On Thursday morning, Spanish coastguards said they had rescued 10 men from sub-Saharan Africa in a rickety boat off Tarifa in southern Spain. Overall, the numbers of migrants crossing the Mediterranean have dropped dramatically this year, mostly because of the rapid decline in the number arriving in places like Greece and Cyprus.
While migrants often camp in crowded tent ghettos upon arrival, a great many disperse throughout Western Europe over a period of time.
"It's possible that Spain will outperform Greece this year", Mr Millman said.
The International Organization for Migration (IOM) - incorporated into the United Nations system in late 2016 - says the number of migrants arriving in Spain is three times higher than it was this time previous year.
The Spanish Red Cross, whose workers are among the first to tend to migrants when they arrive, says it has treated double the number of sea-arriving migrants in southern Spain during the first since six months of 2017 compared to a year earlier.
Many of those opting for the longer route are from west African countries including Senegal, Gambia, Guinea and Ivory Coast.
'Whereas in Spain, the strategy is smaller craft hoping to come in undetected, and undoubtedly some do'.
Spain says around 700 migrants have tried to storm the border crossing between Spain's North African enclave of Ceuta and Morocco, but none managed to make it across.
Both cities are the EU's only land borders with Africa. It seems like it is safer to go through Morocco to Spain than through Libya.
Faced with the number of recent entry attempts, authorities in Ceuta have made a decision to close the border for trade for over a week, to allow guards to better man the fence.
The teenagers will be taken to migrant reception centres, where they can apply for asylum in Spain.
More than 1,600 people have been rescued off the coast of Cádiz since January.