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Gossett, Daniel N. – Studies of Ferruginous Hawk biology. 1993.

Alumni Research

Studies of ferruginous hawk biology:

  1. Recoveries of banded ferruginous hawks from presumed eastern and western subpopulations
  2. Morphological and genetic differences of presumed subpopulations of ferruginous hawks
  3. Sex determination of nestling ferruginous hawks

I examined 537 band recoveries and returns to determine differences in crossover, dispersal, and winter distribution of presumed eastern and western subpopulations of the ferruginous hawk (Buteo regalis). Only 4.1% of eastern banded hawks were recovered west of the continental divide, while 27.5% of western banded hawks were found east of the divide, for an overall crossover proportion of 8.6%. Mean winter recovery latitude of the two groups was not significantly different, but longitude differed significantly. Winter recoveries of both subpopulations were in concentrated areas in northwest Texas, western Kansas, and northern Mexico, although western recoveries were more dispersed. Ferruginous hawks were recovered in highest proportions (48.7%) within their first year and 59.3% of post-fledging birds were recovered within 25 km of banding sites. A high proportion (48.6%) of breeding-age birds were found within 50 km of their natal site during the breeding season. Shooting remains a serious mortality factor; 36.5% of the reported shooting mortalities have occurred since 1972. Of the total recoveries, 3% that had crossed the divide were of minimum breeding age, indicating some degree of potential mixing and gene flow between the two presumed subpopulations.

I trapped 67 adult ferruginous hawks during the breeding season in 1991 and 1992, to examine morphological and genetic differences between the postulated subpopulations. Multivariate analysis of variance (MANOVA) indicated that both sexes of eastern birds were significantly larger in mass, and eastern females were larger for third toe length, bill chord, and gape width than western birds. Discriminant function analysis (DFA) correctly classified 89.6% of females, and 93.33% of males into their respective subpopulations. Mass was the predominate variable influencing four of six misclassifications. Latitudinal variation accounted for most of the morphological differences; eastern capture sites were farther north than western sites. Morphological differences between the eastern and western groups could not be explained by localized ecological adaptation to a different prey base, as previously suggested. No genetic differences were found between postulated eastern and western groups, using the Random Amplified Polymorphic DNA (RAPD) technique, with a survey of 40 primers on 29 birds.

An external morphometric method was developed to determine the sex of nestling ferruginous hawks. MANOVA indicated that five measured characters were significantly different between males and females. For nestlings 24 days and older, DFA classified 100% of 32 nestlings into their presumed sex, based on flexed-hallux, tarsus length, and toe-pad measurements. Nestlings 20 days and older had a classification of 94.7% to their presumed sex with these three variables. A linear discriminant function is presented to classify nestlings to sex using these measurements. Chromosomal C-banding confirmed the gender of five nestlings classified to sex using the morphological method.


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