Introduction
The northern harrier (Circus cyaneus) is a medium-sized raptor found across North America, Europe, Asia, and North Africa. It is notable for its distinctive flight pattern, in which it flies low over open areas while hunting. The northern harrier shows reverse sexual dimorphism, meaning that females are larger than males. This allows the female to more easily capture prey to provision young, while the smaller male can perform acrobatic aerial displays during courtship. Understanding the size of the northern harrier provides insight into its ecology and adaptations.
Wingspan
The wingspan of the northern harrier can provide a sense of its overall size. Wingspan is measured as the distance from the tip of one wing to the tip of the other wing. For the northern harrier, published wingspans range from 90-110 cm for females and 80-95 cm for males (Svensson et al. 2009, Ferguson-Lees & Christie 2001). This makes the wingspan of female northern harriers on par with red-tailed hawks and male northern harrier wingspans similar to sharp-shinned hawks. The relatively long, broad wings allow the northern harrier to soar over open areas and utilize thermals during hunting.
Wingspan by Region
Some regional variations in northern harrier wingspan have been noted. In Europe, average female wingspans range from 100-110 cm while males average 90-100 cm (Cramp & Simmons 1980). In North America, average female wingspans are 95-105 cm and male wingspans measure 85-95 cm (Wheeler 2003). The marginally larger sizes reported in Europe may be attributed to Bergmann’s rule, where body size increases with latitude possibly due to thermoregulatory needs.
Body Length
Body length is another metric that provides perspective on the northern harrier’s overall size. Body length is typically measured from the tip of the beak to the tip of the tail. For the northern harrier, published body lengths range from 37-52 cm for females and 30-40 cm for males (del Hoyo et al. 1994, Ferguson-Lees & Christie 2001). This makes the northern harrier intermediate in length between a crow and red-tailed hawk. The slender body shape provides an aerodynamic profile that aids the harrier’s aerial agility.
Body Length by Region
As with wingspan, some minor regional variations in body length have been documented. In Europe, average female body lengths range from 45-52 cm while males measure 35-40 cm (Cramp & Simmons 1980). In North America, females average 40-48 cm in length and males measure 32-38 cm (Wheeler 2003). Again, the slightly larger sizes reported in Europe may reflect Bergmann’s rule.
Weight
Weight provides a three-dimensional perspective on the northern harrier’s overall size. Published weights for the northern harrier range from 300-700 g for females and 200-400 g for males (del Hoyo et al. 1994, Ferguson-Lees & Christie 2001). This makes the northern harrier slightly lighter than a red-tailed hawk. The relatively light weight facilitates the harrier’s buoyant, acrobatic flight technique.
Weight by Region
Regional variations in weight have been documented for the northern harrier. In Europe, average female weights range from 500-700 g while males weigh 250-400 g (Cramp & Simmons 1980). In North America, female weights average 400-600 g and males average 225-350 g (Wheeler 2003). As with other size metrics, the marginally heavier European harriers may reflect an ecogeographic pattern.
Wing Loading
Wing loading provides an index of the northern harrier’s flight capabilities. It is calculated by dividing body weight by wing area. The lower the wing loading, the greater the aerodynamic power and aerial agility. Northern harriers have wing loadings of 3.3-5 g/cm2 for females and 3.5-5.5 g/cm2 for males (Svensson et al. 2009). For comparison, sparrowhawks have wing loadings around 5 g/cm2 and peregrine falcons can exceed 10 g/cm2 (Chin et al. 2017). The light wing loading allows northern harriers to utilize an energetic, buoyant flight technique.
Metric | Females | Males |
---|---|---|
Wingspan (cm) | 100-110 | 90-100 |
Body Length (cm) | 45-52 | 35-40 |
Weight (g) | 500-700 | 250-400 |
Wing Loading (g/cm^2) | 3.3-5 | 3.5-5.5 |
Sexual Dimorphism
The northern harrier exhibits reverse sexual dimorphism, where females average up to 1.3x larger than males across linear dimensions (Hamerstrom & Hamerstrom 1973). This places the northern harrier among the most extremely dimorphic birds of prey, rivaling accipiters. The degree of dimorphism generally increases with latitude in accordance with Bergmann’s rule (Andersson & Norberg 1981). The larger size of females is an adaptation to allow them to more effectively capture and deliver prey to nestlings.
Hypotheses for Dimorphism
Several nonexclusive hypotheses may explain the evolution of reversed sexual dimorphism in northern harriers (Simmons 2000):
Female Provisioning of Young
Larger females are better able to capture prey and provision nestlings, conferring a selective advantage. Smaller males are more agile and better able to avoid predation during aerial displays.
Migratory Divide
Smaller males may be better adapted for migration, allowing an earlier arrival on breeding grounds to establish territories. Larger resident females can better contend with predators and winter weather.
Intrasexual Competition
Smaller males avoid competing over prey with larger females during the breeding season, reducing conflict over resources.
Growth Rate
The growth rate of northern harrier nestlings provides insight into how they attain adult sizes. Nestlings increase in weight from about 30 g at hatching to 700-800 g just prior to fledging, gaining about 30-40 g per day at peak growth around 20 days of age (Hamerstrom & Hamerstrom 1973). Linear growth of tarsi and culmens follows an S-shaped curve, slowing after about 2 weeks. Nestling wing lengths grow nearly linearly at about 2 cm per week, reaching adult lengths by fledging (Terres 1980). So northern harriers follow altricial growth patterns but attain mature sizes rapidly by fledging.
Factors Influencing Growth
Several factors can influence nestling growth rates (Simmons 2000):
Prey Availability
Abundant prey enables higher provisioning rates and faster growth.
Brood Size
Larger broods show slower growth due to competition for food.
Hatch Order
Earlier hatchlings tend to outcompete later hatchlings for food.
Parasitism
Blood-feeding ectoparasites can retard growth.
Sex Determination
The distinct size dimorphism between male and female northern harriers is apparent soon after hatching. Nestlings can be sexed based on size by about 2 weeks of age (Hamerstrom & Hamerstrom 1973). Foot span is one reliable indicator, with females exceeding 70 mm by this age while males remain under 65 mm. However, molecular techniques allow definitive sexing from hatching based on DNA (Fridolfsson & Ellegren 1999). Cell samples can be collected by minimally invasive methods like blood draws or buccal swabs. Chromosomal analysis or PCR can then identify sex based on the presence of W and Z sex chromosomes.
Significance of Early Sexing
Knowing the sexes of nestling northern harriers early is important for several reasons:
Understanding Growth
Sex-specific growth rates and sibling competition can be analyzed.
Predicting Size
Sex allows prediction of adult morphological traits like size and wing loading.
Facilitating Marking
Sex-specific markers and tracking devices can be deployed.
Managing Populations
Sex ratios provide insight into demographics and breeding dynamics.
Conclusion
In summary, the northern harrier is a medium-sized raptor characterized by distinct reversed sexual dimorphism in which the female is up to 1.3x larger than the male across linear dimensions. Average wingspans range from 90-110 cm, body lengths span 30-52 cm, and weights vary from 200-700 g. The light wing loading enables the harrier’s signature buoyant flight technique. Rapid juvenile growth results in adult size by fledging. Definitive sexing is possible from hatching based on DNA analysis, providing insight into development, morphology, and demography. Understanding the size dimorphism and growth patterns of the northern harrier provides perspective on its ecology as a unique, adaptable raptor.