Black Astalorps dying

[RTorrance]

We had 15 baby chicks about 3months old. 5 Astralorps, 5 Calico Princess, and 5 Red Production. They've been out in the coop for about 3 weeks now, both an indoor part and a fenced in part. They have food, water, and a heat lamp inside (unplugged during the day). They didn't start wondering outside until about a week ago, which we then put water and food out there.
We live in Western Illinois and the temps have been a little sporadic from the 30s up to high 80s last week. Today reached high 70s and the chicks had been checked on throughout the day by my husband. When I checked on them at 700pm, I found all 5 of our Blacks dead in the grass. Only the blacks. Some of the others were energized while others were resting, but alive.
What would cause all five of only the one breed to die all at the same time of day?

 

[oldhenlikesdogs]

Do they have shade?

 

[RTorrance]

Do they have shade?

Yes. They have access to the inside coop, a shaded spot under the coop, tall grass and a tree nearby casting shade.

 

[21hens-incharge]

What feed are you using?
What treats are you giving and how much?

Very odd to lose all five at once.

 

[RTorrance]

Do they have shade?

The Blacks were also the least adventurous; they didn't wander outside as much/often as the others. The coop door stays open 7am-9pm.

 

[RTorrance]

What feed are you using?
What treats are you giving and how much?

Very odd to lose all five at once.

No treats. The only thing they have is like a crushed pellet since they're still chicks. It's what they've been eating since we've had them. No problems that I've seen with the food. Not moldy either.

 

[RTorrance]

No treats. The only thing they have is like a crushed pellet since they're still chicks. It's what they've been eating since we've had them. No problems that I've seen with the food. Not moldy either.

I'm unsure of the brand off the top of my head.

 

[21hens-incharge]

Any potentially poisonous plants they coud have gotten into?

Good that you are using a proper feed.

 

[RTorrance]

Any potentially poisonous plants they coud have gotten into?

Good that you are using a proper feed.

Hmm..I'm going to say no. There's only grass in the fenced in area. My husband did mow the yard today, though and said they were able to be outside while he mowed. But I still don't think there would be anything poisonous near the coop that the mower could have blown in.
What would be some plants that are poisonous to chicks?

 

[LittleCheepers]

I am SO sorry for the loss of your sweet chickens, the poor little things! I can only imagine how upset you must be. I certainly hope you find an answer as to what happened.