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Cain and Abel syndrome, or simply cainism, is the killing of a nesting 
bird by a nestmate, always the first and oldest bird.  It is most 
dramatic in eagles, especially Aquila.  It is the result of the eldest 
sibling either starving the younger sibling to death or actually 
directly killing the younger sibling.  It almost always happens in 
clutches of two to three birds.  In the Verreaux eagle there are always 
only one bird that surivives out of a clutch of two.  It is very common 
in the bigger birds and less common in medium-sized birds such as hawks, 
to rare in smaller birds.  

It is thought that cainism is related to the abundance of food.  When 
there is an abundance of food cainism is rare, but when food is less 
abundant cainism is common.  What cainism really shows is that only a 
few chicks out of a clutch are actually needed to help the species go 
on, the rest are a form of insurance in case of something happening to 
the oldest sibling.  

Matt Troutman

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