The best and most popular birding spot in DC during spring and fall migration is Rock Creek Park. Sure, some birders will try to play the hipster card and say "oh yeah Rock Creek is played out I'll be at Battery Kemble or Fort Dupont or whatever" but that just means they'll be stuck in traffic when ceruleans are reported on the Ridge.
Rock Creek is great but not everyone knows how to do it right. Like, although the park is huge and there are (probably) a lot of great birding spots, the vast majority of us focus our attention on the Maintenance Yard and the Equitation Field. What the heck do those words mean, you ask? Let me show you.
Alright, though the park is the whole green thing there, we're really only focused on what's in the red circle (and west of the Creek, at that).
Why so limited? Well, in general , RCP is the biggest green space in the city, a natural target/trap for migrant birds. But why limited to this circle in particular? A couple reasons. First, these are some of the highest points of the park, so it's sort of a natural landmark for birds. Second, there are some open areas here that provide views into the canopies that aren't found in many other parts of the park.
So, here's a basic layout.
The 1 to 5 path is a common way to bird RCP in the morning: park at and bird the Maintenance Yard (1 and 2), walk down the Ridge (3) to the Picnic Area (4), and continue down the Ridge to the Equitation Field (5). Now, you may not have time to hit all the spots, and there's nothing wrong with picking one spot and sticking there all morning. Some people like to start at the Equitation Field, or on the Ridge, but it's all good and it's all up to you. At the very least, if you hear another birder talking about some awesome bird they saw at the Picnic Area you'll know what they're talking about.
I typically park at the Maintenance Yard, which is just a fantastic place to bird. Here's a map.
So, the first thing to know as visitor to the Maintenance Yard for the first time is that it feels a little weird to get back there. Park your car in the lot and walk towards the big metal light tower. Follow the path at the base of the tower along the fence line and towards the back of the facility, the path is pretty well worn, but it's narrow. Do not try to enter the NPS facility! Here's a garbage screenshot from Google Street View:
Cool. Keep your eyes open along the walk - there are some good opportunities to see forest birds like thrushes and, like, black-throated blue warblers, but the real action starts when you emerge past the fence. Gawk at but generally ignore all the cool old stonework that the NPS is storing back there, it's cool but there are never any migrants. Head down to the dirt.
Once you're in the open, the place is yours. The reason why this area is so beloved is not just because it's a great open space to see migrants, but it's because there's no one but birders back there. No joggers or walkers, no dogs, no horses and their turds, no cars, no nothing. It is a safe space to be a birder.
Most people hang out by the yellow X on the map and scan the trees in front of them. There is habitat for all kinds of birds here: tall treetops, tangled vines, brush, flowering trees and berries, you name it. Enjoy.
Folks also check the grown-over dirt piles in the back. That habitat is good for skulkers and seed-eaters, like sparrows and, in fall, Connecticut Warblers. Also check the tall trees along the edges, there's often good stuff in there. Good? Let's continue.
Back to the map with the numbers on it, here it is again:
Go out past your car and walk down Ridge Road to the intersection (be careful). Follow the Western Ridge Trail and start heading south. Look out for joggers and walkers and horsecrap on this trail because multiple use and all that. You're on the Ridge (only 14 checklists?! That's because most people don't do separate checklists as they walk, which is understandable...there isn't even a separate hotspot for the Equitation Field, and the hotspot for the Ridge conflates it with the Picnic Area).
Anyway, look for birds. Just look for birds all over. Look for them at the Picnic Area (4 on the map), on both the east and west sides.
And then walk down to the Equitation Field (5) and look for them there. The big fenced in area is for horseback riding, I guess, but I don't know really what its used for. Don't look there look in the trees, especially on the west side to catch the morning light. Also check the trees (big and small) standing alone on the mowed grass. For whatever reason this area seems to be the best spot for Ceruleans and Golden-winged Warblers when they're around, and I've had my only Worm-eating and Hooded Warblers here.
Get out there! Enjoy the spectacle of Rock Creek Park!