AUTHOR BLOG: Are All Eggs Created Equal? Saltmarsh Sparrows Support Gender Equality

Bri Benvenuti and Adrienne Kovach

Linked paper: Annual variation in the offspring sex ratio of Saltmarsh Sparrows supports Fisher’s hypothesis by B. Benvenuti, J. Walsh, K.M. O’Brien, M.J. Ducey, and A.I. Kovach, The Auk: Ornithological Advances 135:2, April 2018.

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Three Saltmarsh Sparrow chicks banded as part of a study on offspring sex ratios. Photo: B. Benvenuti

In birds, females have the ability to control the sex of individual eggs; therefore, a mother may be able to choose whether she prefers each egg laid to be a male or female. This means that offspring sex ratios are not usually left to chance. From an evolutionary standpoint, this can be very beneficial, as different circumstances may favor the success of sons versus daughters.

But how does one know if it would be better to have more sons or daughters? Evolutionary theory suggests that if the potential benefits of raising one sex over the other vary in relation to environmental or maternal conditions, then females should favor the production of that sex. Typically, high quality sons are more beneficial to mothers, because they have the potential to produce far more grandchildren than daughters can (males can mate many times, but females are limited by how many eggs they can produce, incubate, and raise to fledging). More grandchildren = greater lifelong success.  Still, there is a risk to biasing offspring production toward sons; if the son is low quality (competitively inferior), he may not reproduce at all.  On the other hand, daughters tend to be “cheaper” to raise than sons. They need less resources to reach maturity, and if they survive, they almost always reproduce. With this in mind, one could logically say that producing daughters represents the “safe bet”—you might get a smaller payout (in terms of numbers of grandchildren), but you know you’ll get something.

Armed with this information, we chose to investigate whether Saltmarsh Sparrows manipulate the sex of their offspring based on environmental or maternal conditions as we would expect based on evolutionary theory. These tidal marsh specialist birds live a stressful life—they build nests in the marsh grasses just inches above the marsh surface that is regularly subjected to tidal flooding. Nests are more likely to escape these flooding events and successfully fledge offspring if they are timed to fledge within the 28 day lunar tidal cycle. Saltmarsh Sparrows are also one of the world’s most promiscuous birds, with almost every egg in a nest having a different father thanks to the scramble of competition among males for access to females. These characteristics provide interesting hypotheses in the context of evolutionary theory, so we asked, would Saltmarsh Sparrow mothers produce more sons, who would be larger and a) more likely to survive a nest flooding event and b) have the ability to produce more offspring through multiple matings? Or would they take the “safe bet” and produce more females?

To test our hypotheses, we collected nesting data from Saltmarsh Sparrow breeding locations in New England marshes over five years. We used DNA analysis to determine the sex of Saltmarsh Sparrow chicks and calculated the offspring sex ratio for our four study sites and across the whole study population. We then used a modeling approach to determine if there was an influence of environmental conditions (year, tidal flooding, precipitation), temporal effects (nest initiation in relation to flood tides, timing within the breeding season), or maternal condition on offspring sex ratios.

Surprisingly, we found an even offspring sex ratio of 1.03:1 (males to females) across all years and sites, and offspring sex ratios did not vary as a function of the environment, tidal flooding risk, or female condition. What we did find was an interesting pattern of annual variation between male and female bias that mirrored the adult sex ratio in the preceding year.

While numerous studies have provided evidence that female birds may have the ability to adjust offspring sex ratios in an adaptive way, we found no evidence for adaptive sex ratio manipulation in Saltmarsh Sparrows in relation to our hypotheses. Instead, the observed time-lagged relationship between offspring and adult sex ratio meets expectations of frequency-dependent selection, whereby females respond to higher frequencies of one sex by increasing production of the rarer sex, which would have a temporary fitness advantage. Our findings overall show support for balanced offspring sex ratios at a population level over time.

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