and heavily disturbed areas (Cruz et al. Reference Cruz, Arriola, Johnson and Beauprez2000, Esselstynet al. 2003). Canopy removal and the resulting increase in light availability have been found to increase L. camara growth, germination, survival, and establishment in Australian forests (Duggin and Gentle Reference Duggin and Gentle1998). Openings created in the canopy of the native forest from natural events, like tree mortality and typhoon damage, would therefore likely lead to the increased establishment of L. camara in Aguiguan’s forests. This, combined with lack of recruitment in native trees, could lead to long-term alteration of native forest on the island.
Supplementary Material
The supplementary materials referred to in this article can be found at journals.cambridge.org/bci.
Acknowledgements
Data analyses were conducted by the Hawaii Forest Bird Interagency Database Project, a project of the U.S. Geological Survey-Pacific Island Ecosystems Research Center (PIERC) and the cooperating agencies - Pacific Islands Office of the U.S. Fish and Wildlife Service and University of Hawaii Pacific Cooperative Studies Unit.We thank the many field biologists who organized and collected data. Also, we thank everyone who helped cut transects over the years, the CNMI Division of Fish and Wildlife for their participation in surveys, issuance of permits, and other assistance, and the Mayor and people of Tinian for their support. Two anonymous reviewers and P. Banko, K. Brinck, L. Mehrhoff, and G. Wiles helped improve the manuscript. We also thank H. Kozuba for preparation of data described here and assistance with generating tables and figures. This study was funded by the U.S. Navy, Pacific Islands Office of the Fish and Wildlife Service, and by PIERC. The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
