My cockatiel was wheezing and died this morning - how can you save your pet bird?

Описание: My male cockatiel spent yesterday's afternoon in his cage in a shaded area in my garden in the company of my other female cockatiel. The last few weeks they have enjoyed the warmer weather of April from inside their cage where they have been able to pick from the grass in the garden, eat their seeds, drink their water and move out of the shade to bask in the sunlight. Around this period of the year I start to place my birds out of my house in the garden to give them a breath of fresh air and let them enjoy the new season. Yesterday it was sunny and warm with almost no wind. When I placed the cage back inside in the evening I noticed the male cockatiel had started making a funny noise - the same noise you can hear in the video. I called some vets but they all told me that they didn't treat birds as patients and they knew nothing about them. My male cockatiel continued the whole night making the same sound, breathing heavily, and this morning right after making this video I watched him run out of air and die of heart attack in his cage. I can't understand what has happaned to him. The female cockatiel that has always lived in the same cage beside him is doing just fine. Help me understand the reason of my bird's sudden demise!