Researchers from Durham University and the British Trust for Ornithology analysed 40 years of data from 286 European monitoring sites and found a marked decline in "migratory fuel loads" among 33 Afro‑Palaearctic species. Fall migrants now carry less fat than in previous decades, while spring migrants appear unchanged. Scientists link the change to hotter, drier summers that reduce insect and fruit availability, and warn lower fuel stores threaten migration success and already declining bird populations.
Millions Of Migratory Birds Are Arriving Lighter — New Study Raises Alarm
Every year millions of birds travel thousands of miles between Africa and Europe in two seasonal migrations that depend on precise timing and abundant food. New research led by Durham University and the British Trust for Ornithology indicates those journeys are becoming harder for many species.
What the Study Found
A study published on Zenodo analysed 40 years of monitoring data from 286 sites across Europe. Researchers examined 33 Afro‑Palaearctic migratory species and documented a significant decline in migratory fuel loads — the fat reserves birds build up to power long crossings of the Mediterranean Sea and the Sahara Desert. The decline is most evident in birds preparing for autumn (fall) migration; spring migrants did not show the same drop.
Timing And Regional Shifts
The study also found shifts in when birds accumulate energy: northern European populations are storing fuel earlier in the season, while many southern populations are doing so later. These timing changes, combined with smaller fuel stores, may reduce the ability of individuals to complete long-distance crossings or reach breeding and wintering sites in good condition.
Likely Causes
Researchers link the reduced fuel loads mainly to hotter, drier summers. Higher temperatures and more frequent droughts reduce the abundance and nutritional quality of the insects and fruit that migrating birds rely on to build fat reserves, and they can also dry out resting and refuelling habitats at key stopover sites.
Why It Matters
Migratory birds provide important ecosystem services: they help control insect pests, disperse seeds, and support agricultural systems. Declines in migration success could ripple through ecosystems and food systems, increasing costs for farmers and weakening natural habitats.
Conservation And Action
Across Europe, conservation organisations are working to protect migration corridors, restore degraded habitats and safeguard important stopover sites. Researchers emphasise that addressing the root causes — notably climate change driven by greenhouse gas emissions — will require broad policy and societal action, while local habitat protection and restoration can improve stopover success in the near term.
Bottom line: Fewer fat reserves before autumn migration mean many birds face a tougher journey. Continued monitoring, habitat protection, and efforts to limit warming are needed to support migratory populations.
