In ${\mathrm{NH}}_3$ bond angle is less than tetrahedral bond angle due to 

In ${\mathrm{NH}}_3$ although the central atom N is ${\mathrm{sp}}^3$  hybridized, the bond angle is $107°$ (instead of $109°28\text{'}$). The decrease in bond angle is due to repulsion between the lone pair of electrons on N-atom and bond pair of electrons between $\mathrm{N}$ and $\mathrm{H}$ atoms.