[DeluxeMoustache]

Lots of people like to talk about this old wives’ tale that baking soda and vinegar can be used to unclog a drain. Not really

Here is the equation
CH3COOH + NaHCO3 -> NaC2H3O2 + H2O + CO2
vinegar + sodium bicarbonate -> sodium acetate + water + carbon dioxide

The fizzy volcano project basically shows CO2 gas being liberated. The pressure increase in the bottle, and reduced diameter across the neck, the gas / fizz escapes with velocity. With no obstruction though. Basically moving from an area of higher pressure to lower

Sodium acetate is an organic salt, not corrosive. So anything you rely on chemically would be perhaps an excess of a very weak acid, vinegar.

* In the right proportions, these products may be able to make a nice fizzy electrolyte replacement drink to spank that hangover

Even if you can seal off an end of the pipe, I suspect you would be able to effect a minor pressure rise, likely practically insignificant, given the volume of the pipe. The liberated gas causing pressure rise would be spread over the available volume within the pipe

If the blockage is a perfect seal, you may have a pressure gradient. If not, the pressure of the gas will normalize on both sides

As for birdseed, I don’t know exactly what it’s made of

In a controlled lab setting, hydrochloric acid can be used for demineralization, and sodium hydroxide (a strong base) for deproteinization

Both are terribly nasty on their own (but fun fact, mixed together, in the right proportions, they would make ordinary water and salt - HCl + NaOH -> NaCl + H2O )

Drano uses sodium hydroxide

If freezing is the issue, hot water or maybe some rock salt.

Depending on whether you have any drainage at all, you could try both an acid and a base treatment, as they would do different things. Just one at a time

Perhaps Muriatic acid - let it sit for a good amount of time and flush (hopefully you are getting at least some drainage)

Then afterwards, something like Drano?

Or try Drano first. I’m not sure how much protein is in the seeds

I feel like either reaction would take a long time, given concentrations available in commercial products, maybe repeated treatments.

Your guidelines for something like Drano are more likely based on dissolving things like food waste and hair.

If freezing isn’t the issue, probably the best bet, in terms of time to resolution, is physically unclogging, using auger or some kind of water jet. That thing AFC mentioned sounds interesting.

Is it possible to fill the pipe and use a plunger?

Good luck