Very sad.
Would the other holders understand should you put a lock on the run to prevent C. from getting to the chickens without at least first letting you know, or would that be an abuse of collective property?
I've read this more than once on BYC and I'm sorry to say it's just not true anymore, the breed has become a victim of it's great popularity. Over french forums many chicken keepers are complaining about them. They say breeders from the Marans Club de France tend to breed for egg color first and SOP second, and hardiness is totally left aside. There has been too much inbreeding and poor strains.
Then on the other side, many dishonest breeders are selling Harko or Harko x Marans as pure Marans. Harko is a RIR x Plymouth rock hybrid, belonging to and commercialized by Hungarian brand Tetra Balbona, sold as a dual hybrid in many european and african countries, and worth half the price if not less of a Marans.
While there are certainly good breeders left , and especially if you travel to Marans or it's region Charentes-Maritimes ( 1000 km away from my home) as was the case for the chickens Shadrach lived with, the chance of getting a good Marans chicken without knowing the breeder in France are low.
I got two pol pullets cross Marans/harko two months and two weeks ago, and while the breeder was terrible, she had at least the honesty of selling them as such. They have been laying like crazy, as much as the leghorns I got from the same place. They are going to be heavy birds and they really don't seem to fare well in our relative heat (34c/ 93f).
Kara. no feathered feet and has a broken beak since we got her.
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Lilly , feathered feet, lays less, seems unwell in the heat.
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That makes sense, and IMO it's one of the most unfortunate things that can happen to any breed of any animal. Popularity, in most cases, ruins breeds, at least to an extent. It's double sad when the quality of a breed lessens significantly in its home country. What's also very sad, is when the line between a breed going extinct from not enough popularity, or having terrible quality from too much popularity. Since you brought up heat resistance, I'm inclined to believe that there is in fact some connection between quality/vigor and heat resistance. But heat resistance is a very big topic, so I don't know if it's worth opening pandora's box