Birds sing for several reasons
Mainly birds sing to attract a mate and declare their territory. Why they sing the same song though is another thing. It has been observed and hypothesized that the singing birds tend to be male, and are crooning to guard their territory and attract a mate. Within the realm of territory, a bird sings to declare “Stay off my lawn!”
Other animals mark their territory by urinating or by rubbing their scent on everything. We humans even mark areas with fences and borders. But “birds don’t do it that way, they will sing and potentially fight to keep their territory free of competition.
And if that song attracts a mate in the process, more power to them. Granted, there are around 10,000 species of birds in the world, and every species is different, but oftentimes the female is the one that picks the mate, not vice versa.
The males’ songs basically express, “Hey, females, if you’re passing by, listen to me because I’ve got a beautiful song! I’m a healthy strapping male! You should stop by and check me out!”
This ritual is costly for both sexes, in that it uses lots of energy. While singing, the male can’t look for food, and his calls make him more visible to predators. For females, it takes a lot of energy to lay eggs and raise young, so she wants to be sure she chooses the right mate.
Once the chicks hatch, people might hear another repetitive bird note. When chicks are hungry, they often call to their parents for food — saying something along the lines of, “Mom, I’m hungry! I’m over here!”.
If this call sounds annoying, that’s because it’s meant to be, she said. Just like a siren, the call is designed to get attention, and fast.
A really interesting fact
The same birds from different regions actually have slightly different songs. It usually happens if you have big physical barriers, like mountains”. “Over time, their song changes just a slightly.”
So if you’re on vacation and hear a slightly different chickadee-dee-dee song, know that it’s likely the same species, but with a different dialect.