Treecreepers are small passerine birds that occur in wooded regions across the Northern Hemisphere and sub-Saharan Africa. This family contains ten species in two genera, Certhia and Salpornis.

Only two species hybridize: Short-toed Treecreeper (C. brachydactyla) and Eurasian Treecreeper (C. familiaris). However, several cryptic subspecies of the Brown Creeper (C. americana) live sympatrically in contact zones across Northern America where they possibly interbreed (Manthey, Klicka & Spellman, 2011). A genomic analysis of a contact zone between two divergence lineages of this species in the Madrean Archipelago sky islands (Arizona, USA) reported complete allopatry during the breeding season and no gene flow (Manthey, Robbins & Moyle, 2016).

Brown Creeper (Certhia americana) 

References

Manthey, J. D., Klicka, J. & Spellman, G. M. (2011). Cryptic diversity in a widespread North American songbird: Phylogeography of the Brown Creeper (Certhia americana). Molecular Phylogenetics and Evolution 58, 502-512.

Manthey, J. D., Robbins, M. B. & Moyle, R. G. (2016). A genomic investigation of the putative contact zone between divergent Brown Creeper (Certhia americana) lineages: chromosomal patterns of genetic differentiation. Genome 59, 115-25.