LINKED PAPER
Ferruginous Hawk movements respond predictably to intra-annual variation but unexpectedly to anthropogenic habitats. Isted, G. H., Thomas, R. J., Warner, K. S., Stuber, M. J., Ellsworth, E., Katzner, T. E. 2023. IBIS. DOI: 10.1111/ibi.13200. VIEW
The availability of remote tracking methods has made it possible to learn a lot more about the movements of animals both spatially and temporally, and has highlighted the amount of intra-annual variation in movement behaviour that often occurs due to differences across the annual cycle. For example, birds experience different sets of processes and stressors during breeding, migration, and non-breeding periods (Sillett & Holmes 2002). Tracking the movements of species across the entire year can help us to evaluate wildlife home-ranges, understand interactions with resources, assess human influences, and ultimately better identify any threats and pressures affecting population, helping to guide conservation actions. However, monitoring of many migratory species is often only limited to one annual period e.g. breeding season (Morrison et al. 2013, Marra et al. 2015).
In a recent study in Ibis, Georgia Isted and colleagues collected GPS telemetry data from 12 Ferruginous Hawks (Buteo regalis) over 207 bird-months in southwest Idaho, USA, to understand patterns of movement ecology and space use across the annual cycle.
Ferruginous Hawk migration
Ferruginous Hawks are migratory, although southern populations may be sedentary or migrate only short distances compared to the middle-distance migration of northern populations, and their migration is typically non-linear (first longitudinal then south) which may be in response to variation in rodent abundance (Ng et al. 2020). The Ferruginous Hawk breeding season has been widely studied but knowledge of other periods of the annual cycle is limited. The researchers estimated home-range area for each bird each month and assessed land-cover association with these home-ranges across the annual cycle. The 12 hawks in the study were eight adults (five female, three male) and four juveniles (two female, two male).
Figure 1. (a) Generalized additive mixed model and 95% confidence intervals showing effect of month on mean monthly home-range of Ferruginous Hawks (95% autocorrelated kernel density estimation, n = 12 hawks (eight captured as adults, four captured as juveniles) and 207 bird-months) tracked between 2016 and 2021 in North America. (b) Number of days analysed where birds were range resident for each month. (c) Distance of home-range areas from the breeding site (NCA) for each month analysed.
Territories and human-wildlife interactions
Results showed that the home-range varied across the annual cycle and that the association with land-cover types by territorial birds varied between breeding and non-breeding months
and was linked to home-range size. As was expected based on previous studies, the smallest home-ranges were seen during the breeding season with larger home-ranges at other times during the year. During breeding months, adults acted as central place foragers staying close to the nest-site, but younger hawks were not tied to a specific nesting site and appeared to be non-territorial, exhibiting larger home-ranges. Larger home-ranges outside of the breeding season may be linked to reduced habitat quality (e.g. due to changes in prey availability) or to reduced defensive behaviour and resource-sharing, but it is noted that the effect on movement of variation in food availability between years could not be tested, and no sex-specific differences in home-range size were detected in contrast to some other studies of raptors.
Figure 2. Effects of proportion of cropland within the home-range (95% autocorrelated kernel density estimation) on home-range size of Ferruginous Hawks inside and outside of the breeding season. (a) Breeding adult hawks during the breeding season (March–July) and (b) range resident hawks outside of the breeding season (August–February). The shaded area represents the 95% confidence interval around home-range size.
An unexpected result of the study was the observation of seasonal variation in movements in relation to cropland. Association with irrigated cropland habitats was negatively associated with home-range size in the non-breeding season, but the opposite was seen in the breeding season, with use of croplands resulting in larger home-ranges. While anthropogenic habitats are often linked with contractions in home-ranges, the tagged territorial hawks travelled surprisingly long distances to visit small parcels of irrigated cropland, increasing their home-range size. As Ferruginous Hawks are rodent specialists, they may have been exploiting these croplands which provide perches, low vegetation density, and high rodent populations, but more information on crop type, levels of irrigation, and foraging success are needed to support this.
Ferruginous Hawks are often nest-site limited (Wallace et al. 2016), and the study area contained a large number of artificial nesting platforms used by all of the tagged territorial hawks. Many of these platforms are located far from the agricultural lands, which may explain the long distances travelled as hawks use both anthropogenic resources (nesting platforms and croplands) during the breeding season. This could also explain the finding that in the non-breeding season, higher association with irrigated cropland habitats was linked with smaller home-ranges as the hawks only use the anthropogenic resource of croplands, without needing to use the nesting platforms. This provides an important insight into the nuance of this human-wildlife interaction, and may be applicable to other species which utilise multiple anthropogenic resources.
This study demonstrates the value of using remote tracking in combination with environmental information to gain an understanding of animal behaviours, and highlights how monitoring migratory species across a full annual cycle can reveal both expected and unexpected behaviours. Such behaviours provide insight into the use of anthropogenic resources, and overall improve our understanding of how wildlife, including those of conservation concern, may adapt to anthropogenic alterations to the landscape.
References
Marra, P.P., Cohen, E.B., Loss, S.R., Rutter, J.E. & Tonra, C.M. (2015). A call for full annual cycle research in animal ecology. Biology Letters 11: 20150552. VIEW
Morrison, C.A., Robinson, R.A., Clark, J.A., Risely, K. & Gill, J.A. (2013). Recent population declines in afro-Palaearctic migratory birds: The influence of breeding and non-breeding seasons. Diversity and Distributions 19: 1051–1058. VIEW
Ng, J., Giovanni, M.D., Bechard, M.J., Schmutz, J.K. & Pyle, P. (2020). Ferruginous hawk (Buteo regalis, version 1.0.). In Rodewald, P.G. (ed) Birds of the World. Ithaca, NY:Cornell Lab of Ornithology. VIEW
Sillett, T.S. & Holmes, R.T. (2002). Variation in survivorship of a migratory songbird throughout its annual cycle. Journal of Animal Ecology 71: 296–308. VIEW
Wallace, Z.P., Kennedy, P.L., Squires, J.R., Olson, L.E. & Oakleaf, R.J. (2016). Human-made structures, vegetation, and weather influence ferruginous hawk breeding performance. Journal of Wildlife Management 80:78–90. VIEW
Image credits
Top right: Ferruginous Hawk (Buteoregalis) | Genghis Conure | CC BY 4.0 Wikimedia Commons
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