Birds breeding in northern Europe but wintering in Africa, south of the Sahara, are declining much more rapidly than resident species, and climate change is an important factor.
Migratory populations may be affected by environmental change at breeding grounds, along migration routes and in wintering areas.
Studies show that summer migrants now arrive earlier – by a week or more – than they did in the 1960s.
However, species may not be bringing forward their arrival enough to compensate for the even earlier emergence of favoured invertebrate prey, which could cause a decline in breeding success through a process known as ‘phenological mismatch’.
Research by Stuart Newson at the BTO has revealed that they also depart later, meaning that they remain in northern Europe for longer.
The impact of a changing climate during periods of migration – such as the devastating droughts in southern Spain – has been linked to population decline in our breeding cuckoos.
For other species there is evidence that the choice of wintering areas has an impact on population trends.
Do you have a wildlife question you’d like answered? Email your question to firstname.lastname@example.org or post it to Q&A, BBC Wildlife Magazine, Immediate Media Company, Eagle House, Bristol, BS1 4ST.
Main image: Cuckoo. © Lillian King/Getty