The Central Pacific Conservation Area contains one of Costa Rica's two viable, but threatened Scarlet Macaw Ara macao populations. For 14 years (1990–2003), we monitored the size of this population. Non-linear models fitted to 1990–1994 observations revealed seasonal and long-term changes in population size. The peak of annual population size occurred in August, with a cyclic range of about 90 birds between the lowest and highest points of the annual cycle. The best model also revealed a decline in population size of approximately eight birds counted per year or 4% of the total population per year (1990–1994). Young-to-adult ratios calculated for the month of August during this study fluctuated around a baseline that averaged 6.1% a year (1990–2003). These ratios exceeded 8% for three different years (1995, 1996 and 2000). All three of these “good” recruitment years occurred after management began, and two of them were associated with zealous anti-poaching efforts that ultimately could not be sustained. After intensive management practices began in 1995, the August counts increased by about 37 individuals in two years (1995–1996) to an average 243 individuals, which remained almost constant up to 2003. Management practices included creation of a local conservation organization that coordinated environmental education, artificial nest construction, networking among stakeholders and with governmental authorities, and artificial and natural nest protection. Although Scarlet Macaw conservation efforts have been inconsistent since 1997, our study demonstrates that collaborative conservation by local stakeholders increased the population of this threatened species in 1995–1996, and from 1996 to present the population has sustained itself.