Pájaro vs. ave
The Spanish language has several words meaning bird, two of which are pájaro and ave. Although they are often used interchangeably, there is a slight difference between them.
Pájaro
A1The word pájaro is a general term for any bird, typically used for small flighted birds.
El gato está cazando un pájaro.
(The cat is hunting a bird.)
Ese pájaro tiene un plumaje de colores brillantes.
(That bird has brightly colored plumage.)
Me desperté al sonido de los pájaros cantando.
(I woke up to the sound of birds singing.)
Ave
B1Ave is a scientific or poetic term for any bird. Typically reserved outside of these contexts for all birds that one wouldn't call a pájaro, i.e., large birds or flightless birds like eagles or penguins.
El avestruz es la ave más grande.
(The ostrich is the largest bird.)
Las aves migran en busca de climas cálidos.
(Birds migrate in search of warm climates.)
Esta especie de ave no puede volar.
(This species of bird cannot fly.)
Summary
While both pájaro and ave refer to birds in Spanish, pájaro is commonly used for smaller flighted birds and ave has a more scientific or poetic connotation, often used for larger or flightless birds.