I'm so sorry for your loss.
There can be a lot of causes.The only way to be sure of what they died from is to go to a vet and do a necropsy.
Chicken mesh is not really good to use for a bird aviary. This is because the cockatiels can get their heads stuck in the wire holes and can break their necks whilest trying to free themselves.
Birds are also very sensitive to chemicals. Maybe they inhaled something toxic. Sudden deaths are known to occur in otherwise healthy cockatiels by inhaling fumes from toxic agents. These are:Non-Stick Cookware
: According to the American Veterinary Medical Association and the American Association of Poison Control Centers, fumes from overheated Teflon® and other non-stick products coated with PTFE (Polytetraflouethylene ) are lethal to birds. Toxicity occurs when pots are left on a hot stove and they become overheated. It was originally thought that cookware had to reach a temperature of 500° Fahrenheit or 280° Celsius before poisoning occurred. Recently, temperatures as low as 285° F have been found to be fatal to birds. Fumes start being emitted as soon as the product starts heating, resulting in the release of toxic particles into the air that cause severe and irreversable damage to a bird's lungs when inhaled. Birds are unable to clear toxic particles by exhaling or coughing and are therefore more susceptible to this type poisoning. Toxic fumes travel through an entire house quickly. Even if birds are in another room they can still die. Death can occur within a few seconds or it can take up to 24 hours after exposure because the fumes can linger in curtains, upholstery and other materials. Death from overheated non-stick cookware fumes is very painful and necropsy of birds that have died after exposure to these fumes show lesions and hemorrhaging of the lungs.
Teflon® and all brands of non-stick cookware as well as many appliances are coated with a fluoropolymer resin. Different companies have different brand names for the same type of resin. PTFE coating can be found on pots and pans, skillets, griddles, cookie sheets, cake pans, and other bakeware, stove drip pans and broilers, self-cleaning ovens, electric fry pans, woks, crock pots, deep fryers, popcorn makers, bread machines, indoor grills, and other products such as irons, ironing board pads, curling irons, blow dryers, space heaters and now some types of heat lamps and light bulbs. Always read the accompanying literature for new cookware, appliances, light bulbs to see if it says "NON STICK" or "PTFE". If it does then it's coated with a fluoropolymer type substance. If you are not sure, call the company and ask. DON'T rely on the say so of the store clerk. Don't take any chances with non-stick cookware or products, thinking it's fine to use as long as it doesn't overheat. Get rid of it. Use stainless steel, aluminum, Pyrex®, enamel or cast iron cookware instead. Dupont has recently been ordered by the courts (2005) to release documentation that has been kept hidden from the public for 20 years. Dupont has known for 20 years that fumes from Teflon® can be hazardous to humans and birds. Other Kitchen Fumes
: Fumes from oven cooking bag, oven liners, self cleaning ovens, silicone bakeware, leaking gas, burning plastic handles on pots, burning food, overheated cooking oil, fats, margarine, butter, from burning food on aerosol spray on "Non-Stick" products can be lethal to birds.Pine Scented Cleaners
: These types of disinfecting agents are toxic. Do not use them to clean cages, food and water dishes and do not use them around your bird. Always read labels on cleaning products to see if there are any safety warnings regarding birds.Chemical Odors
: Fumes from mothballs, pesticides, hairsprays, perfumes, nailpolish remover, bleach, ammonia, paint, varnish, paint remover, permanent markers, nicotine, air freshener (sprays & hanging ones in cars/homes and plugins), carpet fresheners, scented candles, incense or any other product with fumes you can smell, can make your bird sick or cause death. Don't use them around your bird. Birds can absorb nicotine on your hands through their skin. Contact with nicotine will make your bird itchy.Pesticides
: Are toxic. Don't ever use them around your bird.Remodeling
: New carpet backing, paneling, and particleboard contain formaldehyde, and fumes are LETHAL. Paint, varnish and wallpaper paste all contain toxic fumes. Scented Candles
: they contain lethal amounts of carbon monoxide, soot and essential oils.Another cause of death could be related to food. Did he get a snack with something in it he normally may not eat? Did he chew on something? :Heavy Metal Poisoning and Sources of Lead and Zinc
: Metal hardware, wires, coat hangers, pennies, twist ties for plastic bags, stems on artificial flowers, Christmas tree ornament hooks, stained glass items, some wire toys, cages with loose solder and chipped paint, fishing weights, drapery weights, costume jewelry, zipper teeth and paint in older homes, glue on cardboard rolls from paper towels and bath tissue, duct tape, metal ends of light bulbs, flooring, lead water pipes in your home, can all contain zinc or lead. Galvanized metals are also toxic because a coating is applied over a lead or zinc base item. Ingesting heavy metals or rust is LETHAL and will cause death or permanent neurological damage if not treated immediately. If you have lead or galvanized metal water pipes in your home or your source of drinking water comes from an underground well, avoid giving your bird tap water to drink and bath in. The water could contain lead or zinc. Give your bird bottled water instead. Symptoms of heavy metal poisoning include loss of balance, muscle weakness, inability to fly, red droppings, vomiting and respiratory distress. Do not let you bird chew on any type of metal or product that contains a glue-like adhesive. Fresh Peanuts & Moldy Foods
: Fresh peanuts, Brazil nuts and other nuts in shells are often contaminated with a toxic, mold-causing fungus. Aspergillus flavus, and Aspergillus parasiticus are most commonly implicated. These 2 species grow beneath the shell of the nut and produce a mycotoxin (toxin from a fungus) called Aflatoxin. Aflatoxin is undetectable by sight, smell or taste and it can not be destroyed by cooking or freezing. Ingesting food contaminated with a high level of Aflatoxin is lethal and will cause death . Avoid feeding your bird fresh peanuts, other fresh nuts, bulk peanut butter or peanut butter sold in health food stores. All can contain deadly levels of Aflatoxin. Commercial brands and dry roasted nuts are monitored for this. Dried corn, grains and birdseed can also become contaminated with Aflatoxin. Keep your bird safe. Store all seeds, pellets, treats, grains in tightly closed containers, off the floor and in a cool, dry area.
Seeds, grains, fruit, meat, cheese, bread and other foodstuffs can become toxic if contaminated with mycotoxins. Mycotoxins are toxic compounds produced by many species of mold-causing fungi. Under the right conditions, fungi will multiply rapidly, producing high levels of mycotoxins. "Moderate levels of this compound, if ingested, will cause lesions on a bird's organs and may have carcinogenic effects (cancer causing). High levels of mycotoxins are lethal and can cause mortality within 2-3 days." Birds and other pets are especially susceptible to being poisoned by mycotoxins because of poor quality control and improper storage of food. Although some molds are visible, like the fuzz on old bread and strawberries, mycotoxins if present, can not be detected by sight, taste or smell and they can not be destroyed by cooking or freezing. Never feed your bird mushy, discolored, bruised fruits or vegetables or a food that has mold growing on it. "When in doubt, throw it out." Toxic Foods
: Avocados, rhubarb, leaves and stems from potato, tomato, eggplant and bean plants, alcohol, coffee, tea, chocolate, sugar, salt, greasy foods, tobacco, fruit seeds or pits from apples(contain trace amounts of Cyanide within their seeds), apricots, oranges, cherries, peaches, pears and plums are toxic and can make your bird sick. If a large quantity is ingested, some foods can be lethal to birds. Also raw, dry bean mixes can be extremely harmful to your pet. Uncooked beans contain a poison called hemaglutin which is very toxic to birds.