Does migrating earlier help birds? It depends on the journey

Birds are changing their migration calendar, but an earlier journey does not mean the same thing throughout the year. Jin Liu, Wenjing Xu, Richard A. Fuller, Thomas Mueller, Qi Yang and Shaopeng Wang assembled global data and found a crucial difference: advancing migration before breeding was associated with more favorable population abundance trends, whereas advancing migration after breeding was associated with less favorable ones. Their study appeared on 2 October in Nature Ecology & Evolution.

Pre-breeding migration is the journey back toward the places where birds raise young. Post-breeding migration takes them away. The researchers paired 4,351 estimates of migration timing in populations of 549 species with trends in population size and rates of climate change in breeding and non-breeding areas. A theoretical model spanning the full annual cycle helped them explore why the two journeys might differ.

Across the evidence analyzed, migration advanced by a cumulative 13.86 days from 1970 to 2019. This is a historical aggregate, not a claim that every bird, species or route shifted by precisely that amount. When the authors compared populations, faster advancement before breeding was associated with better abundance trends. After breeding, the association went in the opposite direction. The most favorable net pattern occurred when the first journey advanced faster than the second; the association was stronger in larger-bodied species.

That asymmetry makes the entire year relevant to conservation. Birds encounter different conditions on breeding grounds and in places they use during the rest of the year. The analysis links changes in timing to changes in numbers; it does not demonstrate that moving a departure date by itself causes a population to grow. The authors made their data and code available through Figshare as documentation of the same study, not an independent replication.

The evidence supports accounting for the stage of migration when interpreting how populations respond to environmental change. It does not establish that earlier migration is always beneficial or offer a way to alter birds’ behavior. A decisive next step is to measure food, climate, survival and reproduction through the whole year in the same populations, then test which conditions explain the contrasting associations.