The golden-crowned sparrow (Zonotrichia atricapilla) is a large species of New World sparrow found in the western part of North America. University of Nebraska-Lincoln researcher Anastasia Madsen and colleagues previously found that sparrows show consistent flocking relationships across years, and that familiarity between individuals influences the dynamics of social competition over resources. Their new research shows that: (i) golden-crowned sparrows exhibit interannual fidelity to winter home ranges on the scale of tens of meters and (ii) the precision of interannual site fidelity increases with the number of winters spent, but (iii) this fidelity is weakened when sparrows lose close flockmates from the previous year.
The golden-crowned sparrow (Zonotrichia atricapilla) in British Columbia, Canada. Image credit: Alejandro Erickson / CC BY 2.0.
“We found that a golden-crowned sparrow returning to California after a winter migration — one that can stretch as many as 3,000 miles — resettled an average of just 90 feet away from the center of its previous year’s range,” the authors said.
“But golden-crowned sparrows appearing for at least their third consecutive winter began to drift from their preferred locales when their closest flockmates failed to rejoin them down south — hinting that, even for sparrows, home is where the heart is.”
“The fact that they come back to this winter site and then hang out with the same individuals — and it’s important for them to be with the same individuals — is kind of a crazy thing that we’re still wrapping our heads around.”
Dr. Madsen and co-authors undertook the research hoping to untangle the knot of what she called a ‘chicken-and-egg question.’
Many animals, including the golden-crowned sparrow, share space with members of their species. In many cases, they also spend time around and interact with those neighbors.
Golden-crowned sparrows, for instance, have adopted what are called fission-fusion networks, spending minutes or hours congregating in small groups before dispersing, only to later reassemble with different members of the larger flock.
The challenge for ecologists: figuring out how much animals are interacting with their own because they happen to value the same territory versus sharing that space because they value the social bonds and benefits of the friends they find there. Which ties, in other words, actually bind?
“Are they coming together because of a resource? Are they coming together because of social partnerships? And as they come together for resources, do they gain social partners? Or are they using social partners to find resources? It’s a complex question,” Dr. Madsen said.
“But we wanted to know if there was some kind of directionality here, if there was one of those forces that took precedence.”
“Do they care about this one specific patch of bushes, which is a really nice patch that they come back to every year? Or is it their friends, their flockmates, that they’re coming back to spend time with?”
As the overwintering site for thousands of golden-crowned sparrows fleeing the cold of Alaska and western Canada, an arboretum at the University of California, Santa Cruz seemed as fitting a place as any to investigate.
From 2009 to 2019, the combination of multi-colored leg bands and diligent observation by scores of researchers and volunteers helped map both the geographic distribution and social networks of individual sparrows.
The more consecutive winters a golden-crowned sparrow spent in Santa Cruz, the less its average range shifted from the prior year, with the sparrow seeming to home in on and develop an affinity for a particular site.
On its own, that finding might have pointed toward certain spots boasting certain features that were ultra-appealing to the species.
But that decade of data also allowed the team to identify the equivalent of each sparrow’s closest friends, or the 10% of fellow golden-crowned sparrows it was most likely to be spotted with in a given year.
An average sparrow, the researchers discovered, was liable to lose about 52% of its favored flockmates across however many years it migrated to Santa Cruz.
And in the years that its closest social contacts did fail to return, the shift in a sparrow’s home range tended to reverse course, flitting not closer but instead farther from its prior center.
The finding suggests that a sparrow’s loyalty lies not just with a locale, and the resources it might offer, but with the fine feathered friends it comes to expect will be there to greet it.
When the latter wanes, a sparrow’s ties to a specific location appear to wane, too.
“That’s all the more telling, given that resources are generally less scarce, and real estate less prized, when overwintering than when seeking mates and raising a handful of hatchlings in the summer,” Dr. Madsen said.
“It’s really interesting that they’re coming back to such specific sites on their wintering grounds, where it doesn’t seem like it wouldn’t matter that much — especially since, at an arboretum, they have pretty cushy lives.”
The scientists also discovered a quantitative quirk that bolstered the case for the magnetism of friendship: the loss of flockmates didn’t seem to much affect the home ranges of golden-crowned sparrows that were returning for only their second winter in Santa Cruz.
Though it’s not definitive, the discovery could indicate that second-year sparrows hadn’t developed as many close contacts, or as tight-knit of bonds, as sparrows that had been coming back to California for at least several consecutive winters.
“Some of these relationships are being built up over multiple years,” Dr. Madsen said.
“And as one sparrow is returning again and again, not only are they maintaining friendships with all of the flockmates that do return, they also make new friendships.”
“They gain new flockmates from new immigrants, from first-year birds that are coming into the population. So they’re building lots of social capital.”
The results appear in the Proceedings of the National Academy of Sciences.
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Anastasia E. Madsen et al. 2023. Loss of flockmates weakens winter site fidelity in golden-crowned sparrows (Zonotrichia atricapilla). PNAS 120 (32): e2219939120; doi: 10.1073/pnas.2219939120
