1. What do the following words have in common?

IN
SIN
ALLY
CENT
STING
VENUE

2. What do the following words have in common?

OIL
SWAT
STARK
THRASH
STARTING

HINTS:
1. The same two letters, in the same position, will change each word into a new one.
2. It has to do with changing one letter in each word.

Answers:

1. Each word can be preceded by the letters RE to make new words:

REIN
RESIN
REALLY
RECENT
RESTING
REVENUE 

2. Changing one letter in each word will create the names of five birds:

OWL
SWAN
STORK
THRUSH
STARLING