Captive and wild populations of threatened birds tend to be small. As a result, they are vulnerable to genetic drift and inbreeding which can depress hatching success by either reducing parental fertility or embryo survival. Previous research has shown that early embryo death is the main cause of hatching failure in wild populations, whereas infertility is more common in captivity. This difference between wild and captive populations suggests that the way in which genetic risks are manifested is dependent on the environment. The proposed project will assess how and why environmental stressors modulate the expression of genetic risks in wild and captive non-model organisms. Working with ZSL’s captive and wild breeding programmes the student will establish the key factors underlying hatching failure in birds. We will first identify patterns of infertility and embryo mortality within and between species, then investigate the underlying intrinsic (e.g. genetic) and extrinsic (e.g. environmental) causes. ZSL holds detailed pedigree and breeding information on several captive and wild populations. This project will focus primarily on two extensively monitored populations of threatened birds, the corncrake Crex crex (captive, UK) and hihi Notiomystis cincta (wild, New Zealand), which experience high and increasing levels of hatching failure. The student will also have access to data and material from several other populations via ZSL and the supervisor’s network of collaborators. Most field work will be based on offshore islands in New Zealand and lab work in the UK. This CASE project is co-supervised by Laura Gardner, ZSL Bird Collection Manager.