Sparrows drift from favored spots after losing friends

A new study suggests that losing long-lasting flockmates may cause a golden-crowned sparrow to deviate from its usual overwintering spot. This finding highlights the importance of friendly and familiar companions in anchoring these sparrows to familiar environments.

Researchers at the University of Nebraska–Lincoln led the study, which revealed that golden-crowned sparrows returning to California after a long winter migration tended to resettle within an average distance of only 90 feet from their previous year’s range.

However, the behavior of golden-crowned sparrows that appeared for at least their third consecutive winter changed when their closest flockmates did not join them. This suggests that for these sparrows, the sense of home is strongly linked to the presence of familiar companions.

“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,” said lead author Annie Madsen, who recently earned her doctorate from Nebraska.

Madsen and her team conducted this research to investigate the dynamics of social interaction and resource sharing among animals, specifically the golden-crowned sparrow. These sparrows engage in fission-fusion networks, congregating in small groups before dispersing and reassembling with different flock members.

The challenge for ecologists is to determine the extent to which animals interact based on shared territories and resources versus forming social bonds and friendships. Madsen explained, “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 that’s really hard to address.”

The researchers chose the Arboretum at the University of California, Santa Cruz as their study site, where thousands of golden-crowned sparrows overwintered. By using leg bands and diligent observation, researchers and volunteers mapped the geographic distribution and social networks of individual sparrows from 2009 to 2019.

The study found that the more consecutive winters a golden-crowned sparrow spent in Santa Cruz, the less its range shifted from the previous year. This indicated that sparrows developed an affinity for particular sites over time. However, the data also allowed the team to identify a sparrow’s closest friends, representing the 10% of flock members it was most frequently seen with each year.

The researchers discovered that on average, a sparrow lost approximately 52% of its favored flockmates across multiple years of migration to Santa Cruz. When their closest social contacts did not return, the sparrows tended to shift their home range farther away from the previous center. This suggests that sparrows are loyal not only to a specific location and its resources but also to the familiar companions they expect to encounter there.

Notably, the loss of flockmates had a minimal impact on the home ranges of second-year sparrows returning to Santa Cruz. This could indicate that these sparrows had not yet developed close relationships and strong social bonds like those returning for several consecutive winters.

Madsen concluded, “Some of these relationships are being built up over multiple years. As one sparrow returns again and again, they are not only maintaining friendships with all the returning flockmates but also forming new friendships with new immigrants and first-year birds. They are building lots of social capital.”

The study, titled “Loss of flockmates weakens winter site fidelity in golden-crowned sparrows ( Zonotrichia atricapilla ),” is published in the journal Proceedings of the National Academy of Sciences.

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