Why do birds fly so low to the ground when crossing roads? Whenever I’m driving they fly right in front of my car when they could fly at any other height?


Why do birds fly so low to the ground when crossing roads? Whenever I’m driving they fly right in front of my car when they could fly at any other height?

In: Biology


I really think that’s their way of playing – simple as that. Just messing around and having fun.

Some species of birds inhabit grasslands and low shrubs and don’t really fly that high unless they’re migrating.

They’ll fly low across the ground to different feeding or nesting areas and they do that throughout their territory, including across roads.

I can say this for ducks and geese at least from years of observation. Birds fly at a height dependent on how far they intend to go. For short distances they may only fly 10-50′ high. If they are going to the golf course three miles away, they fly much higher to observe the terrain and follow the roads and so forth. If they were migrating south, they would fly even much higher. Most birds are not flying far so they fly minimum height. Yes, birds can crash into things that are stationary too, not just vehicles.

If a bird wants to fly from one point to another, it takes a lot less energy if they “dip” down like this than if they try to maintain a perfectly level flight. This is because the drop at the start helps them to gain speed, and the climb at the end helps them to slow down.

I heard about a study that looked at Cliff Swallows, who used to be fairly rare, but was growing in population as we started building highways in their habitat (1983-2012). They relied on cliffs for nesting sites, but readily adopted to using overpasses instead.

An ornithologist studying them over a few decades started compared the wingspans and found that the wingspan of these birds was shortening. This gave them increased agility at the expense of range. This makes sense, since their nesting sites were much less rare, but you needed to be able to dodge traffic.

Birds have literally evolved in the last century to be able to bob in and out of traffic. This gives them an unnaturally protected habitat and access to the bounty of food source (roads create an “edge”, like you’d normally find in a clearing in the wood, where insects and quick growing plants flourish).


I’m sure that this extends beyond just wingspan but also reaction time. If you’ve ever seen a bird drop out of the sky onto a pile of seeds, you know they have incredible reaction times to begin with. They may seem close to your car to you – but I imagine they have a faster reaction time than you, and they get a bit more of a heads up, as they are probably very sensitive to changes in air current caused by your car (which also give added energy to move them out of the way).

I have been watching this a lot the past couple of years, I personally feel that the lift provided by vehicles saves them a lot of energy. The energy would dissipate at something like 1/E^2, so the closer they get the more lift provided. It would be like hitting the NO2 button.

Have you considered that you only notice the ones that fly in front of your car because you can see them and don’t notice all the ones flying over your car where you can’t see them? Explanations about being low to the ground to see the food and all that make sense, but there’s a lot of confirmation bias in this question.

I think also they aren’t afraid of cars. If you throw an aerodynamic object (like a Frisbee) at a moving car it gets sucked into the slipstream and doesn’t make impact.

I think it might be the same for birds.

Very very happy to be corrected