The Rock Dove had the shortest FIDadj to an approaching observer, the slowest dominant mode of retreat (walking) and the shortest principal retreat distance. However, contrary to our prediction, the Crested Pigeon had a similar FIDadj and vigilance time allocation to the Spotted Dove, and a slower dominant mode of retreat and shorter average retreat distance than the latter species.
Flight initiation distance
Among bird species, longer FIDs are variously associated with greater body size, older age at first reproduction, omnivory and carnivory, cooperative breeding and habitat ‘openness’ (Blumstein 2006). Among urban birds, species that have genetically adapted to cities for many generations have shorter FIDs than recent urban colonists (Møller 2008). However, for the species in the present study, none of these traits explained the observed disparities in FID. The largest species, the Rock Dove (34.5 cm mean total length), actually had a significantly smaller FIDadj than the other species, and the smallest species, the Spotted Dove (30.5 cm), had a similar FIDadj to the slightly larger Crested Pigeon (32.5 cm). All three related species are granivores, differing substantially in their diet only in the volume of human food waste consumed in the urban environment (Higgins and Davies 1996; Mulhall and Lill 2011). None of them has been recorded breeding cooperatively to any significant extent (Ford et al. 1988).
Møller et al. (2014) showed that, particularly in Europe, farmland birds tended to have longer FIDs than those in other habitats. This may be related to both the declining bird population densities and the relative lack of cover (and hence birds’ greater vulnerability to predation) in agricultural land. Rock Doves, which had the shortest mean FIDadj, foraged mainly and equally in fairly open public parks and more visually obstructed streetscapes. However, Spotted Doves and Crested Pigeons, which had a similar mean FIDadj, contrasted in foraging habitat ‘openness’, the former mostly using streetscapes with many visual obstructions and the latter very open sports fields. The Rock Dove, which has probably been present in Melbourne longest (possibly for > 200 years), was the least wary of humans. However, whilst the Spotted Dove, which has been resident in the city probably for c. 150 years, was more wary of human proximity than the Rock Dove, it was also more wary than the very recent urban colonizer, the Crested Pigeon. Our comparison of three dove species in the urban environment substantially controlled for purely phylogenetic influences on tolerance of disturbance by humans. However, it must be conceded that it is probably less likely to reveal the particular influences on FID of the type listed above than a phylogenetically broader comparison.
Natural selection for tolerance of human proximity may have occurred in urban Rock and Spotted Doves, but it cannot plausibly explain the fact that Crested Pigeons were more tolerant than Spotted Doves. It is possible that the observed pattern of tolerance occurred because Crested Pigeons as a species, or the subset of urban founder Crested Pigeons, were bolder than Spotted Doves even after generations of selection for tolerance in the latter species. However, it seems more likely that behavioural flexibility was involved in the greater tolerance exhibited by Crested Pigeons. Such greater flexibility could theoretically be related to relative brain size, which is significantly positively correlated with urban colonizing ability in some birds (Malakov et al. 2011), but was not compared among the present study species. However, Guay et al. (2013) have shown that at least in shorebirds brain volume is not correlated with FID. FID being shorter where estimated human population density was higher in the present investigation could be attributable to either habituation or differential settlement by individuals varying inherently in tolerance of disturbance.
Vigilance time allocation
Anti-predator vigilance is another variable reflecting tolerance of human and predator proximity (Fernández-Juricic and Schroeder 2003). Greater investment in vigilance can increase the distance at which approaching predators and people are detected and/or the probability of detecting them. This enhanced detection ability gives the bird more flexibility to adaptively increase or decrease FID, depending on other relevant variables (e.g. perceived predation risk, Lima and Dill 1990; body mass, Creswell 1995). Fernández-Juricic and Schroeder (2003) showed that Spot-winged Pigeons (Columba maculosa) had a greater proportional time allocation to vigilance and a longer mean FID than co-habiting, smaller Eared Doves (Zenaida auriculata). Presumably for Spot-winged Doves the optimal adaptive response if predators are detected at a great distance is, for whatever reason, to flee early. In our investigation, Spotted Doves and Crested Pigeons seemed to have this greater flexibility to adaptively adjust their FID. They had a significantly greater proportional vigilance allocation than Rock Doves and hence probably a superior ability to detect approaching predators and people, and they retreated from an approaching human significantly earlier than Rock Doves. As Rock Doves in Melbourne are directly fed by humans much more than the other two species, they may have down-regulated anti-predator surveillance as the risk posed by people is perceived as being smaller (Lima and Bednekoff 1999).
The similar vigilance budgets of Spotted Doves and Crested Pigeons are intriguing, because the former forages in a more visually obstructed habitat than the latter and consequently might be expected to spend more time being vigilant (Lazarus and Symonds 1992). However, Fernández-Juricic and Schroeder (2003) found that proportional visual scanning time actually decreased as visually obstructive tree cover increased for urban Spot-winged Pigeons and Eared Doves, so the greater level of visual obstruction in streetscapes might be contributing to the similarity in vigilance budgets of Spotted Doves and Crested Pigeons. Conceivably, at high levels of visual obstruction vigilance may become relatively ineffective and be down-regulated. The observation that vigilance effort of Spotted Doves and Crested Pigeons was less in larger flocks accords with many, but certainly not all, findings for other bird species (Beauchamp 2008), but it is intriguing that the highly gregarious Rock Doves did not exhibit the same relationship. Currently we know too little about natural predation on these three urban doves to factor it into the vigilance formula.
Whilst it is valid and informative to compare bird species’ surveillance for predators by measuring relative vigilance time allocations, two qualifications are pertinent. First, some species’ visual fields probably permit surveillance for predators during head-down foraging (Fernández-Juricic et al. 2008), although this may be less of an issue when, as here, closely-related species (which probably have similar visual fields) are being compared. Second, even when species’ proportional vigilance time allocations are similar, differences in scanning regime structure may nonetheless result in disparate predator detection efficacies (Fernández-Juricic 2012). These aspects of vigilance require examination in the doves investigated here.