Bird siblings:
I fall in love pretty easy. I really like women. Fortunately for all, that is a one way street. Otherwise my life would probably be a greater disaster than it has been, and I might have caused a lot of grief for others that I have not caused.
But there are different degrees, nay different qualities. When I find I am attracted to a suitable relative it … well it’s hard to explain. It’s like the difference between falling out of the door and falling out of a high window.
And sometimes I have not even known we were kin. Given the incredible importance of mating with kin, one might expect that it would be possible to recognize them. I have never known this to be put to the test among humans.
It has, however, been put to the test among birds. (Oh Sibling, Who Art Thou, Andrew Cockburn NATURE vol. 466 no. 7309 August 19, 2010 page 930 and Promiscuity and the Evolutionary Transition to Complex Societies, Charlie K.Cornwallis, Suart A. West, Katie E. Davis and Ashleigh S. Griffin page 969 of the same issue)
What is at stake is that some birds, instead of starting families of their own, will stick around the mother and help her raise other hatchlings. If she is truly monogamous, then the helper will be investing in young that are as nearly related as his or her own offspring. If she is not, things get a bit trickier. Some females improve the free time gained by having the help by flying off to find males with whom to cheat. But by and large fidelity correlates with more help.
Still in all, if one were rationally pursuing a behavior pattern that promoted the flourishing of genes one shared, it would be quite helpful to be able to look at a brood and say, “Yep, they’re all my full siblings all right; guess I’ll pitch in and help Ma like a good boy,” or “Nope, Mom’s been hanging out at the bar again; these guys are only my half-siblings. I think I’ll hop down to that bar and see what I can pick up on my own account.” The decision is of course not rational in birds and probably has a smallish rational component among humans. But is would be handy to be able to tell.
As it turns out, birds generally can recognize a related brood but not generally a related individual.
So it is unclear whether kin recognition is of any consequence for courting purposes even though it is of supreme importance for reproductive purposes.
And even this tenuous kin recognition that is described only applies to siblings. They may not be the best choice. Inbreeding and all, don’t you know. What would be advantageous would be kin recognition out somewhere along second, third or fourth cousin level. That has not yet been described.
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