First I would like to say that this fact does not apply in specific to canaries, it is for most type of birds.
The fact is
Birds are the only living creatures that have the ability to keep sperms alive in their bodies for days or weeks under normal body temperature. After a successful mounting by the male and ejaculation, some sperms will travel all the way to meet the released egg and fertilize it. Most sperms will be stored in a specific gland near the vent called "sperm storage gland". After the egg is being laid by the hen, the contractions and the return of the cloaca to its normal position cause some of the stored sperms in the gland to be squeezed back in the female reproductive tract to meet the newly released egg and fertilize it. The process keep going on after each egg being laid. So, one successful mount and ejaculate is enough to produce fertilized egg.

As we know, most of our pet birds produce 3-5 eggs in a day or two interval, so within this short period of time all eggs under normal conditions should be fertilized.

My opinion is that, with increasing the time of sperm storage the sperms will start to lose some of their fertilizing ability. More explanation, canaries, finches, parakeets.... will finish laying eggs in a week so it is a short period of time so sperms will still be a live, motile, and have a good fertilizing ability. Chicken as we know keep producing eggs and from what I know sperms can be stored in the sperm storage gland for two weeks. Those two weeks "long period" is what I am talking about regard decreasing the fertilization ability, they are good for days but longer period cause them to be less fertile.

I heard many stories from breeders that after breeding, the male died and the female laid eggs and they were fertile and produced chicks. The male died and they introduced a new male, when the chicks were hatched they carry either the first dad characteristics or some of it, which were completely different between the two males.

Please if you need more explanations or have questions, do not hesitate to reply back.

Best wishes to all

Thanks Abu Aljoud. Yes I agree with you, respecting the life cycle of our lovely birds is the key to success.

Best wishes

Thank you my brother

You are more than welcome Dear Abu-Seir