Spearthrower. I owe you a debunking, and here it is.
What do you mean 'get into the genome'? What does that actually mean? It's already in the genome, it's the expression of the genome... it's not magically floating around in potential bird space waiting to be alighted on.
Since it was not present in the UCA, then where is it?
You have a population of birds living across an environment from optimal feeding grounds to non-optimal feeding grounds. As seasons change, as climate changes over millenia, these optimal and sub-optimal grounds shift. Birds would have started moving to wherever the food was or died (actually, numerous, probably the majority of species of birds did just that). That's partially genetically influenced, as in the drive to eat etc, but the actual specifics don't need to be encoded into the gene.
Good so far.
There's not a map written in DNA with an alarm clock saying - move north 2 miles on October 3rd, land, eat, then fly 2 miles further.
That’s curious, and not consistent with the facts. Let me remind you that the swallows arrive at the specific location in Capistrano on the 18th March precisely, every year, and leave on Oct 23rd. They must, therefore, have a GPS and a calendar built in somewhere. So
a. How did it get it wherever it is and
b. What mechanism do you propose for its origin?
Instead, there would have been variation in the population. Some birds would have stayed where they were and eked out a living on sub-optimal grounds and consequently gained adaptive traits that improved their survivability, others would have travelled further and further in search of food.
I don’t think anybody disputes this.
Over the generations, those birds whose genetic traits encouraged this behaviour would be retained, as they'd be passed onto their surviving offspring. Over the generations, these traits would have become more stringent with respect to the changing environment. They tied themselves into a survival strategy, and consequently there would be a strong selection pressure on their ability to traverse the world in this way as it's precisely how they survive.
But I see no explanation of
a. how a specific geographical location 7,800 miles away becomes programmed into the genome and
b. how a specific date becomes programmed in at the same time.
If you said ‘specific season’, I might agree with you, but a specific date? No chance.
The return journey also presents a difficulty. How do the birds acquire the information needed to return, since the geographical and astronomical features are now in reverse order?
And then they pass down the ACQUIRED information to their offspring. Not allowed.
Further, it must be obvious to you that a journey of 7,800 miles poses an enormous threat to their survival. Whether the swallows fly entirely over water is not clear to me, but the godwit flies 7000 miles, and the plover flies 2,800 miles entirely over water, with no stopping points possible.
Therefore those migrations at least were not undertaken bit by bit. There were simply no places to stop.
That means that gradual evolution and natural selection played no part in the production of these behaviours.
So consider your explanation debunked.