Looking quickly at the examples and not thinking too deeply about why I'm modelling the bird as a cube, "The aerodynamic lift developed by the bird's wing varies approximately with the area of the wing; the weight of the bird varies approximately with the bird's volume.", i.e:

thrust = wingspan ^ 2

weight = wingspan ^ 3

Since "Heat production (a function of muscle mass) varies approximately with the volume of the animal; heat loss (primarily through the body surface) varies with the surface area." I guess that means:

heat production = weight

heat loss = wingspan ^ 2

[I'm using "=" instead of proportional to, but you know what I mean]