My thought was the same as all the previous posters.
This link describes the alleged method of toxicity..
https://animal-world.com/can-chickens-eat-lettuce/
Many more toxic things than that are growing in my yard/pasture.
Small amounts of iceberg lettuce are not an issue IMO and experience and I would not currently be concerned at all.
		
 
		
	 
	
		
	
	
		
		
			I’m reading now that it is toxic to them.
		
		
	 
I'm just going to put in here a friendly reminder to
 doublecheck the sources of any information you read about a specific product, method, etc. for bias and misinformation.
For example, this Animal World.com site appears very sketchy.  They don't have an "about us" page, or any typical information displayed at the bottom other than (© 2024 Animal World • All Rights Reserved), and don't cite scientific sources.  Aside from their many, many articles, and a "contact us" page which is just a questionnaire box with no actual contact information their site is essentially blank.
The linked page is marked as "
Approved by Dr. Paola Cuevas" but has no information on it that I can see as to the legitimacy of this Dr. (through further research this Dr. specializes in dolphins according to their LinkedIn page???) or of the site itself.  The page repeats information about 3 times and then continues on to say that 
its sources are other similar sites (aside from the Purina.com one) with, again, no valid scientific citations that I could see.
Anyone can write an article and make claims based on hearsay.  If they don't have proper research to back it up, or the article has an incoherent line of thought with massive grammar/spelling errors throughout, then it's likely not a legitimate source.