Summer typically lasts from June to September.
Summer officially begins on the Summer Solstice, which differs each year although the recognized date is June 21st. Summer officially ends on the September Equinox (also the kickoff for Fall), which also differs each year although the recognized date is September 22nd.
in the 30s and lower
For the same reason that the north pole is cold and north America is hot. The earth is hottest in the middle (the equator) and gets colder the further you get from the middle. It is hottest between the Tropic of Cancer and the Tropic of Capricorn which are the furthest points (north and south) where the sun is directly above it. In addition to rotating on its axis to make days and nights, the earth also tilts to create summer and winter. North of the equator summer occurs in the months of July and August while in the southern hemisphere summer is December and January.
spring & summer
spring,summer,fall,winter
August is in mid Summer.
North America: Arctic/Subarcatic, Very Cold, Cold, Marine, Mixed-Humid, Hot-Humid, Mixed-Dry, Hot-Dry.
Near the North Pole and on top of mountains. Even in the summer, the region near the North Pole in Canada and Alaska, and on the tallest mountains, it still is somewhat cold in the summer.
You answered your own question actually. They migrate because it's cold. Many migratory birds fly to North America for our summer and to South America for their summer. That way they're always in summer. (When it's winter in North America, it's summer in South America and vice versa.) So when it starts getting cold, that's their signal to start migrating to the warmer climate.
still summer
If it is summer in South America then it is winter in north america
If it is summer in South America then it is winter in north america
If it is summer in South America then it is winter in north america
when it summer in north america its winter in south america and when its winter in north america its summer in south america.
If it is summer in South America then it is winter in north america
It is Winter in Zimbabwe when North America has its summer.
It's very temperate. Cold and snowy in winter and warm and sunny in the summer.
In July, North America is in summer.
If it is summer in South America then it is winter in north america