Why do birds get divorced?

Divorce in Savannah Sparrows of Kent Island

by: Michael Calcagno, Meghan Ghazal, Lizzie Poyant & Zarir Sidhwa (Stonehill College Evolution Fall 2017)

In Savannah sparrows (Passerculus sandwichensis) that reside on Kent Island, New Brunswick, Canada an understanding of divorce rates within the species was analyzed to determine the molecular basis of the phenotypic trait. As well as, to determine if divorce is an adaptive strategy for greater fitness for females. In this species, approximately 47% of pairs in which both partners survived to the following breeding season ended in divorce. Neither the lifetime number of divorces nor whether an individual had ever divorced affected the fitness of either sex, thus suggesting little to no sexual selection for the trait. Divorce in the Savannah sparrows appeared to be an inheritable behavior in which expression depending primarily upon an individual’s age, mating status, sex, and size.

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The researchers proposed a few hypotheses to explain the divorce behavior in the Savannah sparrows.  A female’s or male’s decision to divorce could be driven by its assessment of its genetic or behavioral compatibility with its mate; the genetic or phenotypic quality of its mate; its perception of the availability of more compatible or higher-quality mates; or the benefits of producing genetically diverse offspring. Whether divorce is an adaptation, the costs and benefits of divorce are likely to differ for females and males and for individuals of different ages or social status. This is very similar as to how humans consider divorce in their relationships. In this study, the researchers were interested in a large series of questions including: What are the causes and fitness consequences of divorce in a natural bird population? Is the probability of divorce affected by a bird’s age, size, or other factors? What are the costs and benefits of divorce in terms of reproductive success and survival? Are there fitness differences between initiating a divorce versus being divorced? How much additive genetic variance for the tendency to divorce exists, and is the behavior repeatable between breeding seasons?

To explore these questions over the course of a long-term study, the researchers employed a variety of methods. These compared the reproductive success of divorced birds to those that remained with their original mate, quantifying the effect of divorce in terms of the number of fledging’s, recruits, and fledglings per egg. As well as to determine the birds’ long-term reproductive success and ages.

The study site consists of a total of three fields totaling 10 hectares in area. Within the site, all adult sparrows are uniquely banded with a Canadian Wildlife Service aluminum band and three randomly selected plastic color bands while being measured (body mass, tarsus and wing length, bill depth and length). All successful nests in the study site are located, all social pair bonds determined, and all nestlings banded. The population on Kent Island spends the winter in the central and southern United States. In late April or early May, males begin to return to the breeding grounds in Canada and establish territories. Females arrive a few short weeks after males. By late May or early June, females lay clutches of 3–5 eggs in nests on the ground in open habitats within their chosen social mate’s territory. If the first brood successfully fledges, most females lay a second clutch 1–2 weeks later. If the first clutch is lost due to predation, females lay replacement clutches. Replacement clutches that are destroyed may be replaced by a third or even a fourth clutch. The available evidence to the researchers suggested that divorce in Savannah sparrows is most likely initiated by females, and males have not been observed rejecting potential females, seemingly because a male’s fitness increases with the number of mates he attracts.

To determine social pairs and construct a pedigree for animal model analyses, daily observations of individual banded birds were made to understand the behavior of the species. Observed behaviors included mate guarding, copulations, territory defense, and cooperative feeding. Depending on the year, between 10% and 40% of males attract more than one mate. Both monogamous and polygynous males exhibit these behaviors, but polygynous males spend more time with primary females. Usually both members of a divorced pair found new mates the following year, in which case divorce was synonymous with mate switching. It was uncommon for a female to change mates within a breeding season.

To understand the factors that influence the probability of divorce between breeding seasons, we assessed the effect of individual traits and reproductive traits. Individual traits included female and male body size and age, while the age of each sample bird was defined as a categorical variable. Reproductive traits included laying date and three measures of breeding success.

Three sets of analyses were completed regarding the possible consequences of divorce for females.  First, comparison of reproductive success of females who divorced, to females that remained with the same mate. Second, whether divorce incurred reproductive costs, and lastly, the costs of divorce expressed in terms of survival.

The obtained results were rather shocking and disproved many of the expected observations regarding the divorce behavior. It was uncommon for an individual Savannah sparrow to pair with the same mate in successive years because of high annual mortality and high rates of divorce. Over an 18-year period, the annual divorce rate ranged from 30% to 69%, as well as the divorce rate for all years combined being 47.0%. Divorce could have been considered adaptive if the switch to a new mate allowed females to improve their long-term breeding success. It was found that divorce improved breeding success in older females, where females older than 2 years produced more fledglings following a divorce than in the previous season. Remarkably, it was concluded that divorce behavior was not heritable because there was no evidence that supported that the behavior was passed from parent to offspring in subsequent generations. Instead, the decision to divorce in Savannah sparrows was mainly influenced by the interaction between a female’s mating status, age, and breeding success, along with male body size. Repeatability of divorce behavior may be because females that tend to be unsuccessful at breeding, consistently switch mates to improve their situation.

Works cited:

Article:  Wheelwright, N. T., & Teplitsky, C. (2017). Divorce in an island bird population: Causes, consequences, and lack of inheritance. The American Naturalist, 190(4), 557-569.  http://www.journals.uchicago.edu/doi/full/10.1086/693387

Paper author’s contact information: Nathaniel Wheelwright, nwheeler@bowdoin.edu, https://www.bowdoin.edu/faculty/n/nwheelwr/ https://www.maineaudubon.org/nature-moments/

Podcast music attribution: Extinction Love” by Skill_Borrower from http://dig.ccmixter.org/film

Image by Cephas – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6785567

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