Adaptations of the rare endemic Grey Falcon Falco hypoleucos that enable its permanent residence in the arid zone of Australia

Abstract

The Grey Falcon Falco hypoleucos is an extremely rare and little known Australian endemic raptor. The Web of Science lists only two publications for this species, considered to be one of the five rarest Falco species of the world: a literature review and analysis of museum material (Olsen and Olsen 1986), and the results from the preliminary investigation that led to this study (Schoenjahn 2013). The difficulty in finding these rare birds (<1000 mature individuals), distributed thinly across much of Australia’s arid/semi arid zone (~5 million km2), hampers detailed studies and has deterred previous researchers from studying this species.The Grey Falcon is the only species of Falco to have its entire population confined exclusively to a hot arid environment. To understand the processes that help the species to persist in its extreme environment, I explore key aspects of its ecology, morphology, and anatomy, using observational data collected during 14 field seasons (2003–2016), involving 59 breeding events and satellite tracking data from seven individuals tracked for between 82 and 797 days.How do individuals, during the various stages of their lives, cope with extremely high ambient temperatures? Investigating whether the species is specifically adapted behaviourally and anatomically to its environment, I found that Grey Falcons keep physical exertion and thus activity levels low in each aspect of their day-to-day lives, and lack particular morphological or physiological characteristics that would help them to cope with heat better than other bird species do. A possible explanation of the behaviour to keep activity levels low is that this helps to keep endogenous heat production low which enables these birds to respond to extreme and unexpected environmental changes without delay.The breeding ecology of the Grey Falcon is characterized by a behaviour that is unique among birds of prey. The young often stay with their parents for up to 12 months or longer after fledging, i.e. well into the next breeding season, and some of these young were even fed by their parents at this advanced age. I show that this behaviour may be interpreted as a consequence of Grey Falcons of all ages keeping their activity levels low at all times.The movements of individuals is a further key aspect in the ecology of a species, particularly so in desert-living birds. The Grey Falcon lives exclusively in an area where at least 45% of the regularly breeding bird species are considered to be nomadic. I found that the movement patterns of the Grey Falcon are generally nomadic, but are characterized by a distinct and unusual element of reluctance. That is, if conditions become inadequate for breeding these birds rather stay and forego breeding than embark on a search for more favourable conditions that may be far away, and could expose them to risk. Although capable of long-distance movement (one individual moved more than about 1000 km in 9 days), Grey Falcons appear to move on only when environmental conditions become a risk to their survival, i.e. only when absolutely necessary, and when they do move on, they move no farther than necessary. I term this behaviour ‘reluctant nomadism’.An unexpected behaviour brought to light by this study is that Grey Falcons often roost on the bare ground at night. This behaviour was noted on 149 (26%) of 576 nights that Grey Falcons were recorded at locations other than the nest. They roosted on the ground at times even when trees (and their nest) were within view. I propose that this is also a consequence of the species’ behaviour of keeping activity levels low at all times. Night-roosting on the bare ground exposes these birds to predation by recently introduced cats, foxes, and dingoes. A further hitherto unrecognized threat to the species is suggested to be climate change. This threat derives from extreme climatic events that characterize the species’ environment, and is a consequence of the species’ behaviour to remain at all times in the arid/semi-arid zone. This hot environment is characterized by extreme and unpredictable climatic events, and these are predicted to increase in frequency, severity, and duration over time.In summary, in key aspects of its ecology, the behaviour of the Grey Falcon shows markedly low levels of physical exertion. The consistency suggests that the species’ ecology is, consequently, governed by the principle of keeping physical exertion low. I interpret this behaviour as an adaptation of the species to its hot arid environment, one that is characterized by extreme and unpredictable climatic events. Keeping levels of physical exertion low helps to keep endogenous heat production low. This enables these birds to respond to adverse climatic events without delay even under challenging circumstance, thus enabling the Grey Falcon’s permanent residency in the hot arid and semi-arid zone of Australia.