Migrants Losing Fuel for the Journey South
New BTO and Durham University research reveals that long-distance migrants like Swallows and Willow Warblers are struggling to build vital fuel reserves before their epic journeys to Africa
Declining condition before migration spells danger
Many of Europe’s best-loved summer visitors – including Swallows, Willow Warblers and other long-distance migrants – are facing growing challenges as climate change disrupts their ability to prepare for migration. New research from the British Trust for Ornithology (BTO) and Durham University has found that migratory birds are arriving at key departure points across Europe in poorer condition than they did four decades ago, with less body fat to sustain their long flights to Africa.
Using data from more than 33 Afro-Palearctic migratory species, collected through the efforts of thousands of bird ringers at 286 sites across Europe, the study provides the first continent-wide evidence that the amount of energy – or ‘fuel load’ – birds build before their journeys south has significantly decreased since the 1980s. The research also found shifts in timing, with northern breeders beginning to fuel earlier, while birds in southern Europe are doing so later in the season.
Changing seasons and shrinking food supplies
Migratory birds rely on abundant food to build up fat reserves before setting out across the Mediterranean and the Sahara – journeys that can span thousands of kilometres without a single feeding stop. But as European climates warm, food availability is becoming less predictable. Droughts, especially around the Mediterranean basin, are reducing insect numbers and shortening the critical period when birds can feed intensively before departure.
“The timing and quality of food sources are shifting as the climate changes,” explained Dr Jennifer Border, Senior Research Ecologist at BTO. “Our long-term ringing data show that birds now weigh less before migration than they did 40 years ago, which means many are leaving Europe with insufficient energy reserves for their crossings.”
Hidden impacts of a warming world
The study found a strong link between reduced fuel-loads and higher European temperatures, particularly in the south where droughts have become more frequent. According to the researchers, even small reductions in migratory condition could increase mortality, as birds either fail to complete their journeys or are forced to delay, encountering unfavourable seasonal weather conditions en route.
Professor Stephen Willis of Durham University’s Conservation Ecology Group added, “To predict how future climate or land use changes will affect migrating birds, we first need to understand what influences their body condition before and during migration. This work is a crucial step in uncovering those links.”
Consequences for familiar migrants
The findings offer a sobering perspective on population declines already being recorded among many long-distance migrants across Europe. “This study reveals a previously hidden impact of climate change on migratory species,” said Professor James Pearce-Higgins, BTO’s Director of Science. “If warmer years result in poorer condition, fewer birds will survive the Mediterranean and Sahara crossings. That could be a key factor in the long-term declines we’re seeing in species like the Swallow and other familiar migrants.”
With evidence now mounting that climate change is eroding the physical resilience of migratory birds, conservationists stress the urgency of protecting and restoring habitats that provide rich feeding grounds – both in Europe and along their vast migratory corridors into Africa.
November 2025
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