Brother cats fighting after seeing cat outside - help!

[JillyBoel22]

We have two 8 month old brother cats who have recently started going outside (neutered/vaccinated/chipped)

They have so far been enjoying lounging on the conservatory roof and on the walls of the back yard and seem to be just monitoring 'their turf' so to speak and not going much further. They come back when called so are generally no bother and haven't any worries of them being out.

However yesterday another cat came on the roof and began fighting with one of my cats (appreciate this is normal as they are the new cats on the block so to speak) however when they came in, both my cats were growling/hissing/whining/making lots of noise at eachother and being hostile. This lasted about 30mins then they were back to cuddling.

Today, a different cat came to the front garden whilst my two were out there. I chased said cat away as they were making noise at eachother aggressively. Same again when they came in, my two then hissing and making loud whining noises at eachother.

They are currently friends again - but I am starting to get concerned at why they seem to turn on eachother - is it just adrenaline from the stand off with the other cat? Help!

Thanks in advance

I had two very docile Persians, who completely turned on each other if a visiting cat came into their garden. I didn't realise until I spotted the blood. And one of them had taken a chunk out of her sisters ear. Though they were mainly indoors.

My cats bizarrely were absolutely fine with visiting Persians actually in their house. But a local
Moggy in the garden resulted in a punctured eyeball, and an £800 vets bill. 🤔 Strange creatures!

I think it is adrenaline, it didn't affect my cats relationship at all.

Thanks @Toddlerteaplease that's reassuring to know it didn't affect the relationship - cats are definitely bizarre creatures! Hope your two are doing okay (especially the injured one! :( )

Cat relationships are fascinating. I was told my pair were closely bonded. But they weren't at all. One of them died a few years ago and remaining cat is much happier with her new sister. They aren't particularly close, but work together to gang up on me. Which is hilarious!

Cats are odd creatures - ours will scrap (just noise) and then turn round and bathe each other like nothing has happened