Increase the number the bird is carrying (8)
I believe the answer is:
heighten
'increase' is the definition.
(heightening is a kind of increasing)
'the number the bird is carrying' is the wordplay.
'the number' becomes 'eight' (eight is a kind of number).
'the bird' becomes 'hen' (hen is a kind of bird).
'is carrying' indicates putting letters inside.
'eight' put within 'hen' is 'HEIGHTEN'.
(Other definitions for heighten that I've seen before include "Move up" , "Intensify; make taller" , "Intensify, enhance" , "Make more intense - elevate?" , "Lift" .)