Sparrow Supermoms: Why Some Female Birds ‘Double-Brood’

In the grassy fields of Kent Island, New Brunswick, a tiny migratory songbird with a big breeding strategy is helping scientists understand one of the mysteries of reproductive success in the wild: Why do some female birds double-brood? 

Double-brooding is the ability to successfully raise two separate broods of chicks in a single season. It’s a breeding strategy that plays an important role in shaping individual fitness in birds, like the Savannah sparrow, that are short-lived and experience high predation rates.

“To double the amount of offspring you’re producing in possibly the only season you’re alive is huge,” explains Dr. Hayley Spina, a recent graduate from the Department of Integrative Biology whose PhD research focused on the sparrow. “The ones that are successful are like the super birds.” 

But only 36% of the sparrows included in the study double-brooded. So why don’t more Savannah sparrow females do it?

For more than 35 years, the Kent Island Savannah sparrow population has been the focus of a long-term research project aimed at understanding the breeding behaviour of this ground-nesting songbird. Dr. Amy Newman and Dr. Ryan Norris, also from the Department of Integrative Biology, play a central role in this research, along with Spina and many other students.

Spina’s goal was to find out whether baseline levels of corticosterone – a hormone responsible for regulating physiological and behavioural responses – could explain why some female Savannah sparrows double-brood while others do not. Though often labeled as a “stress hormone,” corticosterone is better described as a hormone that helps regulate energy use during energetically demanding times, like breeding. This made Spina and her colleagues wonder: Could the hormone serve as a “biological marker” of a female’s ability to double-brood?

To find out, Spina conducted targeted sampling during either the incubation or nestling stage of females’ first broods. In addition to the data collected over two summers, she used another 10 years of hormone data that had previously been collected, giving her 12 years of data in total.

Spina and colleagues found that female sparrows with better fat stores and body condition had a lower baseline level of corticosterone. Females that had lower corticosterone during the nestling stage of their first brood were also more likely to double-brood that season. Together, these results suggest that corticosterone is an indicator of female quality, with higher-quality females being more likely to double-brood.

“A higher-quality individual might have more access to resources, or be better at foraging, or have something else about them that enables them to get the energy they need to breed without it taking a significant toll on their own health and survival,” says Spina. 

The researchers weighed and measured the birds to determine their fat stores and body condition. To measure corticosterone, the researchers collected blood samples from the birds.

“It gives you a snapshot of their corticosterone levels at that exact time, so we take our blood samples within three, or ideally two, minutes of when we catch the bird,” says Spina, explaining that it is crucial to act quickly because hormone levels can fluctuate rapidly.

“After approximately three minutes, that’s when the stress-related corticosterone will start to circulate in the blood, and that’s not what we want to measure.” 

Catching the female birds at the right breeding stage was no small feat. Because the birds generally tended to breed around the same time, many birds could be sampled the same day. While the blood sample only takes a few minutes, setting up mist nets to catch the birds and waiting for the right moment is time-consuming.

“Sometimes we just couldn’t get all of the birds, and that was okay, we still ended up with a really good sample,” says Spina. 

When spending so much time in the field, some individual birds become very familiar to researchers.

“It was amazing to see the personalities of all the different birds,” Spina recalls.

While some birds were more oblivious, others were quite the opposite. “There was one male that, as soon as you walked anywhere near his territory, he would be up at you, chipping at you, telling you to go away.” 

Summer field seasons on Kent Island with the Savannah sparrows were a highlight of Spina’s doctoral research.

“Getting to go out into the field and be a part of their world, because we’re just plopped into the middle of their little breeding area – it felt like such a privilege. The island is home only to the research station, the birds, and the other critters that live there.”

This research adds to our understanding of avian fitness and the factors that influence it. It also has important implications for grassland songbird ecology and how these birds may enhance their reproductive success in the wild, navigating the pressures of survival one brood – or two – at a time.

The study was funded in part by the Natural Sciences and Engineering Research Council, the Society for Canadian Ornithologists, and the American Ornithological Society. 

Read the full study in the journal Hormones and Behavior.

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