Fig. S2.

Relatedness to the whole brood decreases as brood size increases. The average relatedness of a focal offspring to the whole brood decreases with each additional sibling, until reaching an asymptote r = 0.5 for full siblings (●) and r = 0.25 for half siblings (○). Whole brood relatedness is calculated by averaging a focal offspring’s relatedness to itself and each of its siblings. For example, r = 0.75 for two full siblings (1 + 0.5)/2, and r = 0.5 for three half siblings: (1 + 0.25 + 0.25)/3. Even without any extrapair paternity, the relatedness of a focal chick to the whole brood decreases rapidly as brood size increases: average relatedness to the whole brood is 75% with one full sibling, drops to 67% with two full siblings, and to 55% with ten full siblings.