Beyond Family Labels: A Taxonomic Ornstein-Uhlenbeck Prior for Avian 3D Shape Recovery

Recovering 3D pose and shape of birds from single images is bottlenecked by the long tail of avian diversity: out of $\sim 10{,}000$ species, only a few dozen have 3D-labeled training data, and in-the-wild test sets routinely contain species never seen at training. Generalizing across clades is therefore a load-bearing problem. The current state of the art, AniMer+, mitigates it with a family-level contrastive loss that conveys only a binary same-family indicator per pair and discards the continuous structure of the species tree. We propose a **taxonomic Ornstein--Uhlenbeck prior** on the AVES shape code: a multivariate Gaussian whose covariance decays exponentially with Linnaean rank distance, with a learnable selection strength and noise scale. The prior generalizes family contrastive into a continuous, generative form, exposing the graded cross- and within-family similarity that a binary indicator cannot, and brings continuous comparative phylogenetics in as a biology-grounded inductive bias for the long tail. Our method consistently improves over AniMer+ on every reported avian benchmark cell, with the largest gains on the unseen-species CowBird benchmark. A permutation ablation isolates taxonomic structure from generic Gaussian smoothing, and a rank-granularity ablation identifies cross-order distinctions—the structure beyond a family-vs-not indicator—as the dominant source of the unseen-species gain.