Loving birds:
I had previously mentioned that a study done on mostly-loyal birds showed that when the female was promiscuous and had offspring by some male other than her social, and usually procreative, mate the offspring seemed more fit than those of her first choice.  This I put down to the notion that given the strong selective pressure all animals have to mate with more or less close kin nature probably provides some means for recognizing a mate of suitable consanguinity.  In the hurry of mating season of course she does the best she can.  But perhaps after time for further experience she could find the better mate, meaning one of a more appropriate degree of consanguinity.  An effort to get this idea checked out was in vain. 

Now we get the other side of the story.  (Cheating Cuts Offspring Fitness NATURE vol. 485 no. 7397 May 10, 2012 page 151 reviewing work by Jane Reid et al.  of the University of Edinburgh, UK Am. Nat.http://dx.doi)  They found that in song sparrows the half siblings sired outside the social pair were reproductively less fit than those sired within it. 

Since the evidence runs both ways, there is no proof that consanguinity has anything to do with it although I cannot imagine what else could be at work.  It would require keeping track of the family trees of all the birds in both experiments and see whether the sparrows got it right more often on the first try.  Of course such a study would requite epic investment of resources and would only add a pebble to the mountain of evidence already on offer.

Still, it’s nice to think that at least for song sparrows, fidelity does pay off. 

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