jmc

Let the number of two-legged birds be $x$ and the number of four-legged mammals be $y$. We can now use systems of equations to solve this problem. Write two equations: $2x+4y=522$ $x+y=200$ Now multiply the latter equation by $2$. $2x+4y=522$ $2x+2y=400$ By subtracting the second equation from the first equation, we find that $2y=122⟹y=61$. Since there were $200$ heads, meaning that there were $200$ animals, there were $200-61=\boxed{139}$ two-legged birds.