Hello everyone! Welcome to Katie’s last summer blog post! This post is about the differences in female and male raptors, because it is often different than people expect. Most people know that in birds, the males have bright flashy colors and sometimes very elaborate feather displays, while the females are drab grays and browns.
Almost all of the raptors, with a few exceptions, do not follow this rule – most of the time, females and males are the same color!
So how are we supposed to tell the difference? Did you know that the anatomy that tells us male or female is all inside a bird’s body? That means that the only way to know for sure if you are seeing a female or a male is to do a surgery, or a blood test, or if you see them laying eggs… So how can you tell if you see a pair in the wild? The best way is to look at body size – but who is bigger?
Most people also know that in many animals, the males are larger than the females – and this is true across a majority of mammal, reptile, and bird species. But those awesome raptors are different in this too – in most of the species, females can be 30-50% bigger than the males! This is called RSD, or reverse sexual dimorphism, and is found across almost all of the raptors as well as just a few other species of birds, fishes, mammals, and reptiles. Let’s look at some examples of the size differences!
Some raptors show more differences than others, and scientists are not sure why. Most of the New World Vultures (Turkey Vultures, Black Vultures)
are usually very similar in size, many buteos (soaring hawks) have only slight weight differences, and some accipiters and falcons have huge differences! Red-Tailed Hawks, one of the most common hawks in North America, typically weigh between 1.5-3 pounds, typically with males averaging 2.2 and females averaging 2.7 pounds. So only about half a pound bigger on average, but with the potential to be 2 times bigger!
Meanwhile, it’s not unusual for a Female Sharp-Shinned Hawk to weigh easily 2 times more than her mate, with a potential 65% difference!
Now, no one knows for sure why there is often such a large (and what we might consider backwards) size difference, but there are a few theories that could explain why. One is that a large body is an advantage for the female, so she can defend the nest easier and have more resources with which to lay eggs (which takes a lot of calcium and energy!). Also, if the male is smaller, he is likely to be more agile and fast, so he can possibly catch more food to bring back to feed the mom and the chicks. In addition, when both male and female are in the same area to raise chicks, the can use their different sizes to hunt different-sized prey. This way, they are not competing with each other for food, and they can bring a greater variety of food back to the nest for the chicks. Finally, some scientists think that in some species females may compete with each other for males, so a larger size may be a benefit there too. Whatever the reason, when you next watch a nest-cam or see a nest for yourself with one raptor bigger than the other, know that you’re most likely looking at a big female and a small male!
This whole topic is super fun to talk to our visitors about, and it makes our jobs interesting, too. That’s because we really do not know for sure whether some of our birds are male or female! As I mentioned earlier, unless a raptor fits into the exceptions (like Salem the American Kestrel – we know he is a boy because he has the gray-blue on his wings), the only way to know for sure is to do a surgery or blood test…or unless they lay eggs, which our Golden Eagle Kateri has done for a couple of years now! For everyone else, we have had to make our guess based on their size! As with anything, there is always a chance for mistakes, which is why we thought our Bald Eagle, Jade, was a girl at the beginning of the summer – oops! When Jade came to us, he looked really big, and so we guessed that he was a female. But then it came time to weigh him for a health check, and he weighs only 8 pounds – and Bald Eagles range between 8-13 pounds. So now we are 99% positive that Jade is a male, and we know next time to get a weight first.. Regardless of whether they are female or male, all of our birds are excellent ambassadors for their species!
I hope everyone has enjoyed the blogs I wrote this summer! I had such a great time coming back and seeing the staff and volunteers and the birds of this amazing raptor experience, and I am definitely going to miss everyone again when I leave! Make sure you come and see all of these incredible birds and people!!