CALAIS, FRANCE / AFP – Thwarted at every turn in their efforts to reach Britain, migrants in northern France have once again changed tack, turning to flimsy dinghies to carry them across the Channel. Migrants in Calais have sneaked into trucks or trains, and even tried to walk through the undersea tunnel in increasingly risky — and sometimes deadly — attempts to reach Britain. However, tighter security has stymied these efforts and authorities have reported about a dozen operations to rescue migrants drifting in inflatable dinghies in the Channel since the start of the year. In 2015, there were none. The most recent rescue took place on June 11, when three Iranian migrants were saved from their sinking vessel. Four days earlier, three other migrants were rescued in similar circumstances. Pierre-Joachim Antona, spokesman for the local French maritime authority, said efforts to cross the Channel by boat were “no longer extraordinary or isolated. There have been rare cases in recent years of migrants drowning while trying to swim across the narrow sea, and an Afghan once built a raft with a bed sheet in a failed effort to drift to England. But the phenomenon is now a “real and lasting