7 chicks dead

So I looked in on my freshly hatched babies this am... and there were 7 dead... 2 just laying there... That is hard for me, but i undrstand it happens... but what I found in the cornor of the brooder box just plain makes me angry and confussed... 5 chickies all wet with slime in a staight line... then in the 4 week old pen i found one dead.... I dont get it what is going on I have never had any problems like this before... whatis killing my babies???

 

there is but it is topped with chick wire... there is a spot where there is a bigger hole though... we custom made ours it is a 7 foot long 3 foot wide wooden box with a see though top covered in chicken wire lid... the dogs run around out there so we but so hard steal goat fence attached to the top of the lid... its really heavy... it just kills me... these poor babies