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

Abstract

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.

Song of the Riverbank Warbler Myiothlypis rivularis: Male Constraints and Female Territorial Signaling

“…It is possible that other song parameters, not measured here, function as better proxies of male body condition. Also, as the scaled mass index can vary seasonally (Nip et al 2018), measure it only once may have biased our results. Song parameters may also act in intrasexual communication and modulate behavioral responses of potential rivals (e.g.…”

Inbreeding depression depends on the body size and environmental conditions in a threatened songbird, the aquatic warblerAcrocephalus paludicola

“…SMI is a proxy for relative energy reserves, and is quantified with the body mass and a measure of body length (Peig & Green 2009), for which we used the tarsus length (see point 7). In passerines, SMI corresponds to the amount of subcutaneous fat (Peig & Green 2009, Nip et al . 2018).…”

Breeding season forest fragment size does not create negative carry-over for adult Wood Thrushes on fall migration timing or apparent annual survival

“…To account for structural size variation, we used the scaled mass index developed by Peig and Green (2009) to estimate body condition. The scaled mass index has been identified, as the best way to estimate energy reserves in some passerines (Peig and Green, 2009, Nip et al 2018) by adjusting mass to a standardized body length measure that is positively correlated with mass on a log–log scale. In our case, wing chord was more strongly correlated with mass than tarsus, so we used it for our length measure.…”