Making a Mark At Harmony Hill: A Sharpie in the Forest
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A walk in the winter woods at Harmony Hill, like so many places, allows for the senses to reach farther into the trees. The leaves of the oaks, hickories, dogwoods, and hardwoods are messily scattered on the forest floor. The songs of birds are much softer and are mostly limited to simple, short calls. There are no cicadas or katydids to fill the background with constant sound. And the sun streams more cleanly through air not stagnated by the heat and humidity of summer. Instead, we can see movement of animals normally obscured by vegetation and we can hear small sounds usually dampened by the near endless activity of longer and warmer days. It is in this open and quiet backdrop that our attention was snatched from observing a beetle foraging along a recently downed snag by the sound of wings fluttering quickly and a speeding blur from the corner of our eye.
The sound of wings we heard were those of birds; chickadees, titmice, cardinals, and robins, that spotted the presence of the blur much earlier than we did. Their very lives depend on knowing when this bullet of an animal is around. And it was their alarm calls that let every other bird within earshot know danger was extremely close. The cause for their alarm was a Sharp-shinned Hawk (Accipiter striatus), an accomplished and highly evolved hunter stalking the interior of the forests of Harmony Hill.
This area sees two members of the genus, Accipiter, during various times of the year. The larger of the two species, the Coopers Hawk (Accipiter cooperii), is a year-round resident. The smaller, but just as active member of the genus, Sharp-shinned Hawks, often called “Sharpies” are only found during the winter.
Small does not in any way mean less capable or any less feared by other birds. The male Sharp-shinned Hawk is about the size of a Blue Jay and weighs in at about 3 ½ ounces; but it is adept at taking prey as large as Northern Flickers and American Robins. Female Sharp-shinned Hawks, as is often the case among many species of birds of prey, are considerably larger than males. For this species, they exhibit a size difference of about 30% larger. This phenomenon in which females are larger than males is known as reverse sexual dimorphism. And it seems that more actively a bird of prey species hunts, the more pronounced the reverse sexual dimorphism is.
In vultures, like the Turkey Vultures and Black Vultures found at Harmony Hill, the difference in size between the females and the males is negligible. That could be because vultures don’t have to do much more than look and smell for decaying animal material. In our larger hawks, like Red-tailed Hawks and Red-shouldered Hawks, who actively seek out and chase prey, the size difference is about 25%.
But, as much work as Red-tailed Hawks put in to catch squirrels, rabbits, and mice, they don’t pursue potential meals with the effort and zeal that Cooper’s Hawks and Sharp-shinned Hawks. You see, the Accipiter hawks are built for close quarters attacks and for chasing other birds through tight wooded areas. Larger Hawks often patrol wide open areas or perch where they can scan vast areas for prey, then take the chase to the prey. Sharp-shinned Hawks lurk in the limbs of trees in wooded areas, where prey is obscured by obstacles, vegetation, and structure. When they decide to attack, they know very well the chase is afoot and the ultimate game of tag is on. Dodging saplings, cornering on a dime, and zooming through narrow passages made by limbs, leaves, and vines, Sharp-shinned Hawks are as agile as they are stealthy.
Now, let’s dig into a natural history mystery! Why would female birds of prey be larger than males? And why would the level of activity placed into hunting be a factor in that reverse sexual dimorphism? It seems counter to what we learned about other groups of animals. In most mammals, the male is larger. That size difference is thought to be evidence of reproductive fitness, ability to provide, and ability to defend a territory. We see it in most primates, and we even see it in deer, and even bobcats. Why is it opposite in birds of prey?
The simple answer is, “We aren’t sure!” There are a couple of leading hypotheses, but nothing we know for certain. Perhaps it allows females to hunt for larger and different prey than the male, keeping them from competing against one another and more effectively providing for growing chicks. Perhaps it gives females greater physical reserves for better survival rates and to face the rigors of egg development. Perhaps it could be all of that and even more. The fact is, those reasons make perfect sense; but, we just don’t know. And, as you’ve heard us say before, not knowing is fine. The journey to more knowledge is begun by knowing and embracing what we don’t know. Mother Nature has many mysteries she has yet to reveal to us. We’ll keep looking to the plants and animals of Harmony Hill in search of the answers, reveling in our curiosity as we go!
Now that we’ve looked at the big picture of birds of prey and reverse sexual dimorphism, let’s tighten our focus to our spotlight species. In every way, Sharp-shinned Hawks are built for their jobs, surprise attacks and high-speed pursuits in forests. Looking at this week’s photos, it’s easy to see the physical adaptations they’ve evolved to excel at hunting other birds. The sleek, streamlined body grants them ability to slip through small openings. The short, rounded wings give them the speed necessary to chase, but also the uncanny maneuverability to make quick changes in flight angles. The long, striped tail acts like a feathered rudder, accentuating the body and wings to increase agility. And those incredibly sharp talons and beak deliver the killing blows to prey unfortunate enough to not be fast enough to escape.
That rudder tail is likely the reason for the species name of “striatus”, meaning striped. The genus name of “accipiter” refers to the Latin for “hawk”, as well as the Greek for “swift winged”. Both extremely fitting for our fast hawk with a prominent striped tail!
Make no mistake, the circle of life is real and there is very little warm and fuzzy about surviving day-to-day in nature. At Harmony Hill, we embrace that circle, for it is Mother Nature working her magical ways to seek balance. Predator/prey relationships are a keystone to the web of life and nature. Sharp-shinned Hawks are a fact of life in surviving winters at Harmony Hill for their favorite prey: other birds, and the occasional small mammal or insect. Birds; however, make up the largest percentage of their diet when compared to bats, rodents, and insects.
We don’t know what became of the hunt we caught the briefest glimpse of. But we know hawks like the Sharp-shinned are only successful about 20-25% of the time. For every successful hunt they have, there are at least 3 or 4 others in which they come up empty handed. Whether it obtained a meal or not, we were able to witness one of the coolest birds to grace the trees of Harmony Hill each winter. And we marvel at the many lessons one bird can teach us about size differences, hunting prowess, and the beauty of the struggle of life and death in these gorgeous woods we call home.