The straights of Gibraltar are one of the narrowest crossing points between Africa and Europe and as such are a major route for migrants. The other main routes are from Tunisia to Scilly and thus southern Italy and the third route is up through Turkey and across into Greece.
The reasons birds choose these routes depend on the type of bird. Large birds like Eagles, Vultures and Storks like to soar, they are not that keen on flapping their wings for hours on end. So they find a thermal and use it to gain height, going round and round ( like the joggers in Park life) and when they have got up nice and high they soar off on their journey and will then gradually loose height. Before they hit the ground, they find another thermal and regain height , and then they are off again, and so it continues. However you do not get thermals over water and so, they have to select a narrow crossing point. Then they gain height close to the coast and then soar off across the water and hopefully reach the other shore and then find another thermal before they run out of altitude.
Smaller birds are more able to flap and keep going but obviously they like a bit of a break every so often. They will use ships for this or oil rigs but favourite is an island, so they prefer not to migrate across hundreds of miles of water. They will also make use of prevailing winds. The record is held by the bar-tailed godwit, a plump shorebird with a recurved bill, godwits can fly as many as 7,242 miles without stopping in their annual autumn migration from Alaska to New Zealand.
This is excessive but many small birds can fly non stop for three days and with a tail wind will achieve huge distances. Some birds will get up really high and use the jet stream like Swans and Geese.
There has obviously been some recent movement of people into Europe via this crossing as there are several bits of graffiti suggesting people are welcome. I refer you to this report for more information. spanishnewstoday.