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
The 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
'Ave' 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.