A comparative study of the mechanics of the pectoralis muscle of the red‐tailed hawk and the barred owl

A comparison of the isometric forces and levers of the pectoralis muscle in red‐tailed hawks (Buteo jamaicensis) and barred owls (Strix varia) was done to identify differences that may correlate with their different flight styles. The pectoralis consists of two heads, the anterior m. sternobrachialis (SB) and the posterior m. thoracobrachialis (TB). These are joined at an intramuscular tendon and are supplied by separate primary nerve branches. As in other birds, the two heads have distinct fiber orientations in red‐tailed hawks and barred owls. SB's fiber orientation (posterolateral and mediolateral from origin to insertion) provides pronation and protraction of the humerus during adduction. Electromyographic studies in pigeons show that it is active in early downstroke and during level flight. TB is more active during take‐off and landing in pigeons. The anterolateral orientation (from origin to insertion) of its fibers provides a retractive component to humeral adduction used to control the wing during landing. In our study, the maximum isometric force produced by the combined pectoralis heads did not differ significantly between the hawk and owl, however, the forces were distributed differently between the two muscle heads. In the owl, SB and TB were capable of producing equal amounts of force, but in the hawk, SB produced significantly less force than did TB. This may reflect the need for a large TB to control landing in both birds during prey‐strike, with the owl maintaining both protractive (using SB) and retractive (using TB) abilities. Pronation and protraction may be less important in the flight behavior of the hawk, but its prey‐strike behavior may require the maintenance of a substantial TB for braking and controlled stalling, as it initiates strike behavior. J. Morphol. 2012. © 2011 Wiley Periodicals, Inc.     

Characterization of 10 microsatellite loci in an avian louse, Degeeriella regalis (Phthiraptera: Ischnocera: Philopteridae)

We isolated and characterized 10 polymorphic microsatellite loci in an ischnoceran louse, Degeeriella regalis, which parasitizes the threatened Galápagos hawk (Buteo galapagoensis) and other falconiform birds. The loci were screened across 30 individuals from two island populations in the Galápagos Islands. The number of alleles per locus ranged from two to 28. Polymorphic information content ranged from 0.14 to 0.94 and observed heterozygosity ranged from 0 to 0.67. These markers will be valuable in comparative population genetics studies in this species, which is the focus of a long-term population and disease ecology research program.     

Red-Shouldered Hawk Occupancy Surveys in Central Minnesota,USA

ABSTRACT Forest-dwelling raptors are often difficult to detect because many species occur at low density or are secretive. Broadcasting conspecific vocalizations can increase the probability of detecting forest-dwelling raptors and has been shown to be an effective method for locating raptors and assessing their relative abundance. Recent advances in statistical techniques based on presence—absence data use probabilistic arguments to derive probability of detection when it is < 1 and to provide a model and likelihood-based method for estimating proportion of sites occupied. We used these maximum-likelihood models with data from red-shouldered hawk (Buteo lineatus) call-broadcast surveys conducted in central Minnesota, USA, in 1994–1995 and 2004–2005. Our objectives were to obtain estimates of occupancy and detection probability 1) over multiple sampling seasons (yr), 2) incorporating within-season time-specific detection probabilities, 3) with call type and breeding stage included as covariates in models of probability of detection, and 4) with different sampling strategies. We visited individual survey locations 2–9 times per year, and estimates of both probability of detection (range = 0.28-0.54) and site occupancy (range = 0.81-0.97) varied among years. Detection probability was affected by inclusion of a within-season time-specific covariate, call type, and breeding stage. In 2004 and 2005 we used survey results to assess the effect that number of sample locations, double sampling, and discontinued sampling had on parameter estimates. We found that estimates of probability of detection and proportion of sites occupied were similar across different sampling strategies, and we suggest ways to reduce sampling effort in a monitoring program.     

Possible Causes of Variation in Territory Tenure in a Lekking Pompilid Wasp (Hemipepsis ustulata) (Hymenoptera)

Territorial males of the pompilid wasp Hemipepsis ustulata occupy their landmark territories for from less than 1 day to several weeks. Variation in territory tenure could arise if territories differ in their value and cost to defend (the tradeoff hypothesis). If superior sites attract more male competitors, raising the cost ofterritorial defense of these locations, we would expect (1) tenure times to be shorter at top-ranked sites and (2) tenure times to decline in years of high male density and competition. Although mean tenure times for males able to hold territories for at least 2 days were shorter at top-ranked sites, the difference was only statistically significant in one of three years. In the year of highest male density, a significantly higher proportion of all territorial males failed to hold the top-ranked sites for more than a single day; however, mean tenure duration at all occupied sites did not decline in this year. Thus, the tradeoff hypothesis received only partial support. An alternative hypothesis is that variation in male resource holding power, as reflected in male size or wing condition, contributes to differences in territorial tenure. However, although larger males were more likely to become territorial residents at the top-ranked sites, the duration of tenure at these locations was not positively correlated with body size. In addition, although wing condition declined throughout the flight season in concert with a sharp decrease in the duration of tenure at the top-ranked sites, males with fresher wings did not hold territories longer than males whose wings were in poorer condition when they assumed control of a site.     

