Beak Discoloration in Sparrow

[Turonis]

Hi everyone, I hope you are all safe in this weird times we are living.
I have searched the internet about this issue and the information on sparrows is very scarce, so I decided to post it here because of all the very experienced members of the community.
It's the middle of the Fall season here where I leave and I noticed my sparrows are molting, which was expected. Now I noticed that the male sparrow's beak is changing color. Once completely black, it is becoming light from the part that touches his head.
I don't know what could be causing this. I feed them a diet of seeds and pellets for finches and canaries, plus apple and leaf greens. I make sure they take a sun bath almost everyday, except when it's cold and windy. They also have a cuttlebone at their disposal, which they peck at often.
I let them out of the cage for hours everyday so they can fly and get some exercise.
I've attached some pictures. The last one is from 27th April, when his beak was still completely black.




Let me know if you need more info to assess this.

 

[Kiwi & Co.]

@scrape

 

[iamwhoiam]

During breeding season, late spring and summer months, male sparrows' beaks are black.
During fall the beak color changes to beige. It should become black again during breeding season. This is normal.
How old are your sparrows?

 

[scrape]

Sounds completely normal to me

 

[Turonis]

Both of them are 1 year and 6 months old, with two weeks apart form each other.
That makes complete sense, thanks a lot. I am relieved.

 

[iamwhoiam]

For whatever reason the change can be variable. I know that Spee's beak would generally become very beige (female coloration) off breeding season but sometimes it would stay mostly black or change to light grey. They are very cute, btw. What are their names?

 

[Turonis]

I had noticed that his behavior started to change. He is still very active, but calmer. I heard that in some species of birds the males go sterile when it's Fall/Winter.
The female sparrow is called Thrill, the male is Tron.
Is spee your only sparrow?

 

[iamwhoiam]

@Turonis Never heard about the sterility during the non-breeding seasons. That's interesting.
Have you had Thrill and Tron since they were babies? How did you get them?
Yes, Spee is my only sparrow. I do have other birds, though.