Family success prompts tit divorces

Yes, it sounds backwards, but it’s what the study found: Pairs of long-tailed tits are more likely to break up and find new mates if they succeed in raising young than if the nest fails.

“To our knowledge, this is the first documented case of successful breeders being more likely to divorce than unsuccessful breeders,” report Ben J. Hatchwell of the University of Sheffield in England and his colleagues in the April 22 Proceedings of the Royal Society of London B.

These tits, Aegithalos caudatus, sport their extralong black tails through woodlands from the United Kingdom to Japan. Tracking three populations in Yorkshire from one breeding season to the next, the researchers noted that 63 percent of pairs divorced. Compared with moms whose offspring had died, nearly twice the percentage of females that raised their youngsters to the fledgling stage moved out of the family flock and took mates elsewhere the next season—81 percent versus 43 percent.

Theorists have proposed some half dozen explanations for the widely varying divorce rates among birds (SN: 3/7/98, p. 153: http://www.sciencenews.org/sn_arc98/3_7_98/bob1.htm). The tits that changed partners didn’t seem to be moving on to mates that were more successful or had more relatives willing to help feed nestlings.

Therefore, Hatchwell picks inbreeding avoidance as the best explanation for tit breakups. Males tend to stay near their natal flock, so successful moms that move on limit the risk of inbreeding among their offspring.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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