(CNN) Nightingales may be less likely to survive their annual migration because climate change is causing the songbirds to evolve shorter wings, new research has found.

Scientists discovered that over the past two decades, the wingspan of nightingales from central Spain has decreased.

They highlighted natural selection caused by rising average temperatures in the region as the likely cause of the trend.

Nightingales flock to sub-Saharan Africa every year, but the researchers from the Complutense University of Madrid warned that the birds find it harder to return if their wingspans are stunted. Their study was published Wednesday in the journal The Auk: Ornithological Advances.

"There is much evidence that climate change is having an effect on migratory birds, changing their arrival and laying dates and their physical features over the last few decades," lead author Carolina Remacha said in a statement.

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