Concentration‑dependent Trophic Transfer of Selenium to Bird Eggs: Field Synthesis and Laboratory Benchmarks

Maternal transfer of selenium is the critical exposure pathway for reproductive risk in birds, yet diet→egg transfer varies widely with species, diet composition, timing, and selenium speciation. We synthesized paired diet–egg datasets from wetlands and terrestrial systems together with controlled feeding studies to quantify diet→egg trophic transfer factors and evaluate their concentration dependence. Across field settings, invertebrate or fish prey frequently exceeded dietary screening levels while egg selenium yolk = 0.211factors, often ≤ 1. By contrast, laboratory selenomethionine diets produced higher, steadier trophic transfer factors, reflecting sustained, highly bioavailable exposure. A compartment analysis showed albumen responds rapidly to recent intake whereas yolk integrates over days to weeks, explaining field scatter and lab steadiness. These findings support a three‑zone conceptual model, that is deficiency, regulation band, and high exposure, in which proportional transfer is damped across the regulation band. We provide operational guidance that emphasizes aligning prey sampling to the immediate pre-laying window, considering selenium speciation (especially the selenomethionine fraction), and applying guild- and site-appropriate TTFs for forward and backward translation.