We study the dynamics of hovering mechanisms inspired by hummingbirds and bats to extract practical guidelines for the design and control of aerial robots. Using a dynamical model that incorporates wing flapping, pronation and folding, we determine the parameters for which hovering is possible. We show that the pronation angle (rotational degree of freedom for the wing about its span) significantly effects energy consumption of the system and demonstrate an optimal wing-beat frequency that minimizes the average power consumption. Furthermore, we show that increased body rotational inertia can significantly increase the energy consumption of hovering and can lead to implausible hovering trajectories. Last, we show that a bat-inspired hovering mechanism that allows for wing folding can generate plausible hovering behavior for larger aerial systems.