Taste is crucial in shaping animals’ perception. Sourness, one of the primary tastes, is aversive in mammals, whereas many birds frequently consume acidic fruits, suggesting a potential tolerance. Our study uncovers a mechanism enabling sour tolerance in birds involving changes to the sour receptor (Otopetrin1, OTOP1). We demonstrate sour tolerance is a conserved trait in birds, with avian OTOP1 exhibiting acid-induced inhibition, and OTOP1 modulation affects sour perception and tolerance. Ancestral reconstruction reveals that the increase in acid tolerance evolved at the same point in the songbird phylogeny--before the last common ancestor of honeyeaters and other songbirds, and after the divergence of Australian treecreepers--as the regain of sweet sensing in this clade. We propose this shift might have enabled songbirds to feed on a wider range of fruits, expanding their ecological niches, and may play a role in the evolution and diversification of the songbird radiation. 

68 Otop1 gene sequences (51 birds, 9 non-avian reptiles and 8 mammals, Table S1) were downloaded from GenBank, or obtained from NCBI via basic local alignment search tool (BLAST) using canary and mouse Otop1 genes as queries. Sequences were then aligned using MACSE (v11.05) with the MAFFT algorithm. Ancestral reconstruction was conducted with FastML (v3.11) using the yang model of codon evolution, the sequence alignment and a tree topology representing relationships within amniotes derived from published phylogenies.