According to google, the definition of monogamous is having only one mate at a time. Based on that definition, yes, cockatiels are 100% monogamous. Cockatiels only reproduce with one mate at a time. After hours of research on the topic, I have found very little studies on the subject. The goal, not only as a cockatiel owner, but as a breeder, is to imitate cockatiels natural life cycle and habitat in the wild.
According to animaldiversity.org, in the wild, cockatiels remain together and loyal, as a pair over the course of the year. Cockatiels will often continue with the same mate year after year if they are bonded. Cockatiels are flock birds and can be found in a flock of birds up to 1,000. This gives each bird the opportunity to find their "true love" and they will search until they do. Unfortunately most ethical breeders do not have that many birds nor do they allow them to choose their own mate out of the flock in order to keep breeding lines pure and strong. Unlike in the wild where you typically only see the natural grey cockatiels, in captivity there are many different mutations of different colors and patterns. Many of these mutations are recessive. Breeding of recessive genes must be controlled and carefully calculated to keep each line strong and free of any defects. Due to this breeding style in captivity, it is very difficult, nearly impossible to imitate the mate selection in the wild. However, in captivity, pairs do often bond and have a strong enough bond to cause severe depression when separated. In situations as such, it is the breeders responsibility to keep the pair together (till death do us part).
When a pair is witnessed preening each other, eating together, and even having a clutch together does not mean the pair is bonded. This is common behavior for cockatiels who are extremely social birds. This means the birds are compatible. I did 3 experiments to see if my pairs were "truly bonded" and this is what I have found...
1. I removed one of the two birds (with multiple pairs) from the cages. Flock calling was noted with both birds which is a normal behavior of cockatiels. When placed back into the cage, the birds displayed excitement to again see each other. 2. I then placed two pairs together (both pairs being together already for 2-3 breeding cycles) in a cage. I removed one bird and exited the room with the bird. There was no flock calling noted from the bird whom remained in the cage. My hypothesis is because bird continues to have company in the cage and is not left in solitude as before. 3. The third test, I put several pairs of cockatiels together in an aviary setting. Not only did the pairs (with the exception of two pairs) not actively seek each other out but were courting and showing interest in other birds whom they were not paired up with. Now there were two pairs who sought each other out, one pair was actively nesting and the other is a pair who has a very strong bond to each other. I repeated the 2nd test with the two pairs and found very different results. When the two pairs were placed together in the cage and one was removed, both the bird removed as well as the one who remained in the cage began to flock call and continued until they were reunited. It did not matter that there were other birds present in cage. This was the result with both the nesting pair and the bonded pair. The same test was repeated months later to the nesting pair, at this time their babies have been weaned and they were no longer nesting. This pair did not flock call when test 2 was repeated as they did before. This pair was compatible for breeding but were not truly bonded.
Are cockatiels monogamous? By definition, yes. They only reproduce with one mate at a time. Are they mates for a lifetime? Sometimes, yes. A cockatiel is more likely to find their forever mate in the wild setting than in captivity.
My goal as a breeder is to have forever mates for each pairing. But I must match them up genetically to have strong lines. This may mean I have to create the right genetic match by switching up the pairs from time to time to create offspring for these matches. Once the pair is genetically sounds they will remain together for bonding, very much like an arranged marriage. Though they may not be truly bonded at first, they will eventually grow on one another and the bonds will strengthen over time. We try to shorten the time span of mixing pairs up to find forever mates sooner who will produce healthy, strong babies and at the same time having happy and healthy parents. I feel this is the most ethical way of breeding and the best way for happy birds.
https://animaldiversity.org/accounts/Nymphicus_hollandicus/