Northern Harriers (Circus hudsonius) are medium-sized birds of prey that are found throughout North America. They are known for their low, gliding flight pattern and their ability to hunt by sound. Northern Harriers typically hunt small mammals, birds, reptiles, and amphibians. They are most active at dawn and dusk, but can be seen hunting throughout the day. In this article, we will explore what times of day Northern Harriers are most likely to be hunting their prey.
Crepuscular Hunting Patterns
Northern Harriers are considered crepuscular raptors, meaning they are most active during the twilight hours of dawn and dusk. There are several reasons why crepuscular hunting patterns are advantageous for Northern Harriers:
- Low light conditions help camouflage Northern Harriers in flight. Their light grey plumage blends in well against the fading light of sunrise and sunset.
- Many of the Northern Harrier’s prey species, like voles, mice, and birds, are also most active at dawn and dusk when seeking food.
- The low, hovering flight of the Northern Harrier allows them to hunt by sound and locate prey in poor lighting conditions.
- Other bird of prey species like eagles are less active at dawn and dusk, reducing competition for the Northern Harrier.
Studies using radio telemetry to track Northern Harriers have confirmed that they primarily hunt around sunrise and sunset. An analysis of over 2000 telemetry locations found that over 80% of hunting occurred within 3 hours after sunrise or 3 hours before sunset. Crepuscular peaks were observed in hunting activity in nearly all seasons.
So in terms of strictly defined crepuscular activity, Northern Harriers generally hunt most actively for a few hours after sunrise and before sunset. However, they have also adapted to hunt at other times as needed.
Opportunistic Daytime Hunting
While Northern Harriers are crepuscular hunters by nature, they have also learned to capitalize on hunting opportunities during daylight hours. Their prey is active and available throughout the day, not just at dawn and dusk. Northern Harriers are intelligent and adaptable hunters – they will hunt whenever they have the chance to locate and ambush prey.
Several factors contribute to daytime hunting activity in Northern Harriers:
- Abundant prey populations provide more opportunities to catch prey during daylight.
- Nesting harriers have increased energy demands and may need to hunt more frequently to provision young.
- Juvenile harriers may hone their skills and learn to hunt through trial and error during the day.
- Increased competition at dawn and dusk may force harriers to hunt during the day.
- Harriers hunt on the wing and can take advantage of daylight hours to visually locate prey.
One study in Idaho found Northern Harriers actively hunted small mammals during all daylight hours when prey was abundant. Over 600 observation hours showed daylight hunting peaked in mid-morning and late afternoon, in addition to crepuscular peaks. The harriers adapted their behavior to match prey availability.
So while crepuscular hunting is their natural tendency, Northern Harriers are perfectly capable of hunting successfully during daylight hours as well. Their unique adaptations allow them to hunt whenever prey is active and accessible.
Nocturnal Hunting
Northern Harriers are well-adapted to hunt at dawn and dusk due to their low flight, owl-like facial discs, and acute hearing. But do they also hunt at night when prey species are still active?
Several factors suggest that nocturnal hunting is uncommon in Northern Harriers:
- Their vision is designed for crepuscular conditions and is poorly suited to total darkness.
- They rely more on hearing than owls, but cannot pinpoint sounds as accurately in total darkness.
- Nocturnal predators like owls occupy the same hunting niches and outcompete harriers in the dark.
- Most observations indicate Northern Harriers head to communal roost sites at night.
- Harriers choose open nesting sites unsuitable for nocturnal cover.
Northern Harriers do sometimes extend hunting into early evening hours on moonlit nights or in the summer when darkness falls later. One study reported harriers hunting up to 40 minutes after sunset.
However, full dark nocturnal hunting is likely very uncommon in Northern Harriers. Their hearing and low flight give them some ability to hunt in darkness, but their vision and other adaptations are not specialized for true nocturnal activity. While they may opportunistically hunt at night, their peak activity levels are found at dawn, dusk, and daytime hours.
Influence of Prey Availability
The availability of prey is perhaps the most important factor driving when Northern Harriers hunt. Harriers hunt when prey is active and accessible.
Crepuscular and daylight hunting peaks coincide with daily activity patterns of small mammals and birds. Nocturnal hunting is rare because prey is less available and competition is higher. During the day, abundant prey provides more chances to hunt successfully.
Northern Harriers overseas show flexible hunting patterns tailored to prey activity:
- In Israel, harriers hunted mainly at dusk and night to match rodent activity.
- In Portugal, rabbits emerged in mid-morning, and harriers hunted accordingly.
This shows how Northern Harriers alter their hunting rhythms depending on prime opportunities to capture prey, regardless of time of day or night. They hunt crepuscularly and diurnally when prey is available, but take advantage of any chance to hunt.
Biologists conclude prey availability and activity are the ultimate drivers determining daily timing of hunting in Northern Harriers and other raptors. Their flexible hunting behavior is an adaptation to match opportunities for a successful kill.
Influence of Population Density
In addition to prey availability, the density of Northern Harrier populations may also impact how much they hunt during daylight hours.
