Rick asks: "I have heard that the regurgetation of food in the feeding process is triggered by the visual stimulus of the chicks physical features. In other words: it is the actual look of the young that causes the parent bird to regurgetate the food, in effect feeding it. Is this true?"
The feeding process with birds is an incredibly complex operation, not all birds regurgitate and not all of them would regurgitate just from physically seeing their young.
For example, in many of the Tit and Finch species where their young are born in dark nest chambers or nest boxes they cannot actually see the young, the young do however have a gape flange, a section of the beak where the lower and upper mandible join up, in young birds the gape flange is often brightly coloured, sometimes with distinguishing spots that show up in the dark and direct the parent bird where to place the food.
In addition to this most chicks will also provide some vocalisation to attract the parents to them and often these "begging" calls can be the main factor allowing them to be fed. It is often the most vocal and forthright bird that acquires the most food from its parent!
In large colonies of birds where thousands of chicks and adults live close by such as seabird colonies, penguin colonies etc it can be difficult to understand how parents can find their young let alone give them a feed. Clearly visually the birds might be able to remember where they nested but they would still need to make contact with their young, often the calls of the young are the deciding factor that help to reunite the parent and chick, and once they have been reunited the parent is stiumlated into feeding its young.
The sight alone of a chick might be enough to encourage the adult to provide food, the sole reason the adult left the chick was to get food to bring it back so clearly the parent understands what it needs to do, its parental instincts would naturally kick in upon seeing its chick and feeding would commence, whether this through regurgitation or another way (birds of prey place the food item down and then tear up the meat to feed in smaller pieces to their young).
However, sometimes the chick encourages or stimulates the act of regurgitation through behaviour, Storks and Heron chicks can often be quite forceful in the way they feed almost harassing the adult into opening its beak to regurgitate.
Whether through sight, sound, colour or behaviour the natural instinct would always be for the parent to feed its young, often it comes down to what type of bird and in what situation it is in to how the feeding process is undertaken.
I think the herring gull species is one of the gulls which has a bright orangey-red spot on the side of its bill. The youngsters peck at this, and this action stimulates the parent birds to regurgitate food.
:-)
Seriously thinking about trying harder!
Baby dunnocks have orange gapes and 3 spots on their tongues. I wonder if it's for the same reason.
A closed mouth gathers no foot.