The pied flycatchers, Ficedula hypoleuca, produce more offspring when they switch partners compared to the previous season, according to a study on these birds.
The work was conducted by a research team led by the National Museum of Natural Sciences (MNCN) of the Spanish National Research Council (CSIC), Spain.
To reach this conclusion, the researchers used monitoring data from 1987 for a population in the mountains north of Madrid.
These data included, for example, the date on which each pair begins laying eggs, the brood size and the number of offspring that fledge.
The team was able to determine whether a pair reunites or not thanks to ringing and exhaustive annual monitoring of the population.
In each season, all individuals associated with the nearly 250 nest boxes used by the population are marked.
Moreover, every year, all the offspring born in the boxes are ringed and the breeding adults are captured.
The MNCN researcher, Daniel R. Rodríguez-Solís, explained that each bird has a ring with an individual code, “which functions as an identification card,” allowing its identification when it breeds again.
“Considering that typically four to six offspring are born per nest box, we are talking about more than 1,000 ringed offspring per year, in addition to the breeding adults that are captured and marked, if they have not already been marked,” added Iraida Redondo, also a specialist at the MNCN.
The study, published in the Ibis journal, indicated that, over nearly four decades, only 3.5% of the birds remained together between breeding seasons, and that individuals who bred with a new partner managed to produce more offspring.
The investigation also indicated that both males and females who changed partners managed to produce more offspring than in the previous season, regardless of the age of the individuals or the habitat type in which they reproduced.
In short-distance migratory species, where each breeding season is crucial, the risk of not reuniting outweighs the benefits derived from familiarity or better parental coordination, which could be obtained by mating with the same partner.
According to this investigation, the results suggest that divorce may be an adaptive strategy for these types of birds.
Besides increasing the number of offspring, partner switching may allow greater compatibility, access to a higher-quality breeding territory or mating with an individual of greater reproductive capacity.
“The challenge now is to determine whether the advantages of divorce persist in the long term, for example, in the survival of adults or in the reproductive performance of the offspring,” highlighted Rodríguez-Solís.
