The Northern Harrier (Circus hudsonius) is a medium-sized bird of prey found throughout North America. Also known as the “Marsh Hawk”, it inhabits grasslands, marshes, and other open habitat types. Northern Harriers are distinctive raptors with long, slender wings and a white rump patch. Their acrobatic flight low over fields gives them their other common name of “Hawk Owl”. But are Northern Harriers nocturnal like owls, or are they active during the day like other hawks? Let’s take a look at the evidence.
Nocturnal Adaptations
Northern Harriers do possess some adaptations for nocturnal activity. Their large eyes are positioned on the sides of their heads, giving them good night vision. The disk-like shape of their face also helps channel sound waves towards their ears, improving their hearing for detecting prey in darkness. Finally, the light color of their plumage helps camouflage them in the moonlight. So in some respects, Northern Harriers are physically equipped for nocturnal hunting.
However, these adaptations are not definitive proof that Northern Harriers are nocturnal. Many hawks share these same adaptations, using their keen eyesight and hearing to hunt small mammals and other prey during daylight hours. More evidence is needed to determine if Northern Harriers are actively hunting at night.
Activity Patterns
Studies of Northern Harriers in the wild provide insight into their daily activity patterns. During the breeding season, male Northern Harriers engage in spectacular “sky dancing” courtship displays at dawn and dusk. This behavior suggests they are crepuscular, meaning most active around sunrise and sunset. Females have been observed hunting throughout the day, but seem to be most active in the morning and late afternoon/early evening.
Both sexes typically roost at night, though there is some evidence of nocturnal nest defense against predators. Males may also make brief nocturnal visits to their mates at the nest. Overall, however, the bulk of a Northern Harrier’s hunting and activity takes place during daylight hours. They are not completely nocturnal but instead crepuscular or cathemeral (sporadically active throughout the day).
Prey Availability
Another important factor is the activity patterns of their prey. Northern Harriers feed mainly on small mammals like voles, mice, rats, and ground squirrels. They also take birds, reptiles, amphibians, and large insects. Most of this prey is active during the day, so it makes sense for Northern Harriers to hunt when their prey is available.
Nocturnal prey like bats would be more difficult for harriers to find and catch in the dark. Some nocturnal hunting likely occurs, but the majority of their prey is diurnal. This means Northern Harriers must also be active during daylight to optimize feeding.
Metabolic Factors
Some other hypotheses suggest metabolic factors may limit nocturnal activity in Northern Harriers. They have high mass-specific metabolic rates and rely heavily on vision to hunt. Both of these are energetically expensive compared to owls that are specially adapted for nocturnal hunting. Sustained nocturnal activity could be too physiologically demanding for harriers.
However, the occasional nocturnal behaviors described earlier indicate Northern Harriers are capable of some nocturnality. More research into their precise metabolic rates, oxygen consumption, and energy budgets during the day versus night could provide deeper insight. But current evidence suggests that regular nocturnal hunting may exceed their metabolic capacity.
Conclusion
In summary, Northern Harriers possess visual and auditory adaptations that could facilitate nocturnal hunting. However, observations of their natural activity patterns, prey availability, and metabolic constraints suggest they are not strictly nocturnal. Northern Harriers are best described as crepuscular or cathemeral, with their peak activity occurring in the early morning and evening hours. Limited nocturnal behaviors like nest defense do occur, but the bulk of their hunting happens during daylight when their prey is active and their vision is at its sharpest. So while Northern Harriers are not completely diurnal, they are certainly not true nocturnal owls either. Their activity patterns reflect a flexible adaptation to both dusk/dawn and daylight conditions throughout the year.
References
Activity Patterns
MacWhirter, R.B. and Bildstein, K.L. (1996). Northern Harrier (Circus cyaneus). In: Poole, A.F. and Gill, F.B. (eds) The Birds of North America. Academy of Natural Sciences, Philadelphia, and American Ornithologists’ Union, Washington, DC. https://birdsna.org/Species-Account/bna/species/norhar/
Toland, B. (1987). The effect of vegetative cover on foraging strategies, hunting success and nest depredation of Northern Harriers in southwestern Missouri.
Nocturnal Adaptations
Watson, J. (1997). The Golden Eagle. T. and A. D. Poyser Ltd. London.
Martin, G.R. (1985). Eye. In: King, A.S. and McLelland, J. (eds) Form and Function in Birds, vol. 3, pp 1–54. Academic Press, London, UK.
Prey Availability
O’Connell, T. J., & Apfelbaum, S. I. (1993). Food-handling behavior of the northern harrier. The Wilson Bulletin, 105(4), 675-678.
Bechard, M. J., & Schmutz, J. K. (1995). Ferruginous hawk (Buteo regalis). In: Poole, A. F., & Gill, F. B. (Eds.). The birds of North America. Academy of Natural Sciences, Philadelphia, and American Ornithologists’ Union, Washington, D.C. https://doi.org/10.2173/bna.172
Metabolic Factors
Kerlinger, P. and Moore, F.R. (1989). Atmospheric structure and avian migration. In: Power, D.M. (ed.) Current Ornithology, vol 6, pp 109–142. Plenum Press, New York.
Millsap, B.A. (2018). Nocturnal activity by a nesting Swainson’s hawk documented via webcam. Journal of Raptor Research 52: 192–195.
