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Breaking monogamy: Mate-switching found to have no effect on chick success in Seychelles warblers

A study of Seychelles warblers led by Macquarie University in Australia, with collaborators in the UK and Netherlands, finds no measurable connection between how long bird parents stayed together and the physical condition or reproductive success of their offspring.

Long-lasting partnerships are common in birds, with around 80% of species being socially monogamous. Previous research has focused largely on how these pairings affect adult survival and annual reproduction success. Social monogamies are often speculated to enhance offspring health by improving parental coordination between parents and reducing early-life stress, yet these long-term effects remain understudied.

In the study, "Mate-switching is not associated with offspring fitness in a socially monogamous bird," in Proceedings of the Royal Society B, researchers analyzed a 25-year dataset to investigate whether parental pair-bond duration or separation influenced offspring fitness in a natural population.

Working with 25 years of exhaustive data on Seychelles warblers from the island of Cousin, the team analyzed 1,109 offspring. Each juvenile's parentage was genetically verified, and breeding pairs were observed seasonally.

Offspring were monitored for early physiological markers like telomere length, hematocrit, and body condition, as well as long-term indicators including lifespan and lifetime reproductive output.

Each offspring's parental pair was categorized by whether they stayed together into the next breeding season or separated through death, divorce, or translocation. Pair-bond duration was recorded as the number of days between the first and last breeding seasons in which the pair raised offspring.

Findings ran contrary to expectations. Offspring whose parents stayed together fared no better in either short-term health or long-term success than those whose parents separated. Neither telomere length, hematocrit nor body mass showed meaningful links to parental bond stability. Lifespan and lifetime reproductive success of offspring also showed no association with how long parents remained bonded.

Researchers noted one minor yet intriguing find—offspring whose mothers died had shorter telomeres. It was also not linked to differences in survival or reproduction later in life.

Researchers concluded that pair-bond stability does not play a major role in shaping offspring success in Seychelles warblers.

In this population, rapid re-pairing and the presence of cooperative helpers may buffer the effects of disrupted partnerships, which could explain the absence of measurable parental-bond effects.

Experimental studies in other species with more exclusive parental care may reveal different patterns. Until then, the evolutionary rationale for monogamous fidelity may lie more in adult survival than in benefits to offspring.