There is a long history of recording bird observation in Acadia National Park, ME. These studies and resulting long-term data sets provide evidence of changes happening within and around the park, as well as an opportunity to compare local dynamics with trends detected in regional to continental-scale studies. Over 51 years (1971–2021), community volunteers in and around Acadia National Park engaged in annual Christmas bird counts (CBC), collecting valuable information to assess winter bird-population dynamics and species trends. We analyzed the cumulative data from 2 CBC circles that encompass Acadia and the surrounding lands and waters to generate a combined summary of early winter population-trend estimates for 162 species. We found a 43% reduction in the total number of birds over the 51-year study, with 42 species exhibiting declines, and 33 species showing increasing abundance. The annual number of species observed has declined by over 7%; however, the cumulative species in the full dataset continues to increase as newly observed species are added in most years. Our study complements many other studies from Acadia documenting ongoing changes in the physical environment and coastal biota. Conservation and management actions take place at the local level (e.g., Acadia National Park), and local resource data and trends are critical to synthesize and share for effective decision making.