Females: Larger than Males in many Species

By Dave Hanks

Predatory birds, many snakes, many fish, insects and spiders, the Blue Whale, and even the Spotted Hyena; are species where the female is the larger of the sexes. This is especially true with species that prey on vertebrates, or species that lay tremendous amounts of eggs. Obtaining food for herbivores is relatively easy, as long as the vegetation has not been denuded.

Natural selection has favored bigger females in many species, as it is a fecundity advantage. It takes more energy to produce eggs than to produce sperm. This requires the female to carry more weight, and the female must be larger to carry the extra weight that is required for that energy. Also, she is on the nest much of the time. Males are smaller because there is less demand on them to care for the young, and smaller sized males are more agile, and therefore more maneuverable when pursuing prey.

In predatory birds, such as hawks and eagles, the size difference reduces the competition for food between mated pairs. The bigger female will take bigger prey, which the male has trouble capturing. This division of what’s hunted puts less stress on the various prey species.

The NORTHERN GOSHAWK is a large falcon (21 to 26 inches) – larger than its cousins the Sharp-Shinned and Cooper’s hawks. It is a northern species that is not common. It mostly preys on larger birds (even ducks), but will take squirrels and hares. It is a resident of mixed coniferous woodlands, where it fiercely defends its territory and nest. This bird is a “home body” and rarely migrates, except from extremely cold regions.

We were most surprised to see one in our big maple tree. It had killed a Collared Dove. Feathers were floating down, as it plucked them out in preparation to eat its catch. We had only seen this bird (briefly) on two previous occasions.

(By the road leading down into Teddy Roosevelt Park, North Dakota)