Publication Date


Date of Final Oral Examination (Defense)

November 2008

Type of Culminating Activity


Degree Title

Master of Science in Raptor Biology



Major Advisor

Marc J. Bechard, Ph.D.


Mark R. Fuller, Ph.D.


James R. Belthoff, Ph.D.


I analyzed regurgitated pellets, prey remains, and video recordings to describe Ferruginous Hawk (Buteo regalis) diet in two grasslands in New Mexico, USA, the Estancia Valley and the Plains of San Agustin, that differed in anthropogenic alteration. Video monitoring revealed Ferruginous Hawks provisioned nestlings with more biomass than pellet analysis estimates from the same nests. Three mammalian prey species, Botta’s pocket gopher (Thomomys bottae), Gunnison’s prairie dog (Cynomys gunnisoni), and desert cottontail (Sylvilagus audubonii)contributed similar proportions to Ferruginous Hawk diet in percent biomass while Botta's pocket gopher dominated diet in percent frequency. Ferruginous Hawks breeding in the anthropogenically-altered Estancia Valley consumed more Gunnison’s prairie dogs than Ferruginous Hawks in the rural Plains of San Agustin, regardless of calculative method, while Ferruginous Hawks in the Plains of San Agustin consumed more desert cottontails. From 1998-2005, Ferruginous Hawks in the Estancia Valley produced significantly more fledglings per nesting attempt than Ferruginous Hawks in the Plains of San Agustin. This indicated that the persistence of colonial mammals like Gunnison’s prairie dogs, which offer minimal predatory search time, may have increased Ferruginous Hawk reproductive output and helped to offset the effects of moderate levels of human development. Intact Gunnison’s prairie dog colonies should be conserved in the Estancia Valley to enable maintenance of current Ferruginous Hawk productivity levels in the midst of increased human development, a threat that is becoming more pronounced. Further documentation of behavior and food requirements of Ferruginous Hawks in anthropogenically-altered landscapes is necessary to conserve this grassland obligate whose habitat is rapidly diminishing.

Descriptions of Ferruginous Hawk (Buteo regalis) parental roles and feeding ecology are limited to anecdotal accounts using out-dated methods. In 2004-2005, I collected and analyzed 1,373 hr of video from six time-lapse video-monitored nests in two New Mexico grasslands with different levels of anthropogenic alteration to establish baseline data on key components of the nesting behavior of a grassland raptor. Ferruginous Hawks delivered 93.3 grams of biomass per hour to each nest (g/hr), 42 g/nestling/hr, and 208 g/prey delivery, with the majority of provisioning occurring in the morning and afternoon hours. Males delivered most prey items (72%) and provisioned nestlings at a higher prey-delivery rate but females delivered more mass per delivery. Females delivered most desert cottontails (Sylvilagus audubonii), but there was no clear delineation of differential prey use between sexes. Females spent more time at the nest than males during the nestling stage, more time at the nest on days when males delivered more mass to the nest and less time at the nest as nestlings grew older. Ferruginous Hawks in the human-altered Estancia Valley delivered more prey items in the morning while hawks in the rural Plains of San Agustin delivered more items during mid-day.

Mass-provisioning rate was greater in the Estancia Valley throughout the day. Numerically, hawks in the Estancia Valley supplied nests with 87% more mass per delivery (271 vs. 145 g/delivery) and 60% more mass per nestling per hour (52 vs. 32 g/nestling/hr) but 25% fewer prey items per hour (0.51 vs. 0.41 items/hr) than adults in the Plains of San Agustin. These measures suggest that video-monitored hawks in the Plains of San Agustin spent more time foraging but delivered smaller prey than hawks in the Estancia Valley. Further, hawks in the Plains of San Agustin foraged more during mid-day hours, potentially because sufficient food resources could not be obtained in the morning. On average, females in the Plains of San Agustin spent twice as much time away from nests than females in the Estancia Valley following a disturbance that caused them to flee. The presence of a colonial food source, such as Gunnison's prairie dogs (Cynomys gunnisoni), may help decrease variables associated with optimal foraging theory, such as predator search time and prey handling, thereby increasing provisioning efficiency and reproductive output.

As natural habitat is increasingly modified, land managers must devise plans that include restricted zones around sensitive sites to minimize deleterious effects caused by human disturbance. Here I provide empirical data on the nest defense response of Ferruginous Hawks (Buteo regalis) to a human intruder as related to anthropogenic influence, parental investment, and repeated intrusions. A distance of 610 m prevented 95% of nest-attending Ferruginous Hawks from flushing in response to a human intruder. I recorded a 20% increase in average flushing distance on consecutive visits. This suggests that hawks may become more sensitive with intrusions. Female hawks defended nests with significantly more aggression than males by diving and calling more while I was near the nest tree, and calling more during my approach. Hawks nesting in a humanaltered environment defended nests with similar intensity to hawks in a rural setting, but the former allowed an intruder to approach an average of 120 m closer to the nest before flushing. This suggests some response mediation may have occurred among hawks that nested close to humans. There was no relationship between Ferruginous Hawk nest defense response and nestling age, but overall response levels were comparable to those documented for sympatric buteos in other studies. Establishing disturbance free zones surrounding nesting sites and conserving extant prairie dog (Cynomys spp.) colonies should be objectives of any management plan focused on the long-term maintenance of Ferruginous Hawk populations.

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Ornithology Commons