What does it mean when budgies keep biting at the bars on the cage?

i have 4 birds, 2 male and 2 female. one male and one female in each cage. i got the other two bout 2months ago so i havent put them together yet. But anyways ive noticed the new birds keep biting at the bars on the cage. Then i seen my other start doing the same. they both have a coddle bone and mineral bar in their cages. Do i need to put in something else? thanks for the help, im very new to birds.