Coenonympha nipisiquit McDunnough (Lepidoptera: Nymphalidae) is an endemic butterfly species that has been recorded only in 10 salt marshes of eastern Canada. Despite obtaining the endangered status and benefiting from a conservation program, the populations of this species appear to be declining. However, a lack of information about the ecology of the species and current demographics undermines potential in-situ conservation efforts. To identify the environmental factors affecting the emergence phenology and flight period of this species, daily visual counts of adult C. nipisiquit were carried out on a daily basis from 2011 to 2015 during the flight period along transects in four marshes where the butterfly maintains a continuous presence. A series of environmental parameters was collected during the monitoring. Additive modeling indicated that the accumulation of degree-days above 7 °C from mid-May contributed to the synchronization of adult emergence and flight in C. nipisiquit and identified a number of abiotic factors having an impact on the activity and monitoring of this insect. Data also indicated that some demographic aspects of this butterfly differ between sites and confirmed that C. nipisiquit has been declining from 2011 to 2015. In conclusion, this study not only generated information critical to the adjustment of the conservation program for C. nipisiquit, but also indicated that the extirpation of this species from its natural range should remain a concern.