Hybridization in Darwin's finches is rare or frequent?
I actually just had to read a Grant &amp; Grant paper, so hopefully, I can get this right. My understanding is that hybridization events have repeatedly seen to occur, but the events themselves usually do not repeat within a lineage, i.e. once a lineage becomes partially hybridized, it is unlikley to continue to breed with adjacent populations.

Due to the reproductive barriers that come from song patters of hybridized individuals (Grant &amp; Grant, 2009), Repeated hybridations are unlikely to occur within a lineage, even when in the presence of a species with which hybridization can occur. 

Grant, P.R. and B.R. Grant. 2009. The secondary contact phase of allopatric speciation in Darwin's finches. PNAS 106 20141-20148