Migration is an amazing event in the bird world.  We are fortunate to be in an area where so many birds pass through on their way from their wintering grounds to their breeding grounds.  Some stop to nest here in Iowa, while others keep going, making Iowa a true flyover state.  One of the tantalizing things about migration is that it is so ephemeral.  Birds may be here for a few hours to a few weeks to re-fuel on their way north.  Often their fate lies with the weather--the wind direction and speed, rainfall, and availability of food.  Most songbirds and shorebirds make their long flights at night, and spend the day searching for food.  They are less prone to predation that way, and often the wind is calmer.  They take off once it's dark and as the sky begins to brighten, they look for a place to spend the day.  They prefer a tailwind or no wind at all, and a north wind or rainstorm may keep them in place until the weather changes.  You can actually see birds taking off and landing on weather radar.  If there aren't a lot of clouds in the sky, the colors you see may represent birds.  Go to http://radar.weather.gov/radar.php?rid=DMX&product=N0Z&overlay=11101111&loop=yes for Des Moines area bird migration.   If you check it after sunset or soon after sunrise, you may see an expanding circle in the evening (birds taking off) or a contracting circle in the morning (birds landing).  As birds go up or down they break the radar beam and show up as blips.  The more birds there are migrating, the more intense the color will be.  For a national view, look at http://radar.weather.gov/Conus/index_loop.php .  This will show birds taking off or landing as the sun rises and sets in their part of the country.  It can be fascinating, and give you an idea of how many birds are out there migrating.