The crop is where the food is ground up to a fine paste.
It is a muscular organ and in adults contains grains of grit to help break down food.
You can check if there is food in the crop by holding the pigeon and gently feeling the chest
     below the neck and above the keel, or breastbone.
Using the thumb and forefinger, delicately feel for any grain or food. 
If it contains food the crop will feel like a partly deflated soft balloon.
It takes experience to judge, but if you can't feel anything, the bird will need a feed.

When very young, pigeons and doves must be fed by a tube into the crop or by moist pellets of food placed on the back of the tongue.
Kaytee Exact or a similar parrot rearing food is recommended but in an emergency, Ready Brek or soaked porridge oats with a tiny
pinch of Avipro and/or a vitamin & mineral supplement can be substituted.
Measure the tube against the outside of the bird, to get an idea of the tube length needed.
Gently wrap the bird in a small cloth to restrain the wings and make it easier to hold.
Take great care; open the beak very carefully and introduce the tip of the tube.
Holding the beak open with the fingers of one hand, gently pass the tube over the tongue and down the gullet.
It should go smoothly down, but do not to force it too far down into the crop. 
The tube should reach from the tip of the beak, down the length of the neck to about an inch below the base of the neck.
This should be about halfway down the crop.
If in doubt, do not insert all the tube and feed slowly, making sure the mix is not regurgitated.
You can gently feel the crop to check it is not too full.

Feed 2 hourly from 8am to 8pm if very young, giving 1-2 mls in each feed.
Always check their crops before each feed;  if the crop is not completely empty, try slightly more water in the feed;  if this
    does not work, lengthen the time between feeds.
They must have warmth, quiet, and regular feeds.

e.g. Kaytee Exact.  This must be mixed up freshly for every feed.
A very small amount can be made up freshly for each feed (e.g.half a 15ml scoop is enough for two 50g squabs)
Use hot, previously boiled water and let the mixture stand for a few minutes to thicken and cool to blood heat.
It should be the consistency of double cream but if the crop is not completely empty you may be making it too thick;
make it the consistency of single cream for one or two feeds, this should make it more digestible.
Use a clean syringe and tube for each pigeon.
Afer feeding, wash the tube and syringe well in hot, soapy water; rinse and place in sterilizing solution.

150g dry chick crumbs
420ml hot water (previously boiled and slightly cooled)
Soak chick crumbs in hot water for at least 15 minutes

Quick Mix
If for immediate use: add 300ml hot (boiled) water and after 5-10 mins soaking, add 120ml cold boiled water.
After mixing,  proceed as follows:

Mix well, liquidize for at least 30 seconds then spoon into a sieve.
Push the mixture through the sieve using a large rounded spoon.
Do not contaminate the sieved mixture by putting the spoon used for sieving into the finished mix!
Keep sieving until the mixture has virtually all disappeared into the bowl below.
It takes time, but if this is not done thoroughly you will not get an even mixture.
The finished product should be thick and creamy, and will easily draw up into a tube and syringe.
If it doesn’t then repeat the liquidising and sieving stages.
Store the mixture in a sealed container in the refrigerator.

Very small Squabs:     1-2 mls in 2-3 hourly feeds
Squeakers:   3-5 mls in 3 hourly feeds
Juveniles:      5-10 mls in 3-4 hourly feeds
Adult pigeons or doves / wood pigeons:   10-15 mls in 3-4 hourly feeds

Remove an estimated amount from the bowl (depending on the number of birds to feed) and warm up over hot water or in a microwave.
After feeding, throw away any left over mixture.