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Ferruginous Hawk

Limiting Factors

Limiting factors for the Ferruginous Hawk include those that reduce reproductive success, either directly or indirectly, or increase adult or juvenile mortality. This section stresses those factors that are influenced by human activities such as Habitat Alteration, Food, Availability of Nest Sites, and Human Disturbance.

Habitat Alteration:  Post-settlement habitat alteration has resulted in three factors that have negatively affected Ferruginous Hawk populations. Soils within the northern part of the Ferruginous Hawk's range have been extensively cultivated, thus rendering this habitat unsuitable for the species . Fire suppression has further altered this landscape by allowing woodland habitat to expand and replace some remaining patches of grassland. Finally, following the conversion of grassland to woodland, Swainson's and Redtailed Hawks have expanded their range to compete with Ferruginous Hawks for space, food and nest sites.

Within its current range, the Ferruginous Hawks' fragmented distribution is strongly linked to remaining grassland. In areas where it still occurs, the Ferruginous Hawk may be highly sensitive to any further loss of habitat and may also be unusually vulnerable to disturbance. For instance, alternate hunting grounds may be unavailable to the hawks in the case of a disturbance. Therefore, the survival of these hawks in a large portion of their range is dependent on a traditional ranching economy where native range prevails.

Perhaps the most far-reaching recent influence in favour of the Ferruginous Hawk is the discontinuation or re-examination of various direct or indirect subsidies that encourage breaking of grasslands. For example, the Canadian Wheat Board removed acreage quotas for barley and wheat in 1993-94 and the Western Grain Transportation Act's subsidy for the transport of grain was discontinued in 1995. Thus the Ferruginous Hawk and other prairie wildlife have likely benefited from the encouragement of diversification in agriculture and a return to permanent cover on some marginal lands. However, crop insurance programs can indirectly encourage the cultivation of marginal lands by providing insurance for those years when yields are low.

Food:  Based on abundant and reasonably strong evidence it appears that Ferruginous Hawk conservation in Alberta is inextricably tied to maintaining their ground squirrel prey. Selective poisoning of ground squirrels to guard against damage to cereal crops does not appear to have a major influence on the hawks through secondary poisoning. However, the removal of ground squirrels could affect hawks that utilize small parcels of grassland within intensive farming areas where they have little or no opportunity to shift to alternate hunting grounds.

Remarkably, there is no evidence that Ferruginous Hawks have been impacted by pesticides. While many deferred or indirect deaths likely go unnoticed, it is possible that Ferruginous Hawks are largely uninfluenced by pesticides. Unlike other raptorial birds, such as the Peregrine Falcon, that rely on migratory bird species as their main prey, the Ferruginous Hawk's simple food chain has few linkages (plants-sciurids-hawks), and only marginally includes insect prey or aquatic habitats where agrochemicals are readily transported.

Availability of Nest Sites:  Although the Ferruginous Hawk has vacated the northern parts of its range where trees have invaded, in the south the species largely depends on trees or similar raised structures for nesting. The Ferruginous Hawk prefers elevated nest sites because of the protection they provide and frequently uses trees in abandoned farmsteads. As these trees die, however, the hawks may be required to relocate to new sites where possible. In areas where trees have died and fallen or in areas where few trees are available, artificial elevated nesting platforms have proven to be a useful management tool.

Human Disturbance:  The effect of different kinds of human disturbance on Ferruginous Hawks varies. What is evident, however, is that Ferruginous Hawks nesting in disturbed areas fledge significantly less young than hawks in relatively undisturbed areas.

While it is relatively easy to monitor and avoid disturbance at nest sites, the subtle effects of a general disturbance over a larger area are much more difficult to manage. A study in Idaho revealed that fewer raptors, including Ferruginous Hawks, used an area in years when military exercises were carried out. The raptors that did use the area made only half as many prey capture attempts compared to those in undisturbed areas.

Other limiting factors affect Ferruginous Hawks to varying degrees. Twenty-five known or suspected causes of death of recovered Ferruginous Hawks include shooting, collision with vehicles, injury or illness, striking a powerline, electrocution and starvation.

Reprinted from Alberta Wildlife Status Report No. 18 (1999), with permission from Alberta Sustainable Resource Development.

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