Genes, Not Colony Life, Steer Cory's Shearwater Migration

What do the hybrids do?

The hybrids provided the decisive evidence. When the two shearwater species interbreed, the resulting individuals follow migratory patterns that sit between those of the two parent species — an intermediate route.

"When the two species hybridize, the resulting individuals follow migratory patterns that lie between those of the two species. This provides solid evidence that the long-distance migrations of these species have a genetic basis," Ramos said.

In plain terms: if migration were purely cultural, hybrids would copy one colony's route or the other. Instead, they split the difference, which points to inherited genetic instructions rather than imitation.

What changes for conservation practice?

Hybrids displaying intermediate strategies mean hybridization can produce individuals whose movement patterns match neither parental species. That has a direct consequence for how practitioners interpret connectivity — the links between populations established through movement.

"Therefore, it is necessary to take into account contact zones and introgression processes—the transfer of genes from one species to another—when interpreting connectivity between populations, and when developing conservation strategies that preserve these evolutionary processes," said Jacob González-Solís, head of the Seabird Ecology Lab.

For practitioners, three practical implications follow from the study:

• Track ancestry, not just location. Genomics plus tracking data, used together, identifies which marine areas each genetic lineage depends on across the year.
• Map contact zones deliberately. Hybrid zones such as the Strait of Gibraltar generate novel movement patterns that single-species surveys will miss.
• Protect the process, not only the populations. Conservation strategies should preserve introgression — the flow of genes between species — as an evolutionary process in its own right.

How far do the findings travel?

The authors frame the shearwater result as a general signal for migration biology. "Beyond the example of the shearwaters, this study shows that, in other migratory species, individuals sharing the same breeding colony can have different migratory strategies depending on their genetic origin," they write.

Caveats remain. The study covers one contact zone and two closely related species; whether the same genetic dominance holds for songbirds, waterfowl or other seabirds with different learning ecologies will need separate testing. The mechanisms — which genes or how many are involved — remain unresolved.

Still, for anyone managing marine protected areas or modelling flyways, the message is concrete: two birds from the same cliff may winter on opposite sides of an ocean, and their genomes, not their neighbours, decide it.

The paper is Beñat Garcia-Urdangarin et al., "When Hybrids Take an Intermediate Route: Genetic Ancestry Shapes Migration Across a Seabird Contact Zone," Molecular Ecology (2026), DOI: 10.1111/mec.70598.