There are some disadvantages of long distance travel for migrating birds says a scientific study.
The New Scientist’s LIFE magazine (November 2022) reports on the high rate of break-ups – ‘divorce’ – in bird species with longer migration routes. But why would there be a high rate of bird divorce?
An analysis of bird species found that birds that migrate long distances tend to be more likely than other birds to break up with their partner.
About 90% of bird species are socially monogamous – they have babies with the same partner. Some bird couples stay together for life, while others get ‘divorced’ and mate with a new partner.
Sun Yat-sen University researchers in China studied 232 bird species from data collected in the world’s largest bird database called Birds of the World. The researchers found that species that migrate long distances each year to find food between breeding season have a typically higher rate of breaking up.
The Great Blue Heron (Ardea herodias), for example, migrated more than 3,000 kilometres (1,864 miles) and has a divorce rate of 100%. That means that no partners stayed together.
The researchers think that the further a bird travels for migration, the harder it is for a couple to return home at the same time as each other, and in time for the annual breeding season. This may be due to the many challenges along the way – storms, strong winds, predators, etc. Breeding seasons are short, so there is only a short window of time to find the same partner and breed.
Gisela Kaplan at the University of New England in Armidale, Australia, says, ‘If you’re the one who gets back home first, it’s risky waiting for your partner because you don’t know if they will show up – they may have died or been blown off course.’ She says that, for the birds that travel exceptionally long distances, finding a new partner is less risky than spending time waiting for and finding their previous partner – and they don’t want to miss out.
The Black-Tailed Godwit (Limosa limosa islandica), for example, migrates about 1,000 kilometres (621 miles) from Iceland to the United Kingdom each year but it has a low divorce rate. However, the Black-Tailed Godwit is known to synchronise flight times with its partner even with separate migration paths, says Anne Peters at Monash University in Melbourne, Australia.
Bird species that migrate for only short distances were more likely to fly home together, or to find each other easily and quickly if they took different flight paths home.
Therefore, break-ups are more common in bird species with longer migrations, probably because partners return home at different times and don’t wait for each other to breed. The researchers found other factors to include low breeding success, shyness in the male partner, and climate factors.
Journal reference: BioRxiv, DOI: 10.1101/2022.10.13.512018
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM