Sparrows in the Mist—Lyon Lab

Alexis Chaine (PhD, 2006) measures the crown color with a spectrophotometer. Color bands on the bird's legs allows for individual identification.

Along with former EEB graduate students Alexis Chaine and Dai Shizuka, professor Bruce Lyon has been studying patterns of social organization in golden-crowned sparrows that winter in the UCSC Arboretum. Initially, the project sought to determine whether the highly variable black and yellow crown patches function as "badges of status": does variation in the color or size of the crown patches help the birds determine their relative dominance in their winter flocks?

Two recent experiments, where the size of the black or the yellow crown plumage patches were altered, revealed that plumage patch size alone can determine the outcome of competitive contests over food, and thus that the plumage patches serve as "badges of status." And gender does not matter: females with naturally studly plumage win contests with wimpy-plumaged males.

The research group is now focusing on understanding the structure and dynamics of the winter social organization itself, which is surprisingly poorly understood in most migratory birds. The sparrows spend much of their time each day foraging in small flocks (2-8 birds). However, because the sparrows are all banded with unique combinations of colored leg bands for individual recognition, observing the population over time revealed that membership in a given flock often changes dramatically across days, or even hours.

This pattern of "fission-fusion" flock dynamics raises the question of larger scale social structure: are the flocks a subset of some larger social group or are they random collections of individuals from the entire population? Social network analysis methods are ideal for understanding the degree and pattern of fission-fusion in societies, and they have already revealed some intriguing patterns in the sparrows. The sparrow flocks are not random subsets of the entire population but form from larger social groups, or communities. These communities are also spatially distinct. Each of the three communities detected in 2009-2010 used a different part of the arboretum, with little spatial overlap between communities.

The social structure of the UCSC Arboretum golden-crowned sparrow population, based on co-membership in flocks. Each circle represents an individual bird, and the lines connect individuals seen in short term flocks together. Three "communities", represented here by different colors, were identified.

The birds have a pretty high return rate across consecutive winters (about 50%) which makes it possible that communities might show long term stability. Remarkably, most of the birds that returned in 2010-2011 from the previous year returned to the same communities as the previous year, and associated with the same individuals, on the same small part of the arboretum.

The discovery of distinct social communities arising from fleeting flock associations gives rise to many novel questions. Nina Arnberg, a current graduate student, is using genetic tools to tackle a further question—might these communities represent kin associations? If not, what are the factors that cause communities to arise in wintering populations of birds? In addition, the intriguing social structure raises questions about when and where the plumage badges are needed because the individuals within communities should be very familiar with each other.

The sparrow study is revealing surprising levels of social complexity in the lives of these seemingly modest sparrows— basic aspects of their fission-fusion societies are similar to those described for some primates, including chimpanzees.