Tracking the consequences of interbreeding as a hybrid zone moves north

Two decades ago, my students and I began studying hybridizing chickadees at Hawk Mountain Sanctuary, an extension of our broader research program.

Recently, former M.S. student Robert Driver took the lead on publishing a massive paper documenting how hybridization leaves its mark on the population as the contact zone has slid north in connection with climate change. Specifically, we showed that average hatching success steadily declined as the proportion of admixed chickadees increased.

We also tested whether the deficit in hatched eggs affected female embryos disproportionately. Haldane’s Rule predicts this pattern, because female birds are heterogametic, carrying W and Z sex chromosomes, and are thus likely to experience problems of development and viability.

We did see that effect: male nestlings outnumbered female nestlings in the nests where many eggs (some of which presumably would have been female) failed to hatch. Surprisingly, the ancestry genotype of the father accounted for nestling sex ratio more strongly than did the mother’s ancestry.

This work involved many hard-working former students … and it’s great to see it now in circulation!

Driver, R. J., V. Ferretti, E. S. Burton, M. W. McCoy, K. L. Cornell Duerr, and R. L. Curry. 2022. Spatiotemporal variation in hatching success and nestling sex ratio track rapid movement of a songbird hybrid zone. The American Naturalist 200:264–274.