The Spanish word 'pajarito' meaning 'little bird' comes from combining the word 'pájaro' (bird) with the diminutive suffix '-ito'. The word 'pájaro' itself has an interesting evolution, tracing back to the Latin word 'passer' meaning 'sparrow'. This evolved into the Vulgar Latin 'passar' which generalized to mean any bird, not just sparrows. This then became 'pássaro' in Old Spanish, eventually developing into the modern Spanish 'pájaro'. The diminutive '-ito' is a common Spanish suffix used to indicate smallness or endearment.

The most obvious related Spanish word is 'pájaro' (bird), which is the base word from which 'pajarito' is formed. Spanish speakers frequently use diminutive forms like this to express smallness or affection. You might also encounter other variations with different diminutive suffixes, such as 'pajarillo' or 'pajarico', all meaning 'little bird' with slightly different connotations.

While English doesn't have any common words directly related to 'pajarito', there is an interesting scientific connection. The Latin word 'passer' from which it derives is still used in scientific nomenclature - the house sparrow's scientific name is 'Passer domesticus'. The term 'passerine' in English, referring to perching birds (which make up more than half of all bird species), also comes from this same Latin root.