The Grey-Headed Flying Fox is found in subtropical moist forest, open forest, closed and open woodlands, Melaleuca swamps, Banksia woodlands, mangroves, commercial fruit plantations (Hall and Richards 2000), and increasingly in urban landscapes (e.g., Williams et al. 2006, Plowright et al. 2011, Boardman et al. 2021, Meade et al. 2021).

The diet consists of nectar, pollen and fruits of a wide range of species, most commonly native trees in the families Myrtaceae, Moraceae, Proteaceae and a small number of species from other families. (Eby 1998, Hall and Richards 2000, Eby and Law 2008, Schmelitschek et al. 2009). The primary native food sources are the nectar and pollen of Eucalyptus, Corymbia, Angophora, Banksia, Melaleuca species, and fleshy rainforest fruits (Eby and Law 2008, Eby 1991); and they are known to consume leaves, bark, and invertebrates (Clulow and Blundell 2011, Smith et al. 2020, Schmelitschek et al. 2009). They also forage in agricultural areas, including orchards, and in introduced vegetation in urban areas (McDonald-Madden et al. 2005, Boardman et al. 2021).

The majority of native diet plants have regular seasonal flowering phenologies, but do not flower every year (Eby and Law 2008). Grey-headed flying foxes experience seasonal food bottlenecks during winter and spring when few diet plants are productive and the distribution of native foraging habitat is limited (Eby and Law 2008). Vegetation communities that contain plants that flower in winter and early spring are of particular importance to the species during critical periods in the reproductive cycle and have been identified as habitat critical to survival (Eby et al. 1999, Eby and Law 2008, DAWE 2021).

The Grey-Headed Flying Fox lacks biological adaptations such as torpor to withstand food shortages, instead individuals are thought to move in response to changes in local food abundance (DAWE 2021). It is a highly mobile species with individuals estimated to travel up to 2,500 km annually within a network of roosts located across southeastern Australia (Eby 1991, Tidemann and Nelson 2004, Roberts et al. 2012a, Welbergen et al. 2020). On a nightly basis, individual bats may forage as far as 50 km from their roost. However, mean foraging distances are between 6 and 11 km, with foraging distances shorter in urban than in non-urban areas (Meade et al. 2021). There are few areas within the range where nectar and other food sources are available continuously, except perhaps in urban areas. While many flying-foxes feed in urban areas and urban roosts may be permanently occupied, a proportion of the individuals within these roosts move regularly between urban and other more natural settings (Meade et al. 2021).

It roosts by day colonially in canopy vegetation and many roosts have a long history of use (Lunney and Moon 1997). Individuals move nomadically throughout the species' range within a network of roosts, and change roosts frequently (Roberts et al. 2012a, Welbergen et al. 2020). Some roosts may temporarily contain >100,000 individuals, often coinciding with periodic flowering of preferred tree species (Eby 1991, Eby et al. 1999).

The Grey-Headed Flying Fox regularly cohabits roosts with the black flying fox (P. alecto) and Little Red Flying Fox (P. scapulatus) with which it is partially sympatric (Timmiss et al. 2021), and rarely with the Spectacled Flying Fox (P. conspicillatus) in the north of its range (Parsons et al. 2010). Of the 310 known camps in regular use, almost 75% are shared with other species and only 25% are occupied exclusively by the Grey-Headed Flying Fox (Timmiss et al. 2021).

The majority of camps (58.7%) are located in urban areas (Timmiss et al. 2021). From 1998 to 2019, the number of active roost sites increased by approximately 120% and average population size at camps declined, with 94% of new roosts formed in urban areas (Eby et al. submitted). The pattern of increase in roost numbers has been stepped rather than continuous, in association with periods of food shortage (Eby et al. submitted). The drivers of increasing urbanisation of the species are unclear. Increased temporal stability (Markus and Hall 2004, McDonald‐Madden et al. 2005, Parry‐Jones and Augee 2001, Plowright et al. 2011, Williams et al. 2006) and availability (Markus and Hall 2004, Plowright et al. 2011, Williams et al. 2006) of food resources from urban plantings may be responsible for the recently observed increased presence of flying-foxes of urban areas (Meade et al. 2021), however, broadscale reductions in feeding opportunities in native habitats may also play a role (Parry-Jones and Augee 2001, van der Ree et al. 2006, Williams et al. 2006, Tait et al. 2014, P. Eby unpublished data).

