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Little skulls & cuckoos

> This points toward a complex series of instinctual behaviors.  I can
> see why cuckoos would instinctually switch from one host species to 
> another when the first began to reject foreign eggs, but why would the
> host species _lose_ that trait so quickly?  Is there _any_ benefit to
> having your offspring killed and then caring for a baby that isn't 
> yours? Perhaps there is less of brood parasitism and more brood
> _symbiosis_ than we think.

Delurking briefly, 
  The cost of cuckoo-egg-rejection behavior is that occasionally a host
bird will accidentally destroy one of its own eggs, misidentifying it as a
cuckoo egg.  There has been a fair amount of mathematical modelling &
field studies of the evolution of cuckoo egg rejection and it is partly a
trade-off of "risk of accepting cuckoo egg" vs. "risk of accidental
destruction of own egg". The risks, of course, vary with appearance &
variation of cuckoo eggs & host eggs, alertness of the host to other cues
(e.g. presence of a cuckoo female in the territory), cuckoo population &
host choice by cuckoos. Ditto for other brood parasites....
  As for symbiosis, I haven't heard of any observed benefits to hosts
from brood parasites (pipe up, anyone), while the costs (death of
offspring & "overworking" of parents) are well-documented.
- Kathleen