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The causes of the reversed sexual size dimorphism (RSD; females larger than males) in birds of prey are subject to a centuries-old, passionate debate. A crucial difficulty is to distinguish whether the postulated benefits derive from the proposed causal process(es) or are incidental. After reviewing the existing literature, we present a methodology that overcomes this difficulty and renders unnecessary any speculative a priori distinctions between evolved function and incidental effects. We can thus justify the following novel version of the well-known nest defence hypothesis as the most likely to explain the phenomenon in all birds of prey that show RSD: if the female predominates in actively defending the eggs and young against predators, then she is the heavier sex, and her relatively greater body mass is adaptive. That is, heavier females are favoured (independently of males) by natural selection. The attractiveness of this hypothesis is that it has the potential to explain the phenomenon in all raptors exhibiting RSD, can deal with the exceptional cases in this group, explains the direction of the dimorphism, focuses on a key factor in the reproductive success of most raptors, is parsimonious, i.e. does not require supporting hypotheses, and is supported by a substantial body of evidence.

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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 km 2 ), 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)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016), involving 59 breeding events and satellite tracking data from seven individuals tracked for between 82 and 797 days.