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I hope, dear reader, that you’ve had a great spring migration so far. Depending on where you live, the second week in May is generally either right in the thick of the good stuff or even just a bit post-peak. Regardless, these are good times.

But as you might have noticed by now, even in the midst of migration season, not every morning is a great morning. Some days you’ll get out there ready to swat warblers away like mosquitos only to find almost nothing around. Other mornings your neck will hurt from all the swinging and craning.

So what gives? Why are some migration mornings so good and others, well, not? Check the weather.

Let’s think about the marvel that is migration for a quick minute. Each spring millions and millions of songbirds need to make the trip from wintering grounds in Central and South America to their breeding grounds in North America. The total distance traveled varies depending on where the bird is coming from or going, but it’s at least a couple thousand miles no matter what. I can barely walk up a flight of stairs without getting winded, but millions of tiny birds fly from Brazil to Canada. It’s absolutely incredible.

It’s also dangerous. Birds traveling so far—often over open ocean with nothing to perch on in an emergency landing—have little margin for error or exhaustion. They need to save every bit of energy they’ve stored up during the winter. In fact, certain songbirds go through a host of physical changes before embarking on their journey, including molting into fresh feathers, increasing the oxygen capacity in their blood, and just totally pigging out until their body weight nearly doubles. (That last part doesn’t sound so bad.)

But even with those physical changes and all that hard-earned body fat, the birds still need some luck to help them get to their breeding grounds without interruption. That luck, good and bad, often takes the form of the weather patterns the birds encounter. Weather can delay, detour, or even speed up migrating songbirds, and the prepared birder can look at the forecast to get a pretty good sense of when birds are moving and where they might show up.

As you can imagine, wind speeds are an important factor. There’s a pretty simple rule here: It’s easier for birds to migrate with a tailwind than a headwind. During spring migration, a period of sustained north wind will keep birds on the ground for days or even weeks. (Quick note: I always used to get this confused, but "north wind” means the wind is coming from the north, not blowing northward. Got it?) A steady north wind over the Gulf of Mexico will keep birds grounded on the Yucatan Peninsula, for example, depriving American birders of those migrants until more favorable flight conditions permit the birds to head off.

Lots of weather sites can help you with wind forecasts, but the most elegant way to check wind speed and direction in your area is with this . Birds are most likely to migrate when they have a moderate tailwind; anything over, say, 25 mph is still pretty dangerous for these little birds, even if it’s pushing them in the right direction.

The other major weather variable birds encounter is precipitation. No one likes being stuck in the rain, including migrating birds, who’ll stay put when it’s storming. You don’t need me to tell you that birding during a rainy day isn’t the best idea, but what you might not know is that sometimes foul weather can produce the most incredible migration phenomenon of all: fallouts.

When a whole lot of migrating birds suddenly encounter bad weather, they have no choice but to land as soon as they can. Where birds only have a small patch of territory to choose from, it can produce incredible concentrations of birds—mythic fallouts that birders speak about in reverent whispers. Fallouts occur with some regularity along the Gulf coast, the first piece of land many struggling migrants reach. These at the legendary High Island, a migrant magnet on the coast of Texas, gives an idea of the sensory overload: orioles hanging like ripe oranges from a tree, puddles of Indigo Buntings, and colorful warblers sprinkling the ground. The most insane set of fallout photos I’ve seen are from Machias Seal Island, Maine, in May 2011. The conservationist in me gets a bit sad seeing so many birds tired and hungry, but the birder in me says, "MAMMA MIA, THINK OF MY LIFELIST"!

Whether you’ll ever see a fallout as spectacular as those (I never have), good migration mornings are fairly easy to predict. The best days will be the ones where lots of birds have moved overnight (i.e. a nice south wind with clear skies) but a shift in wind direction toward dawn or a line of fog or rain grounds them en masse. Those living just to the south of a line of precipitation are best positioned for a heavy showing of birds in the morning. Conversely, if your area or areas to the south of you have seen sustained north winds or lots of precipitation, those beautiful flocks of warblers and other songbirds are likely to be stuck, and the birding around you could be pretty quiet.

For example, let me tell you about the Big Day I just attempted in Mississippi back on April 24 (). We were hoping for a nice migration along the Mississippi coast, and by the middle of the week leading up to the attempt, things were looking interesting. From the forecast, we could see south winds coming out of the Yucatan, which encourage migrants to take flight over the Gulf. What we needed was some precipitation or a shift in wind direction to put birds on the ground, right where we could easily spot them. 

Birders on the w. Gulf should keep an eye on the forecast—possible fallout setup on Sat afternoon: N winds over TX but S over the Gulf.

— BirdCast–Cornell Lab (@DrBirdCast)

We almost got it. As you can see in the above tweet and map from the best bird-migration forecasters in the game (we’ll get to them in a minute), there was some precipitation and change in wind direction—only it occurred over the coast of Texas and western Louisiana, not where we were in Mississippi. (Wind direction in that map is indicated by the music note-looking marks, which point in the direction the wind is moving.) Birds cruised on the south wind over the Gulf but then were knocked down into great birding mornings in places like , and . As you can see in the map, the winds stayed southerly over the Mississippi coast, and migrant birds just kept sailing north, somewhere right over our heads. We barely saw any migrants at all on our Big Day.

You can forecast your own migration if you want. For example, say you felt like taking a personal day from work to do some birding, but you weren’t sure which day to choose. Well, you could head over to a weather site like the and start looking at the week ahead. You want to avoid days where it’ll be pouring on you, of course, and also days with heavy winds from any direction. Just rule those out. Look for a day, if possible, preceded by some calm days or light south winds, but where conditions might shift overnight to northerly winds, or where there is precipitation or clouds to your north. It’ll take some sleuthing, but it’ll work.

Or you can just let some smart people do all the hard work for you and check the . The Cornell Lab of Ornithology produces forecasts during spring and fall migration to let birders know what they should be expecting for weather and species in their area. The page is updated every week (sometimes more frequently, when things are getting exciting) and is an excellent companion for the birder looking to the week ahead. I’ve found the species information particularly interesting: Individual species generally migrate within certain windows, and it’s helpful to know what to keep a particular eye or ear out for as you leave for your favorite morning migration spot.

Right now, as you’re reading this, millions of tiny birds are still working their way over the open water, past hungry hawks and cats, around tall buildings, and up the continent towards their breeding grounds. If the timing is right and the weather cooperates, you might have a chance to see them on their way. Wish them well, and enjoy the rest of spring migration!