Bone marrow fat content analysis confirms starvation as a cause of death for Australasian bitterns ( <i>Botaurus poiciloptilus</i> ) in New Zealand

ABSTRACT Wildlife managers conserving threatened species rely on information about causes of population declines. Finding carcasses to record cause of death, sickness or injury in a timely manner is challenging. We used fat content in bone marrow from deceased Australasian bitterns ( Botaurus poiciloptilus ) to determine whether birds had starved by comparing results among 34 bitterns where the cause of death was assigned via independent autopsies. Fat content was near zero in the eight specimens that died of starvation. Many other specimens had depleted fat content, 17 of which were recorded as dying from impact trauma. This evaluation provided confidence to interpret our database of 126 Australasian bittern mortality records evaluated via autopsy or observation. The most common causes of death were impact trauma (56.3%; mainly vehicles and powerlines) and starvation (17.5%) with a further 5.8% having characteristics of both starvation and impact trauma raising the issue of whether starvation contributed to the deaths of individuals where impact trauma was the primary cause of death or vice versa. Although this dataset is biased by specimens more easily observed by the public, the occurrence of starved/near starved bitterns is of conservation concern and indicates wetland food supplies are likely under stress.