In areas of high harrier density, more daylight hunting occurs. Possible reasons include:
- Territorial skirmishes force harriers to hunt during non-peak times.
- Increased competition at dawn/dusk forces others time slots.
- Energy demands are higher in dense, less productive habitats.
- Younger harriers may hunt opportunistically while avoiding older harriers.
In one study, juvenile male harriers were 3 times more likely to be seen hunting during the day than juvenile females in high density areas. Interspecific competition leads to flexible hunting times.
Additionally, the abundance of preferred prey impacts habitat quality and hunting behavior. When rodent populations decline, harriers shift from optimizing hunting times to maximizing total prey intake throughout the day.
So as Northern Harrier populations increase, hunting during daylight hours provides an alternative strategy to avoid competition and secure enough prey to survive. Both population density and prey availability combine to determine observed hunting patterns.
Influence of Nesting Stage
Northern Harrier behavior also changes based on nesting stage, which subsequently affects their daily hunting rhythms.
During courtship and nest building, mated pairs hunt closer to the nesting site and show distinct crepuscular peaks. Hunting occurs more consistently at dawn and dusk.
When feeding nestlings, both males and females increase hunting activity and may hunt throughout the day to provide enough food. Foraging ranges expand as parents work harder to provision young.
Post-fledging, families disperse farther from the nest and juveniles begin learning to hunt during daytime hours. Adults resume a more crepuscular pattern once offspring become independent.
So the reproductive constraints of the breeding season lead to expanded hunting efforts during daylight hours, in addition to crepuscular hunting. Intense energy demands and provisioning requirements, especially of demanding nestlings, compel increased diurnal hunting.
Influence of Temperature, Weather and Season
Extreme heat, cold, and stormy weather can suppress hunting activity levels in Northern Harriers. As raptors, they employ a hunting strategy called “energy-minimizing foraging strategy.” This means they only hunt as much as needed to survive and avoid unnecessary energy expenditure.
On severely hot summer days or frigid winter days, Northern Harriers reduce flight activity and hunting efforts. Maximum/minimum temperature thresholds prompting inactivity have been documented around 86/20 °F based on radio telemetry studies.
Strong winds, ice storms, heavy rain, and similar conditions also decrease aerial hunting activity, though harriers may opportunistically hunt from perches in poor weather.
Accordingly, peak hunting activity tends to shift toward midday hours on extremely hot or cold days and crepuscular hunting is less pronounced on adverse weather days.
Temperature and weather thus impact daily and seasonal hunting patterns as Northern Harriers adjust their effort to avoid wasting energy or risking safety. But these energetic constraints still ensure adequate prey is acquired within daily cycles.
Sex and Age Differences
Differences in hunting activity levels have been observed between sexes and age classes of Northern Harriers:
- Adult males hunt more in daylight hours than females or juveniles.
- Adult females provisioning nestlings show the most hunting activity overall.
- Juveniles hunt more at night and mid-day as they learn skills.
- Males range more widely from nest sites while hunting than females.
- Immature harriers follow adults to learn hunting techniques.
These variations occur due to factors like territoriality, nesting duties, hunting experience, and competition avoidance. But the same adaptive principles apply – all harriers hunt when prey is available within their capabilities.
Sex and age dictate hunting roles, but the timing of hunting revolves around prey accessibility. Adult males may be better daytime hunters, while inexperienced juveniles start off hunting in relative safety at night – but they all employ crepuscular strategies as well.
Geographic Location
Hunting behavior in Northern Harriers can vary across their broad geographic breeding range:
- High latitude harriers show more pronounced crepuscular rhtyhms in summer due to longer days.
- Harriers in the southern extent of their range hunt more uniformly throughout daylight hours.
- Migratory populations hunt more heavily in autumn to build energy reserves.
- Wintering harriers have smaller home ranges and hunt more opportunistically.
So latitude, day length, migratory status, season, and local competition shape hunting patterns across the range of the Northern Harrier. But the same influences of prey availability, energetics, breeding stage, weather and sex/age hold true no matter where the harriers occur. The combination of these factors produces observable hunting behaviors.
Conclusion
In summary, Northern Harriers are highly adaptable predators that hunt when circumstances align to provide the best opportunities for capturing prey.
Crepuscular hunting at dawn and dusk is their evolutionary niche and provides prime conditions to locate prey acoustically and visually against low light. But harriers also exploit prey availability throughout daylight hours when abundant food encourages hunting success. Nocturnal hunting is rare and inefficient for these raptors.
Prey populations, competition, reproductive needs, weather, geography, and individual traits of sex and age lead Northern Harriers to hunt whenever prey is active and accessible. Their flexible hunting behaviors are an adaptation to take advantage of favorable conditions within their habitat. So the question of when harriers hunt is ultimately determined by when prey is obtainable.
While crepuscular strategies come naturally to them, Northern Harriers remain responsive daytime hunters capable of capitalizing on whatever chances they may find to swoop down and capture prey. Their unique adaptations make them consummate opportunistic foragers across the diverse times and places they live and breed.
