A study from the University of Helsinki examined which one has a greater effect on annual population sizes in passerine birds: adult overwintering survival or reproductive success in the breeding area. Doctoral Researcher Inari Nousiainen from the University of Helsinki’s Finnish Museum of Natural History notes that although both adult survival and breeding success positively influence population variation in passerines, the former is the more important driver.
“For our common garden birds, such as the European pied flycatcher and the Eurasian blue tit, the annual abundance depends more on adult overwintering survival than on the number of nestlings born in the previous summer,” she says.
Impacts on bird conservation
Adult survival had an especially strong influence on annual population sizes in long-distance migrants that overwinter in the tropics, such as the European pied flycatcher. The role was weaker among species that remain in Europe over winter, such as the Eurasian blue tit. This is likely because long-distance migrants spend less time in their breeding grounds and usually raise only one brood per summer, while many of the species overwintering in Europe can raise two broods. The study confirms that long-distance migrants need protection beyond their breeding grounds.
“Migratory birds do not recognise national boundaries, and therefore conservation of the long-distance migratory songbirds requires international collaboration”, states senior curator Aleksi Lehikoinen from the Finnish Museum of Natural History.
Colder climate increases variation in population sizes
The study also found that adult survival and breeding success both have a stronger role for annual population change in northern compared to southern Europe. The researchers suggest that this is due to differences in climatic conditions: population sizes tend to fluctuate more in colder climates, where year-to-year weather variation, for example, harsh winters and cold springs, affects both survival and reproductive success.
Unique, high-quality dataset
Led by researchers from the University of Helsinki’s Finnish Museum of Natural History, the study was based on a collaboration of a large international team and published in October in the scientific journal Oecologia. The dataset comprises bird ringing records with over 1.2 million captures of 33 species across 11 European countries.
“This is a unique dataset, collected with the same standardized method in every country over two decades. I’d like to thank all the volunteer bird ringers and, of course, all the birds for taking part in the study,” says Nousiainen.
Original article:
Nousiainen, I., Bosco, L., Lehikoinen, P. et al. Adult survival has a stronger role than productivity in the annual population change of European songbirds. Oecologia 207, 173 (2025).