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Range DescriptionThis species is endemic to Guam (to USA) and the Northern Mariana Islands (to USA), and was introduced to Oahu, Hawaiian Islands (USA) in the early 1960s. On Guam, it was very abundant but, from the mid-1960s to the early 1970s, it underwent a precipitous decline (Jenkins 1983). In 1999, only three colonies remained, the largest holding c.700 birds (total population c.800) (G. Wiles unpublished data). A 2008 report by the U.S. Navy estimated that the population on Guam has increased to around 1,150 individuals (Grimm 2008). In the Northern Marianas, populations disappeared from Rota and Tinian in the 1970s, although, on Tinian, it was possibly only nomadic (Engbring et al. 1982). In 1982, estimates were 9,120 on Saipan and 1,022 on uninhabited Aguijan (Engbring et al. 1982) but, more recently, estimates are c.5,000 (five colonies) and 416 (seven colonies), respectively (J. Cruz in litt. 2007, G. Wiles unpubl. data). On Oahu, it survives in a single known breeding colony in a small tunnel in the Ko`olau Mountains, although similar irrigation tunnels are common and thus other small colonies may exist (Chantler and Driessens 1995). Observations there in 1997 suggested a minimum of 17 breeding pairs (66 birds in total [Wiles and Woodside 1999]), whilst monitoring during 2009-2010 recorded up to 50 nesting pairs (E. Vanderwerf in litt. 2012). The number of nesting pairs and fledglings on Oahu was lower in 2011 because rats invaded the nesting tunnel after rat control was interrupted (E. Vanderwerf in litt. 2012).