CHANGES OVER TIME AND SPACE IN A RAPIDLY MOVING CHICKADEE HYBRID ZONE: VARIATION IN GENOTYPES, HATCHING SUCCESS, AND NESTLING SEX RATIO

ROBERT L. CURRY; ROBERT DRIVER; FERRETTI VALENTINA

Puerto Iguazu

Congreso; Ornithological Congress of the Americas; 2017

Aves Argentinas, Association of Field Ornithologists y Sociedade Brasileira de Ornitologia

Climate warming is causing rapid pole-ward movement of some avian hybrid zones.These systems provide powerful opportunities for investigating fitness consequencesof hybridization. We studied variation in hatching success and nestling sex ratioassociated with northward movement of the contact zone between Black-capped andCarolina chickadees (Poecile atricapillus, P. carolinensis) using long-term data (2000 ?2017) from Hawk Mountain, Pennsylvania, along with comparative information fromthree other regional sites. Based on eight species-diagnostic single nucleotidepolymorphism (SNP) markers, the Hawk Mountain breeding population changed overthe 18 years from pure Black-capped Chickadee to strongly mixed, as CarolinaChickadees moved northward. Mean hatching success declined from > 90% to < 60%.Pair-level analysis confirmed that very low hatching success in clutches produced bymixed pairs drove the overall decline in hatching success. Contrary to expectationsfrom Haldane?s Rule, within-brood nestling sex ratio did not vary with geneticcompatibility of parents or with hatching success; however, sire genotype wasassociated with brood sex ratio, with hybrid fathers overproducing sons. Ouradditional data were consistent with Hawk Mountain results: at two sites that wereoutside the hybrid zone throughout the study, hatching success did not change; at afourth site, population composition shifted from mixed to nearly pure CarolinaChickadee, and annual mean hatching success improved over time. Our study showsthat interbreeding by chickadees confers substantial fitness costs in a pattern matchinghybrid-zone movement, but our data do not yield clear evidence for Haldane?s Rule.