Date: 5/7/18 9:53 am
From: 'Joann O'Shaughnessy' via NHBirds <nhbirds...>
Subject: [NHBirds] UNH study sheds new light on saltmarsh sparrows | New Hampshire

UNH study sheds new light on saltmarsh sparrows | New Hampshire

DURHAM — In many bird species, females have the ability to control the sex of individual eggs.

“This means that offspring sex ratios are not usually left to chance,” a University of New Hampshire news release states. “From an evolutionary standpoint, this can be very beneficial, as different circumstances may favor the success of sons versus daughters.”

But researchers with the New Hampshire Agricultural Experiment Station have found that saltmarsh sparrows do not appear to manipulate the sex of their offspring.

“Typically, high-quality sons are more beneficial to mothers, because they have the potential to produce far more grandchildren than daughters can as males can mate many times, but females are limited by how many eggs they can produce, incubate, and raise to fledging. More offspring equals greater lifelong success,” said experiment station scientist Dr. Adrienne Kovach, assistant professor of natural resources and the environment.

Bri Benvenuti, a 2016 graduate who conducted this research as part of her master’s degree under Kovach’s supervision, said there is a risk to biasing offspring production toward sons.

“If the son is of low quality such as being competitively inferior, he may not reproduce at all,” Benvenuti said. “On the other hand, daughters tend to 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 offspring, but you know you’ll get something.”

A stressful life

Benvenuti and Kovach said that sparrows build nests in the marsh grass 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.

Almost every egg in a saltmarsh sparrow nest has a different father, thanks to the scramble of competition among males for access to females.

Benvenuti and Kovach wondered if saltmarsh sparrow mothers would produce more sons, who would be larger, more likely to survive a nest flooding event, and have the ability to produce more offspring through multiple matings. Or would they take the “safe bet” and produce more females?

To test their hypotheses, Benvenuti and Kovach collected nesting data from saltmarsh sparrow breeding locations in New England marshes over five years. Study sites were located in Chapman’s Landing in Stratham, Lubberland Creek Preserve in Newmarket, Eldridge Marsh in Wells, Maine and the Parker River in Newburyport, Mass.

Using DNA analysis, they determined the sex of saltmarsh sparrow chicks and calculated the offspring sex ratio for the four study sites and across the whole study population.

Benvenuti and Kovach 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.

According to the news release, they found an even offspring sex ratio of 1.03:1 of males to females when averaged across all years and sites. Offspring sex ratios did not vary as a function of the environment, tidal flooding risk, or female condition.

“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,” Kovach said.

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. The findings overall show support for balanced offspring sex ratios at a population level over time.

The study adds to a growing body of literature characterizing the nature of sex allocation in birds. In addition, balanced population-wide sex ratios may have important conservation implications.

“Saltmarsh sparrows are threatened by sea level rise, which results in higher tidal water levels and higher rates of nest flooding,” Kovach said. “As sea-levels rise, these environmental changes on the marshes are expected to continue.”

If saltmarsh sparrows manipulated their offspring sex ratios in response to environmental conditions, then the consistent environmental changes predicted by rising sea levels could result in skewed population-wide sex ratios, which can be detrimental for declining species and small populations.

The research was supported by the NH Agricultural Experiment Station, through joint funding of the National Institute of Food and Agriculture, U.S. Department of Agriculture, and the state of New Hampshire. Funding also was provided by the U.S. Fish and Wildlife Service.
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