Background
The northern harrier is a raptor (bird of prey) that breeds throughout much of Canada and the northern US. This species breeds in wet meadows, grasslands, abandoned fields, and coastal and inland marshes ^4,5,7. In Massachusetts, they breed primarily along the coast and are regularly seen in in coastal marshes in the winter ⁵. They nest alone or in loose colonies, and can occasionally exhibit polygyny (when a male has more than one female mate) ². Northern harriers prey primarily on small mammals, including rodents, shrews, and rabbits, but can also eat reptiles, amphibians, insects, and small birds ². Population trends and productivity are strongly influenced by the availability of the microtine voles (a small rodent) that the harriers rely heavily on in the early spring ⁷. Northern harriers are partial migrants that can range in migration distance depending on their breeding latitude ⁷.

The northern harrier has experienced population declines through much of its North American range, and is a species of national management concern listed by the US Fish and Wildlife Service due to its dependence on rare and vulnerable habitats ⁴. Threats to northern harriers include destruction of freshwater and estuarine wetlands in breeding and wintering areas, agricultural conversion of native grassland prairies, nest destruction from mechanized agriculture and livestock, and declines of prey species and toxicity exposure through insecticides and rodenticides ⁷. In the northeast, the destruction of wetlands, reforestation, and increase in nest predators are key factors in population declines ⁷.

Climate Impacts
Climate change may negatively impact northern harriers in a number of ways. Precipitation and percentage of wetland area are the best predictors of the abundance of the northern harrier in the Midwest ³. Prolonged periods of rain can also destroy nests and reduce productivity for northern harriers ⁵. Habitat loss is already a serious threat to this species, and the marsh habitat that these species rely heavily on in Massachusetts 5 is considered particularly vulnerable to climate change induced sea-level rise ⁸.  A number of raptor species, including northern harriers, have had significant poleward shifts in their wintering distributions since 1975 in response to changing climate conditions ⁶. Many raptors, including northern harriers, also appear to be arriving earlier in the spring and leaving later in the autumn from their breeding grounds, though it is unclear whether this trend is beneficial, detrimental, or neutral ¹.

Some raptors do appear to be positively affected by climate change. For instance, in the western US, the American kestrel (Falco sparverius) significantly reduced its migration distance in response to warmer winters, which resulted in earlier nesting and increased reproductive success ⁶.