Seasonal and temporal variation in scaled mass index of Black-capped Chickadees (Poecile atricapillus)


  • Emma J. Nip McGill University; University of Guelph
  • Barbara Frei McGill University; University of Ottawa
  • Kyle H. Elliott McGill University



Black-capped Chickadee, condition index, fat stores, scaled mass index, temperature, temporal, body mass


Avian body mass reflects a trade-off between risk of starvation and predation, and may vary with ambient temperature, age, and time of day. Seasonal variability in body mass is a common occurrence in northern temperate regions, including adaptive fattening. Previous evidence suggests that seasonal variability is less pronounced in tree-feeding bird species, as their food sources during winter are less limited and variable compared to ground-foraging species. We determined fat scores of tree-feeding Black-capped Chickadees (Poecile atricapillus) captured year-round between 2004 and 2015 (n = 4248) in southern Quebec, to test the relative strength of possible drivers of variability in chickadee body mass, including time, date, and year of capture, age, and temperature. First, we demonstrated that scaled mass index (SMI) was the body condition index, out of four possible indices tested, which most strongly correlated with fat scores measured in the field. We used SMI subsequently as our estimator of body condition to avoid observer effects associated with fat scores. Similar to other studies, time of capture significantly affected SMI, in which birds captured later were heavier, indicating that chickadees experience overnight weight loss and subsequent weight gain from foraging throughout the day. SMI was constant from April to November, then peaked in late winter, but was not influenced by daily temperature after accounting for month and year. SMI was not significantly affected by age. We concluded that adaptive fattening is an evolutionary response to risk of starvation in winter, rather than a proximal response to immediate ambient temperature.

Author Biographies

Emma J. Nip, McGill University; University of Guelph

I earned my BSc. in Environmental Biology with a specialization in Wildlife Biology at McGill University in 2016. I worked in Kyle Elliot's lab (natural resource sciences) doing research on mass variation in black-capped chickadees at the McGill Bird Observatory. I currently working towards an MsC degree with the University of Guelph, in the Animal Biosciences Department, but I have continued to work with Drs. Barbara Frei and Kyle Elliott on this chickadee manuscript. 


Barbara Frei, McGill University; University of Ottawa

Postdoctoral fellow at the University of Ottawa, in the department of Biology.

Kyle H. Elliott, McGill University

Assistant Professor at McGill University in the department of Natural Resources.