I am August Abbott, certified avian specialist and owner of BrokenBirds.org - I'm sorry for the delay; there seems to be a glitch in getting notified from this site so please don't hold it against me
There is very little more painful for us than to find our feathered companion passed away with little or absolutely no warning. We can carry the guilt, even if it’s not our fault, for years.
Let me assure you, sometimes these things happen and in many cases not only is it nearly impossible to know ahead of time that something’s wrong, but quite often there’s nothing you could do fast enough to change the outcome.
When your bird starts to act ‘off’, chances are it’s been ill for quite a lot longer than you imagine. Their natural, hard-wired instincts to hide illness/weakness overrides their bonding and trust in you every time. In the wild a weak or ill bird will end up another animal’s dinner, very quickly.
Even with a necropsy (the animal version of an autopsy), many times the cause of death is never found.
Probably one of the most common is a toxin. It might anything from Teflon/nonstick cookware fumes (yes, even if they’ve been well tolerated for years, it may have been building up in the system) to gnawing on or even swallowing a toxic metal in or on the cage. Cleaning products, carpet fresheners/shampoos, pesticides used inside or outside the home, scented candles, all sorts of things we take for granted could harm a bird with their specialized and very different respiratory systems.
Sometimes sources of toxins are insidious and sneaky. These days many hand held blow driers, toaster ovens, drip pans on stoves, self cleaning ovens, waffle makers, portable heaters, just way too many things to name.
Other causes could be disease or internal tumors. Liver disease is relatively common, especially if the bird has been on a predominantly seed diet.
There are just any number of reasons something this tragic might happen and blaming yourself isn’t going to change anything. It’s already hard enough to accept this loss, you don’t need to feel worse.
Now, the biggest concern is the remaining bird. He MUST be seen by your vet right away to test for infections or disease
This is often as easy as an oral swab and sampling of droppings to check for yeast/fungus, bacteria, viruses etc.
A culture may be done (and encouraged) - whereas a sampling is then 'grown' in a petri dish over a couple of days to see if it's positive for anything else
In the meantime, clean the cage meticulously just in case your dear departed did have something infectious. Fully change out food and water dishes, washing in hot water with plenty of soap
Wash cage bars down with a mixture of 1 part bleach to 9 parts hot water
Toys or anything porous should be thrown out and replaced
Finally, yes, your remaining bird is in mourning - they actually understand death and loss. Talk to him, interact with him more and as strange as it seems - tell him what happened and that his partner had to fly on ahead to wait for him - but he can't take all the time in the world, no rush
You'll want to be sure he's eating normally and to check this, watch his droppings
You'll want to see the same overall color and consistency
Again though - it's truly necessary to have him seen and checked out
You'll do that?
Please accept my deepest condolences and believe me, I’ve been where you are – though you may never stop missing your friend, the hurt you feel right now will get better in time.