Following Fristoe (2015, GEB https://onlinelibrary.wiley.com/doi/epdf/10.1111/geb.12262):
$B = cM^b$
where M is body mass
"I use values provided from empirical studies for passerine birds under natural condi- tions of c = 10.5... and b = 0.713 (Nagy, 2005; McNab, 2009)."
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