Migrants hoping to reach the United States camp out at a bus station in Oaxaca state, southern Mexico. "I've been sleeping on the street for three days," says one Venezuelan, who adds she has relied on aid from Mexicans to get by while she waits to travel north. A spike in the number of people -- mostly from Central America and Venezuela -- who are seeking to cross Mexico into the United States, is causing a political headache for US President Joe Biden and fresh costs for Mexico. IMAGES AND SOUNDBITES