The Tigers went 3-0 up after just 33 minutes but conceded twice thereafter.
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Six minutes later the Hull winger tried his luck again, but failed to his keep his left-footed shot on target.
Moments later Wade Elliott went close for the home side.
Aluko gave Hull the lead on 14 minutes when he latched on to Jay Simpson's pass and shot home from close range.
The winger doubled his side's advantage in the 26th minute when released by Simpson again, firing in from 12 yards.
It became 3-0 to Hull on 33 minutes when James Chester headed home Aluko's delivery from close range.
However, five minutes later Ravel Morrison pulled one back for Birmingham with a right-footed shot from around 12 yards out.
Then a minute into the second half the home side scored their second goal of the afternoon, Marlon King shooting home from 18 yards.
Simpson came close to scoring for Hull again just after the hour mark, but failed to keep his shot on target.
Aaron Mclean too went close for the visitors with eight minutes of normal time left, but his header went over the Birmingham crossbar.