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Simple Summary: Para-aminopropiophenone (PAPP) is a bait poison used for the control of several introduced predators in Australia and New Zealand. In highly susceptible species (e.g., feral cats, red foxes and stoats), small doses of PAPP reduce the ability of haemoglobin to carry oxygen, resulting in rapid unconsciousness and death. Determining the degree to which different wildlife species may be susceptible to PAPP normally requires the use of lethal trials. Instead, we developed a non-lethal method to estimate the doses of PAPP that would be fatal to high conservation value birds such as takahē, weka and kiwi. Using a series of small but sequentially increasing PAPP doses, we monitored progressive changes in a number of blood markers, enabling us to predict the dose required to achieve a lethal threshold. This approach also allowed us to define the largest PAPP dose that produced no observed shift in the blood values, as well as the lowest dose associated with adverse effects. Because our blood sampling methods were refined to be minimally invasive, we monitored the progress of birds for at least 72 h and were able to define PAPP doses associated with protracted adverse effects. Para-aminopropiophenone (PAPP) is a methaemoglobin (MetHb) forming compound used for the lethal control of invasive carnivores and mustelids. By measuring the dose-dependent inhibition of O2 transport arising from the oxidation of haemoglobin (HbFe2+) to MetHb (HbFe3+), we determined the sensitivity of nine bird species to PAPP. A methaemoglobinaemia absorbance index (MAI) was validated in five common bird species to determine thresholds associated with a 99% probability of survival (ST99) and a 50% probability of mortality (LT50). Dose–response trials in high conservation value birds sought MAI values below the ST99 threshold, projecting the LT50 value and avoiding the need for lethal outcomes. Black-backed gull (LT50 = 1784.7) and eastern rosella (LT50 = 1074 mg kg−1) were the most tolerant species, while brown kiwi (LT50 = 8.4 mg kg−1) and weka (LT50 = 9.3 mg kg−1) were the most sensitive. Takahē were of intermediate acute sensitivity (LT50 = 51 mg kg−1), although protracted impacts on haemoglobin were observed in takahē up to 72 h later and associated with PAPP doses as low as 25.6 mg kg−1. In pukeko (LT50 = 138.4 mg kg−1), protracted declines in haemoglobin 72 h later occurred at doses as low as 29.5 mg kg−1, while at higher doses (253 and 112 mg kg−1), deaths resulted after 4–6 days. Based upon PAPP doses that caused acute and protracted responses, we provide estimates for the lowest observable adverse effect level (LOAEL) and no observable effects level (NOEL) for nine bird species. [ABSTRACT FROM AUTHOR]