Think seriously about the chicks (5)
I believe the answer is:
brood
This could be a double definition.
'think seriously' is the first definition.
'brood' can be an answer for 'think' (thesaurus). I'm not certain of the 'seriously' bit.
'the chicks' is the second definition.
(number of young birds hatched at the same time)
'about' is the link.
Can you help me to learn more?
(Other definitions for brood that I've seen before include "Worry over" , "Meditate moodily" , "Offspring" , "Sulk; incubate" , "Ponder - family" .)