Why migrating birds arrive in cities earlier than the wild

HostI was sitting out on my back step the other morning and I heard a songbird. It felt way too early for that kind of sound, especially with the ground still being so cold. It made me wonder if the birds are actually changing their plans or if I'm just losing track of the seasons.

GuestYou're definitely not imagining it. Birds that fly long distances for the winter are showing up in cities quite a bit sooner than they used to. What's really wild is that they reach the city streets days or even a full week before they land in the woods or on the farms just a few miles away.

HostA whole week is a huge gap for a bird that has been flying for thousands of miles. Is it just that they like the city life more?

GuestThey probably don't care much for the noise or the traffic, but the city has a few things they can't ignore. The biggest one is the heat. If you think about all the brick, the black roads, and the roofs, they all soak up the sun during the day. Then at night, they let that heat back out. It makes the city like a big, warm brick in a cold room. Even a few degrees of extra warmth can make the plants start to bud and the bugs start to crawl much earlier than they would in a damp, shaded forest.

HostI can see how that would help once they get here, but how do they know to show up early while they're still halfway across the world? The heat from the city surely doesn't reach all the way to the warm lands down south.

GuestNo, they don't know about the heat while they're still down south. Most birds start their flight based on how long the days are. Their bodies just know when the sun stays up long enough to signal it's time to move. But as they get closer, they start to feel those warm spots. Some researchers think the birds that happen to land in a city find plenty of food and a warm place to rest, so they stay put. The ones that go to the woods might find it's still too cold and have to wait or move more slowly. Over time, the ones that hit the city first are the ones that get the best nesting spots.

HostSo the city is kind of like a shortcut to spring. But it's not just the heat, right? Cities are bright all night long.

GuestIt really is the light too. All that fake light from street lamps and office buildings messes with their inner timers. Birds use the sun and the stars to find their way, but they also use the amount of light in the day to track the seasons. When a bird is in a city, that extra light can trick its brain into thinking the days are longer than they actually are. It speeds up their internal clock. Their bodies start getting ready for spring earlier because they think the sun is up, even when it's just a glowing billboard.

HostThat sounds like it could go wrong pretty fast. If they get here early because they're tricked by a light bulb, and then a big snowstorm hits, they're in search of food that's not there. It feels like they're taking a massive risk just to get a head start.

GuestYou're right to worry about that. It's a big gamble. If they show up and the trees haven't started making leaves yet, there won't be any bugs for them to eat. They call this a timing mismatch. The birds are on one schedule and the food is on another. If the birds get too far ahead of the bugs, they can starve. But for now, the city heat is actually helping the bugs and plants keep up, so the birds are winning the bet most of the time.

HostBut is this something the birds are learning? Like, does a bird remember that the city was warm last year and decide to head there sooner this time?

GuestIt's less about a single bird learning a lesson and more about which birds survive and have babies. If the early birds in the city get the best spots and raise the most young, then the next year, there are more birds that have that same drive to move fast. It's like the city is picking the winners. But there's another thing we shouldn't forget, and that's us. People in cities love to put out bird feeders. Having a steady pile of seeds waiting for you makes the risk of a late frost a lot less scary.

HostSo we're basically making the city a five star hotel with a heated pool and a free breakfast. It makes sense why they would rush to get here.

GuestIn a way, yes. But it's a weird kind of hotel where the lights never turn off and the walls are made of glass that they can't always see. We're changing the way they have lived for thousands of years just by being here and keeping our heaters on.

HostIt's strange to think that our porch lights and our driveways are reaching out and pulling birds across whole continents faster than they used to go.

GuestSome birds are now landing in city parks nearly a full week before their cousins arrive in the deep woods nearby.

HostThat robin on my porch is definitely ahead of the curve, even if the trees in the forest are still fast asleep.