**4. Discussion**

Of the 258 birds species recorded in the Darwin area (escaped or introduced species and vagrants excluded), approximately 40% are considered resident [46] with the remaining species described as mobile. It has previously been thought that mobile species cope better with urbanisation than sedentary species [47,48], although other studies have suggested that highly mobile species may be more vulnerable to habitat changes due to their dependence on larger habitat patches and generally far-reaching home ranges [49–51]. Furthermore, there is also evidence to suggest that the fragmentation of habitats caused by urbanisation is going some way to changing the status of some birds from 'visitor' to 'resident' [52–54].

That there is a significant difference, albeit slight, in the proportion of those species that primarily feed on insects in areas where greenspace has decreased may possibly be attributed to the establishment of urban gardens. Although the size of the greenspace has decreased overall in these locations, in 73% of the grid cells analysed, the proportion of species that preferred insects increased; in contrast, only 47% of grid cells contained an increase in fructivores. It may be that the plants in these gardens attract a larger array of insects, thus providing more appealing spaces for insectivores. Numerous studies have indicated that the bird species that tend to thrive in urban environments are those that nest in cavities or canopies (sites less likely to be disturbed by human activity) and are more likely to be granivorous or omnivorous, whereas those species that avoid urban areas are those that predominantly nest in shrubs or trees or at ground level and have a more specialised diet, frequently insectivores [29,30,55]. However, these characteristics have been determined from research undertaken in either the Northern Hemisphere or the temperate zones of the Southern Hemisphere, places where garden vegetation is often vastly different to that of the original habitat. Suburban gardens of the Darwin region often contain native plant species alongside exotics, and this mixture may be enough to maintain, or even increase, insect numbers.

That there is no significant difference in the levels of feeding specialisation in bird assemblages is something of a surprise, as it would be fair to assume that more generalised species would be increasing and those species with one preferred food source would be forced out, particularly in areas of urban increase. Homogenisation of habitat is an oft-quoted result of urbanisation [5,56,57], but from this broad overview, it appears not to be affecting the composition of bird communities in the Darwin region. Further research into the characteristics of species making up urban assemblages will go some way to better understanding why this may be.

Invasion by feral species is a frequent result of an increased human presence in an area [29,30,58]. Many bird species that are unable to adapt to the suddenly changed environment will often move out of an area resulting in diminished biodiversity and allowing invasive species to move in [4,8,9]. Other species can benefit from urbanisation; these are often birds with a more generalist diet and life-history traits that are conducive to living in a fragmented habitat [10].

The only feral species in the records was *Columba livia*—the common pigeon, rock dove or rock pigeon. This species was recorded in the 1998 records only, most likely due to an eradication programme undertaken by the Parks and Wildlife Commission, NT, beginning in 1996, that saw the species considered eradicated from the Darwin area by 2004 [59,60]. The lack of established feral bird populations in the Darwin region could, potentially, suggest that native bird species are filling the 'urban exploiter' or 'suburban adapter' niche that is commonly filled by exotic species in other cities, but there is little or no research to show what these species may be. Additionally, a unique aspect of the Darwin region is that it is the only capital in Australia where urban fires are a regular annual occurrence. The city is located within the monsoonal tropics; a landscape dominated by tropical savannas, experiencing frequent fires during the prolonged dry season from May to October, often within just metres of suburban housing areas [61]. In addition to wildfires, many of these fires are prescribed, being undertaken by various land-management bodies as part of a yearly landscape management approach [62,63]. Whether these annual fires are in some way supporting and maintaining the local bird assemblages whilst preventing invasive species from establishing populations is an intriguing question. Research into the combined impact of urban fires and human modification of the environment on local bird populations over time is ongoing; it is hoped that the results will go some way towards answering this question.

### **5. Conclusions**

Whilst very transient, the population of Darwin and its surrounds has grown significantly between the years 1998 and 2018, and urbanisation has expanded more than 50 km from the city centre. Despite this, this general overview of bird assemblages shows that although species numbers fluctuate, avian communities appear to be stable. The increase

in the insectivorous species may be due to the establishment, maturity and vegetative composition of suburban gardens. This may potentially signify a shift in the species diversity of the area; however, further research is needed to investigate this.

**Author Contributions:** S.E.F. conceived, designed and performed the experiments. S.E.F. analysed the data. S.E.F. and A.C.E. wrote the paper. P.W., S.T.G. and T.G.W. provided guidance and comments on the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study is part of a larger Ph.D. project that is supported by Charles Darwin University and the Australian Government via the Research Training Programme scholarship.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** The authors confirm that the data supporting the findings of this study are available within the article, its Appendices, and Australian Bird Data Version 1.0 (https://doi. org/10.6084/m9.figshare.1499292\_D8, access date 10 September 2019) at http://www.birdsaustralia. com.au/ (access date 19 June 2019) and https://www.ala.org.au/ (access date 1 September 2019).

**Acknowledgments:** The authors would like to thank Andrew Silcox and BirdLife Australia for access to bird data records.

**Conflicts of Interest:** The authors declare no conflict of interest.
