Some of the issues may be minimized by allowing the cage mates to see the dead bird, though you do not want to take the risk of exposure to disease if that is suspected. You should expect the remaining birds to call for their missing companion and search for them.
Don't Rush To Replace Their Mate
Seeing as the grieving process can be a few weeks, immediately after the death of a mate might not be the best time to introduce your budgie to a new friend. Especially if they are behaving abnormally, they aren't likely to bond with a new bird and might even be aggressive towards them.
Introduce a Buddy
If you decide to bring home a new bird, quarantine him in a different room for about a month to be sure he isn't ill. The two birds will get used to hearing each other's voices. Then place their cages side-by-side for a week to let them get acquainted.
Can birds sense death? No, but they do experience loss, especially the more intelligent ones such as corvids and parrots. Parrots go through deep depression with the loss of a mate or a caregiver. Corvids such as crows and ravens are known to honor their dead by holding crow funerals.
The other one should be fine – it will probably grieve for a while. If this bird isn't tame, and is just a caged bird, then you might try getting it another bird after a few weeks or so. You can't just put them in the same cage right away.
If you want a pet bird, then a single bird is best. When you have two birds sharing a cage, even if they are the same sex, they will bond with each other and almost always stop wanting interaction with people. A bonded pair is unlikely to be tame at all.
Let's let John Marzluff, noted corvid researcher at the University of Washington, have the last word: "Birds certainly possess the capacity to mourn — they have the same brain areas, hormones and neurotransmitters as we do, they can feel what we feel"— but that doesn't mean we know when it's happening.
Once a bird of any species is dead, other birds have no more interest in it than they do a rock; they either walk around it or over it. I've seen this with dead Starlings, Pigeons, Sparrows, and Collared Doves, and I've seen the same reaction (or lack of any reaction) to the dead bird from every species in my garden.
Once they die, their body decays and goes back to the earth. Of course, decomposition happens only if scavengers don't get to them first. Scavengers – Scavenger animals like crows feed on the carcasses of birds in order to survive.
These reactions vary widely between different species, but many birds show forms of grief and mourning in response to the loss. Some common behaviors include altered vocalizations, changes in feeding habits, and increased aggression towards other animals.
You should expect the remaining birds to call for their missing companion and search for them. This may continue for several weeks, and you can help calm them with soothing words and giving them more attention.
Wild Bird Rehabilitation
Anyone who rescues a wild bird should get it to a licensed wildlife rehabilitator as quickly as possible. This will give the bird the best possible chance of surviving and ultimately being free again. There are lists of licensed wildlife rehabilitators provided by most state wildlife agencies.
Cockatiels die suddenly due to ingestion of poisonous foods, developing respiratory diseases, or being exposed to toxic fumes. In addition, poor nutrition, injury, trauma, and undiscovered illnesses are potential causes of sudden death in these birds.
Will My Bird Come Back If It Flies Away? Birds are social animals who need companionship. They'll eventually search for people and birds they know and will attempt to come back to their owner in most cases. Furthermore, captive birds will have difficulty finding food in the wild.
Bird Sadness and Grieving
There are many instances of birds expressing grief and even engaging in mourning rituals, showing that sadness isn't just a human state. It's interesting to note that birds exhibit many of the grieving behaviors we do: their posture droops, they appear listless, and often cry real tears.
Birds may feel for others (have empathy) and even console them, may have a sense of justice, may show deep affection for their partner and grieve for their loss. I witnessed the mate of a fatally injured tawny frogmouth not moving from the spot next to its dead partner for three days, and then dying on the fourth.
So birds certainly possess the capacity to mourn—they have the same brain areas, hormones, and neurotransmitters as we do, “so they too can feel what we feel,” Marzluff says—but that doesn't mean we know when it's happening.
From transduction to transmission, modulation, projection, and perception, birds possess the neurologic components necessary to respond to painful stimuli and they likely perceive pain in a manner similar to mammals.
Not only are birds capable of becoming depressed, but prolonged depression can lead to self-destructive behaviors, lowered immune response, and an array of other problems.
Birds Can Tell If You Are Watching Them -- Because They Are Watching You. Summary: In humans, the eyes are said to be the 'window to the soul,' conveying much about a person's emotions and intentions. New research demonstrates for the first time that birds also respond to a human's gaze.
By studying the behaviour of ravens hiding food, experts found the birds were able to understand that they could be watched, even without seeing another bird, and behaved sneakily as a result. Known as theory of mind (ToM), the ability allows us to understand things from someone else's perspective.