Migration: New Study Shows Climate Change Is Starving Migratory Birds Before Their Long Flights

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Migration: New Study Shows Climate Change Is Starving Migratory Birds Before Their Long Flights

13 November 2025
By James Hamilton

Redstart by Edmund Fellowes / BTO

New research from the British Trust for Ornithology (BTO) and Durham University highlights a worrying trend: many of our cherished migratory birds are struggling to adapt to a rapidly changing climate.

For species like the Swallow and the Willow Warbler, migration is a finely-tuned routine. These birds depend on good food and suitable habitat across continents — from Europe’s breeding grounds to Africa’s wintering sites — with each leg of the journey relying on precise timing. But the rhythm of those migrations is being disrupted.

A Long Journey Made Even Harder

Across Europe, many long-distance migrants are already in decline. To understand why, scientists looked at over 30 species that migrate between Africa and Europe, paying particular attention to how birds build up “fuel” for the journey.

Before tackling such feats as crossing the Mediterranean Sea and the Sahara Desert, a bird must increase its body weight significantly — the so-called migratory fuel-load. This fat-reserve enables birds to fly thousands of kilometres without stopping.

These birds evolved to align this fuelling time with seasonal peaks in food availability. But now that the climate is shifting, so too are those food peaks. Hotter, drier summers (especially in southern Europe) are reducing insect abundance and altering the timing of food availability. That means birds may arrive at stop-over sites too early, too late or simply with less food to fuel up.

What the Data Shows

Using four decades of ringing data from 33 Afro-Palaearctic species at 286 sites across Europe, the researchers found two key trends:

A large-scale decline in autumn migratory fuel-loads over the past 40 years.
A shift in timing of fuelling: for birds breeding in the north, fuelling is happening earlier; for those at southern sites, it’s happening later.

These changes point to alterations in the quantity, quality and timing of food resources across Europe — changes that are very likely linked to rising temperatures and increased drought. With less fuel, birds face a greater risk: either they fail to complete the journey, or delays expose them to bad weather and other hazards en route.

Why This Matters

Dr Jennifer Border (BTO) explains how this massive data-collection effort made the findings possible: “Collecting long-term body-condition data on migrating birds is extremely challenging. Thanks to decades of ringing, we now see how things are changing.”

Professor Stephen Willis (Durham) adds: “To understand how future climate or land-use changes will affect migrants, we first need to know what influences a bird’s condition before migration.”

And Prof. James Pearce-Higgins (BTO Director of Science) warns: “This shows a hidden impact of climate change. Poor condition in warmer years may stop migrants completing Mediterranean and Sahara crossings — feeding into long-term population declines in birds like the Swallow.”

Every spring and autumn, millions of birds traverse continents — a sight many of us cherish as birdwatchers. Yet this research reminds us how fragile these epic journeys are. If the fuelling stops matching the journey, the risk to those migrations rises.

Protecting these birds means protecting their stop-over sites, ensuring rich insect habitat en route, and supporting conservation efforts across borders. Because when migration falters, we all feel the loss.

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Migration: New Study Shows Climate Change Is Starving Migratory Birds Before Their Long Flights​

Migration: New Study Shows Climate Change Is Starving Migratory Birds Before Their Long Flights