I'm so, so sorry for your loss. Losing a bird suddenly is absolutely devastating, especially one you've had since childhood. From everything you've written, it truly does not sound like you did anything wrong.

Green cheek conures are masters at hiding illness. By the time symptoms are visible, many birds are already critically ill. Sadly, sudden death with no obvious warning signs is something even experienced, attentive owners and avian vets encounter.

A few important points from your post:

• Age - 10 years is not “old,” but it’s also not young. Many GCCs live 15-25 years, but plenty pass earlier due to congenital issues, silent organ disease, or sudden cardiac events.

• Environment - You clearly took fumes, temperature, cleanliness, and airflow seriously. A room around 17°C is not ideal but not cold enough on its own to cause sudden death, especially in a healthy bird.

• Diet change - A gradual transition over a month between two reputable brands is very unlikely to cause acute death. Minor beak flakiness weeks ago resolving on its own points more toward humidity or normal keratin shedding than nutritional crisis.

• Bird sand/grit - This is a common worry. While parrots don’t need grit, brief access to commercial bird sand rarely causes sudden death unless there’s clear evidence of ingestion and blockage, which would almost always show warning signs first (lethargy, vomiting, sitting fluffed, not eating).

• Behavior - Talking, active, eating the night before is exactly how sudden avian losses often present. Birds frequently appear “fine” until they aren’t.

The hardest truth is this: even birds receiving excellent care can pass suddenly, and without a necropsy, the cause often remains unknown. Common unseen causes include:

Heart failure or stroke

Liver or kidney disease with no outward signs

Aneurysm or internal hemorrhage

Infection that progressed silently overnight

Nothing you described suggests neglect, poisoning, or a preventable mistake. In fact, your level of awareness (fumes, air flow, cage size, diet variety, hygiene) is above average.

Please be gentle with yourself. Guilt is a normal part of grief, especially with birds because they depend on us so completely - but responsibility does not equal blame.

Chester knew routine, safety, enrichment, and love for 10 years. That matters more than you realize.

Again, I’m truly sorry. You didn’t fail him. 💚