Hybridization fluctuates with rainfall in Darwin's tree finches

Hybridization in natural populations may be an adaptive response to shifting climatic regimes, but understanding this can be limited by temporal sampling effort and confident hybrid identification. On the Galapagos Islands, Darwin’s finches regularly hybridize, and the islands show extreme annual variation in rainfall, however the effect of annual rainfall on the frequency of finch hybridization is little known. Across a 19-year period on Floreana Island, we compare patterns of hybridization in sympatric Darwin’s tree finches (N = 425; Camaryhnchus spp.) and test for an effect of annual rainfall on, 1) the frequency of hybrids (C. pauper × C. parvulus), and 2) the percentage of male hybrid birds produced per year (hybrid recruitment). Annual rainfall correlated with recruitment positively for hybrids, negatively for C. parvulus and not at all for C. pauper. Furthermore, the percentage of hybrids (range: 12-56%) and C. parvulus did not change with sampling year, but the critically endangered C. pauper declined. Our findings indicate that hybrid recruitment is recurring and variable according to annual rainfall in Camarhynchus Darwin’s finches.

Methods
Our study was conducted in the highlands of Floreana Island, Galápagos Archipelago during February and in some cases March/April during 1997, 2005, 2006, 2010, 2012, 2013, 2014, and 2016 (8 years of sampling spanning 19 years). The focal tree finch species were small tree finch (Camarhynchus parvulus), medium tree finch (C. pauper), and the recently discovered hybrid group that arises from pairings between C. pauper females and C. parvulus males. DNA was extracted and genotyped using nine microsatellite loci designed for Darwin's medium ground finch (Geospiza fortis: GF01, GF03, GF04, GF05, GF06, GF07, GF11, GF12, GF13, Petren, 1998), with protocols described in Kleindorfer et al. (2014) and Peters et al. (2017).  Analyses are as described in the corresponding paper with this data.