We've arrived at the portion of the school year in which it is dark when I walk the Free-Ride offspring home. This means that a good bit of our observation during the walk depends on our ears instead of our eyes.
Elder offspring: (in response to the high-pitched screech-y song of a bird-like shadow swooping above us) I wonder if that was a nighthawk.
Dr. Free-Ride: I don't know. I'm no kind of expert on bird songs. I'm not even sure how I'd tell a bird from a bat when it's this dark.
Younger offspring: A bat is a mammal.
Dr. Free-Ride: I know that a bat is a mammal. But, in the dark, I'm not sure how I'd determine simply from how an animal sounds whether it's a mammal or a bird.
Elder offspring: You know that bats are the only mammals that fly.
Dr. Free-Ride: Yes, in fact I did know that. Flying squirrels don't count, huh?
Elder offspring: Nope. Flying squirrels glide.
Younger offspring: Bats, and also angels.
Dr. Free-Ride: Please tell me you haven't been talking about angels in science class.
Younger offspring: We haven't.
Dr. Free-Ride: That's a relief. So, tell me more about nighthawks.
Elder offspring: Well, they're nocturnal, and they like to eat insects.
Younger offspring: Like bats.
Elder offspring: And sometimes they hover near streetlights with their beaks wide open and they can eat lots of insects at once.
Dr. Free-Ride: Hee! Because the insects are attracted to the streetlights. That's pretty clever.
Elder offspring: And some of them live in the desert, and sometimes they'll open their beaks not to catch bugs but to release heat.
Dr. Free-Ride: Ah, thermoregulation!
Younger offspring: Like when dogs pant?
Elder offspring: Yeah. Nighthawks are also ground-nesters. Like killdeers.
Dr. Free-Ride: I assume a killdeer is a bird. I hope it doesn't actually kill deer.
Elder offspring: I think it's just a name.
Younger offspring: Rats and squirrels nest.
Elder offspring: But bats don't. I think they hang upside down from trees and in caves.
Younger offspring: Do mares nest?
Dr. Free-Ride: No, "mares' nest" is just an expression I use for really tangled hair.
Younger offspring: Oh.
Dr. Free-Ride: This information about the nighthawk is interesting. It sounds like it occupies a similar niche to the bat.
Younger offspring: What?
Elder offspring: They live in the same places and eat the same things and come out at the same time and they both fly.
Younger offspring: Oh.
Dr. Free-Ride: But I'm still not clear about how you're so sure that the thing that swooped above us couldn't have been a bat.
Elder offspring: Bats don't screech like that. They make sounds that are too high pitched for our ears to hear and use them for echolocation.
Dr. Free-Ride: Yeah, I know about echolocation. But for some reason I was pretty sure that bats made screeches that could be heard with human ears.
Elder offspring: I don't think bats' eyes are very good, and they need to be able to find their way around in the dark, so their hearing has to be really sensitive.
Younger offspring: Dogs have really sensitive hearing, too. Also, really good sense of smell.
Elder offspring: Yeah, so like dogs can hear sounds that are too high pitched for us to hear, so can bats.
Dr. Free-Ride: But we can hear a dog's barks.
Younger offspring: Dogs don't use echolocation.
Dr. Free-Ride: I'm not saying that dogs echolocate. I'm just saying that the fact of a creature being able to hear really high pitched sounds that we can't hear does not mean that that creature only makes really high pitched sounds that we can't hear.
Elder offspring: Hmm. I think it would hurt a dog's ears to have really high pitched barks.
Dr. Free-Ride: So why wouldn't it hurt a bat's ears to make really high pitched screeches?
Younger offspring: Maybe it does hurt them.
Elder offspring: I bet when they're flying the screeches travel a long way before they bounce back, and by the time the sound gets back to the bats it's not so loud to them.
Dr. Free-Ride: I still suspect that we can hear at least some of the noise a bat makes when it's screeching. I'm pretty sure I've actually heard a bat screech.
Younger offspring: Maybe you just saw it in a cartoon.
Dr. Free-Ride: I can't entirely rule out that possibility.
A bat using echolocation inside a house. Not our house, though.
The sprogs might get some fun (and learn some interesting things -- like the why killdeer eggs are bigger than robins', even though the birds are about the same size) by looking up more info on the killdeer. Yes, it's a ground nesting bird. Their name comes from one of their frequent calls.
Adult killdeer are known for their "broken wing act," used to lead predators (and curious humans) away from their vulnerable nests.
Killdeer are small birds that look a lot like Plovers, if that means anything to you; they're about the size of robins, but not the same shape. If I remember correctly, they're called killdeer because their call sounds somewhat like someone crying "kill deer." Fun fact (because I really like killdeer): when a suspected predator comes near their nests, the parents will pretend to be wounded to distract it.
Smaller bats use echolocation, the sound starts within the human hearing range. They like cave type places to rest and sometimes can be found in arenas and auditoriums (and have been known to disrupt NBA games and other events). Larger bats eat fruit and nectar and don't use echolocation.
Nighthawks (known in some confused areas as "bull bats") have white patches in the middle of their wings. In dusky to dark conditions, you can usually make those patches out. Once you get a sense of what they look like, you'll learn to recognize their call.
Other fun nighthawk facts: They have specially modified hair around their mouths that make them better at catching insects, and their family name (Caprimulgidae) translates to goatsuckers. They also have odd reflective coatings on the backs of their eyes.
At this time of the year in Northern California nighthawks should be long gone- they would have migrated south a few months ago. Barn Owls, on the other hand, are year round presences in many areas and make a very distinctive screeching call while they fly. There are mp3s of the call on this page. Could that have been what you have heard?
Incidentally, I remember watching Barn Owls feeding on moths in the floodlights at my little league grounds when I was growing up, just like nighthawks.
Something more for the sprogs.
When cats, deer, even gorillas prepare a place to give birth this is known as "nesting". Humans also engage in nesting. Such as when mom was expecting you. All that stuff mom and dad got for your first year of life? That was nesting.
We have little brown bats, and I've never heard them screech. I have heard one confused and recently defenestrated bat make upset squeaking noises, but nothing like a screech.
Bats have social vocalizations well within human hearing, they sound like mice (or at least mouse-sized bats do). I would guess that bats are about as capable of screeching as any similar sized rodent.
Owls and other raptors have screeching to a fine art. As do many humans when confronted with a harmless little bat.