/ər/ spelled ir, er, ur
So, just to keep you on your toes, sometimes the letter r changes the sound of the vowel.
ir bird
ir spelling is usually found in the middle of a word
bird, first, dirt, chirp
er tiger
er spelling is found in the middle or the end of a word
fern, herd, letter, otter, tiger
ur surf
ur spelling is usually found in the middle of a word
turtle, burn, turnip, curb, nurse