In the roosts, males establish ‘mating territories’ that are actively maintained by aggression, which is especially evident during the mating season (March-May, Martin et al. 1993; Martin et al. 1995; Eby 1995; Nelson 1965; Welbergen 2010, 2011). Males mate repeatedly with the same female during a mating session and can have multiple mating sessions with the same female over the course of several days (Welbergen 2005). Males, but not females, accumulate body mass prior to establishing their territories and can lose more than 20 percent of their mass during the mating season (Welbergen 2011).  
The most likely time of conception is April (Martin et al. 1993, Martin et al. 1995). Pregnancy in P. poliocephalus is seasonal, lasts for approximately 27 weeks (Martin et al. 1987) and the majority of females give birth to a single young annually between late September and early November, although small numbers of females give birth outside this time (Justin Welbergen pers. comm.). Lactation lasts approximately 5–6 months (Martin et al. 1996), with the majority of young being weaned by the start of the mating season in March (Welbergen 2011).

Twins are rare and there are no observations of twins surviving to weaning (Peggy Eby pers. comm.). Sexual maturity is reached in the second year (females) or third year (males) after birth (Welbergen 2010), and generation length is estimated to be between 6 and 8 years (Woinarski et al. 2014, A. Divljan pers. comm).

The geographic range of the species has altered in the past 20 years as documented by range-wide monitoring (Westcott et al. 2012), satellite telemetry studies (Roberts et al. 2012a, Welbergen et al. 2020), a systematic search of field records (Roberts et al. 2012b) and public reporting. While latitudinal range boundaries have remained generally stable (Roberts et al. 2012b), there has been a distinct inland expansion. As of 2021 the range spans the coastal lowlands, tablelands, and slopes of eastern Australia, from Finch Hatton near Mackay (Queensland) in the north to Melbourne, Geelong (Victoria) in the south (Roberts et al. 2012b, Woinarski et al. 2014) and inland to Port Augusta in South Australia, Cooma in the Snowy Mountains of New South Wales (New South Wales), and areas of central and southern New South Wales and Victoria including Dubbo, Wagga Wagga and Albury (New South Wales), Numurkah and Bendigo (Victoria). There have also been vagrant sightings outside of this range, including occasional sightings in Tasmania and Bass Strait islands (Atlas of Living Australia 2021, DAWE 2021). Sightings in Tasmania and islands in Bass Strait are associated with periods of acute food shortage (Eby et al. 2021).

The great majority of observations and camps of this species are from the coastal lowlands and slopes of south eastern Australia below altitudes of about 200 m (DAWE 2021, Welbergen et al. unpublished data). However, known roosts vary in elevation from sea level to 800 m (Cooma), and occasionally vagrant individuals have been reported from altitudes up to 1,600 m in the New South Wales and Victorian high country (M. Pennay pers. comm., Welbergen et al. unpublished data). Along the southern periphery of its range, the species has increased in abundance in urban areas including Canberra (Australian Capital Territory), Melbourne, Geelong and Bendigo (Victoria), and Adelaide (South Australia) (Woinarski et al. 2014).

The species is typically found from sea level to 800 m asl, with vagrants being documented up to 1,600 m asl.

This species is known to use a broad range of areas reserved for conservation, which make up 12.6% of the area surrounding known camps, however, only 5.2% of the camps occur within conservation areas (Timmiss et al. 2021). Given the species' reliance on dispersed food resources, conservation areas are unlikely to protect sufficient habitat for the species.

A range of management actions and research to assist conservation of P. poliocephalus into the future has been identified in the National Recovery Plan for the species (DAWE 2021). They include: identify, protect and increase roosting habitat and native foraging habitat that is critical to survival, define and improve precision in current population monitoring to determine reliable trends in population size, build community capacity to coexist with flying foxes and improve management of conflict at roosts in urban areas, reduce the impact of electrocution on power lines, and entanglement in netting and on barbed wire, significantly reduce levels of harm in commercial fruit crops, support research that will improve conservation and management, develop and apply ameliorative care procedures to minimise deaths during extreme heat days, and management of fire (DAWE 2021).

In recent years, governments have funded and administered a number of programs of subsidies to improve conservation outcomes for P. poliocephalus. Plans of Management for several roosts have been developed by land managers in consultation with affected stakeholders (Roberts et al. 2012). The plans set out programs for managing contentious roosts under various conditions, and can serve as effective mechanisms for both reducing conflict and educating the public about the conservation and management challenges that face P. poliocephalus. Government subsidies in New South Wales have enabled commercial orchardists to install full exclusion netting to protect their crops from flying fox damage in advance of a halt to licenced culling on crops. The New South Wales government has also initiated a program that funds restoration of both roosting and foraging habitat.