The Downsides of Sharing: Savannah Sparrow Females that Mate with ‘Taken’ Males Pay a Cost

Birds are perhaps the most famously monogamous creatures in the animal kingdom. It’s easy to imagine idyllic scenes where a single male and female work in tandem to feed their hungry young.

In reality, this is only partially true! Most bird species are indeed socially monogamous, meaning that a single male and female will often form a bond and raise their young together, but they may still mate with other individuals.

Some bird species also display a phenomenon called “social polygyny,” where a single male mates and raises young with multiple females. However, according to a recent study published in the journal Behavioral Ecology, this behaviour comes at a significant cost to females who mate with already-mated males.

Sarah Mueller with a Savannah sparrow. Photo by Cora Dow. 

The study lead, Sarah Mueller, is a PhD candidate in Dr. Ryan Norris’s lab in the Department of Integrative Biology. Her doctoral research is centred on a population of Savannah sparrow that has been monitored on Kent Island, in New Brunswick, since 1987.

“Savannah sparrows on Kent display a mix of monogamy and polygyny,” explains Mueller. “Some males socially pair with a single female, while other males pair with a primary female first, then later pair with one or more secondary females.”

Mueller and her co-authors suspected that secondary females are at a disadvantage compared to their primary and monogamous counterparts.

“Male and female Savannah sparrows share the role of bringing food to their offspring,” explains Mueller. “So, when a male has mated with multiple females, he may provide less attention and food to his offspring with secondary females, lowering their chance of survival.”

Secondary females may also lay fewer eggs to avoid being overworked, or they may put their survival at risk if they expend too much energy caring for a nest with little help from a male.

On the other hand, secondary females could also be gaining an advantage since they can freely choose the male with the best genetic traits and the territory with the most plentiful food resources.

Overall, whether polygyny has a net cost for female fitness remained a mystery.

“Prior studies have tended to evaluate the cost of polygyny based on only a few fitness outcomes,” says Mueller. “As well, they haven’t always distinguished between primary and secondary polygynous females, even though primary females may receive more help from males.”

Alexis Van Esch and Sarah Mueller with a Savannah sparrow. Photo by Dan Mennill. 

To finally unravel the enigma of polygyny’s impact on female Savannah sparrows, Mueller, along with Norris and a team of other researchers, analyzed over three decades' worth of records from the long-term study on Kent Island. The birds on the island are marked with unique combinations of coloured leg bands, allowing individual birds to be monitored and their nests located.

“We looked at several fitness measures,” says Mueller. “For example, we wanted to see if secondary females had smaller clutches of eggs, and if their offspring had a lower chance of surviving to see subsequent breeding seasons.”

The verdict? Mueller and colleagues ultimately determined that secondary females did indeed pay a cost for mating with already-mated males.

“These females raised fewer nestlings to the fledgling stage than their monogamous and primary counterparts,” recalls Mueller.

Younger secondary females are also less likely to survive to the next breeding season than older secondary females, which may be due in part to their lack of experience.

This then begs the question: why would female Savannah sparrows on Kent Island mate with an already-mated male and become secondary when this reduces their fitness?

Mueller believes that the answer comes down to demographics on the island. Each year, there are more females on Kent Island than there are males, so polygyny is likely the only way for some females to rear any offspring.

“Female Savannah sparrows are likely just making the most of an unfortunate situation.”

The researchers note that there are still unanswered questions about polygyny on Kent Island. A particularly interesting avenue for future research now lies in male behaviour. Mueller suspects that they provide less care to their offspring with secondary females, but this has yet to be confirmed.

This will require further research in the grassy fields of Kent Island — a task that will no doubt be eagerly embraced by the intrepid researchers fortunate enough to study the bird community on this fascinating island each year.

Read the full study in the journal Behavioral Ecology.

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