The number of birds migrating south from chilly North America to warmer South America this time of year has dropped sharply compared to 2014, probably because of climate change, an environmental group said Friday.

"Climate change must be having an effect because right now we have more extreme storms and this is affecting the conditions they require" to make the long trip, said Rosabel Miro, director of Audubon, a non-governmental association in Panama that counts birds passing through Central America.

Miro said the number of birds tallied over the past seven weeks was 18 percent lower than for the same period in 2014 -- though she stressed that "last year was an extraordinary year" in terms of migratory numbers.

Audubon's counters on a hilltop overlooking the Panama canal totted up 2.6 million birds passing overhead. Last year the annual migration figure was 3.2 million.

The Americas this year are feeling the impact of El Nino, a cyclical warming of the Pacific Ocean that causes extreme weather patterns impacting temperatures, precipitation and increasing the force of tropical storms.

Scientists say the current cycle is the most intense in more than 15 years. However there is as yet no scientific consensus that El Nino is a consequence of climate change.

Audubon, which has been counting the migrations for the past 11 years, says the birds need to fly on thermal currents created on sunny days to soar along so many thousands of kilometers (miles). Too many stormy days affect their flight.