Predation upon the subterranean rodent Ctenomys talarum (tuco-tucos) by Buteo polyosoma (red-backed hawks) in coastal grasslands of Argentina

Differential predation upon tuco-tucos (Ctenomys talarum) by red-backed hawks (Buteo polyosoma) is reported, considering the characteristics that could lead to prey vulnerability and to predator selectivity. Predation was evaluated through analysis of 100 pellets and 20 prey remains collected in coastal grasslands at Mar de Cobo (Buenos Aires, Argentina) during winter (May to August) 2003–2004. The tuco-tucos represented the main component in the diet of red-backed hawks, registering a high consumption of sub-adult individuals (49.3%). Juvenile individuals (42.0%) were overrepresented and adults (8.7%) were underrepresented in relation to their field frequencies (14.4 and 43.1%, respectively), whereas sub-adults did not show significant differences. This high consumption of smaller tuco-tucos would be based on the higher vulnerability of young individuals to predation rather than raptor's handling constraints on bigger individuals. Our results indicate that a strong predator–prey relationship between tuco-tucos and red-backed hawks is established at the study area, and that tuco-tucos represent a key component for the survival of wintering hawks.

Reportamos la depredación diferencial sobre el tuco-tuco (Ctenomys talarum) por el aguilucho común (Buteo polyosoma), considerando las características que podrían determinar la vulnerabilidad de la presa y la selectividad del predador. La depredación fue evaluada mediante el análisis de 100 egagrópilas y 20 restos presa recolectados en pastizales costeros de la localidad de Mar de Cobo (Buenos Aires, Argentina) durante el periodo invernal Mayo–Agosto de 2003–2004. El tuco-tuco representó el principal componente en la dieta del aguilucho común, registrándose un alto consumo de individuos subadultos (49.3%). Los individuos juveniles (42.0%) fueron sobrerrepresentados y los adultos (8.7%) fueron subrepresentados en la dieta en relación a sus frecuencias en el campo (14.4 y 43.1%, respectivamente), mientras que los subadultos no mostraron diferencias significativas. Este elevado consumo de individuos más pequeños radicaría en una mayor vulnerabilidad a la depredación de los juveniles más que en restricciones de manipuleo del predador sobre las presas más grandes. Nuestros resultados indican que existe una estrecha relación predador–presa entre el tuco-tuco y el aguilucho común, y que los tuco-tucos representan un componente clave para la supervivencia invernal de los aguiluchos.     

Can a ‘wintering area effect’ explain population status of Swainson's hawks? A stable isotope approach

It has been suggested that declines in breeding populations of Swainson's hawks ( Buteo swainsoni ) in California, Oregon, and Nevada may be due to differential mortality of hawks on their wintering grounds. Although massive mortality incidents reported on the wintering grounds partially support this suggestion, there are no data showing differential use of wintering areas by breeding populations of Swainson's hawks. We used stable-hydrogen isotope analysis of feathers to determine whether large flocks of hawks wintering in Argentina consisted of a mixture of individuals from across the North American breeding range or consisted of individuals from discrete breeding populations. We found that flocks of wintering Swainson's hawks consisted of a mixture of individuals. The lack of connectivity between populations of breeding and wintering hawks suggests that high wintering mortality, either natural or human-induced, is unlikely to have direct consequences on a single breeding area in North America. The demographic effects of winter mortality should be 'diluted' across the entire breeding range of Swainson's hawks.     

Does breeding population trajectory and age of nesting females influence disparate nestling sex ratios in two populations of Cooper's hawks?

Offspring sex ratios at the termination of parental care should theoretically be skewed toward the less expensive sex, which in most avian species would be females, the smaller gender. Among birds, however, raptors offer an unusual dynamic because they exhibit reversed size dimorphism with females being larger than males. And thus theory would predict a preponderance of male offspring. Results for raptors and birds in general have been varied although population‐level estimates of sex ratios in avian offspring are generally at unity. Adaptive adjustment of sex ratios in avian offspring is difficult to predict perhaps in part due to a lack of life‐history details and short‐term investigations that cannot account for precision or repeatability of sex ratios across time. We conducted a novel comparative study of sex ratios in nestling Cooper's hawks (Accipiter cooperii) in two study populations across breeding generations during 11 years in Wisconsin, 2001–2011. One breeding population recently colonized metropolitan Milwaukee and exhibited rapidly increasing population growth, while the ex‐Milwaukee breeding population was stable. Following life‐history trade‐off theory and our prediction regarding this socially monogamous species in which reversed sexual size dimorphism is extreme, first‐time breeding one‐year‐old, second‐year females in both study populations produced a preponderance of the smaller and cheaper sex, males, whereas ASY (after‐second‐year), ≥2‐year‐old females in Milwaukee produced a nestling sex ratio near unity and predictably therefore a greater proportion of females compared to ASY females in ex‐Milwaukee who produced a preponderance of males. Adjustment of sex ratios in both study populations occurred at conception. Life histories and selective pressures related to breeding population trajectory in two age cohorts of nesting female Cooper's hawk likely vary, and it is possible that these differences influenced the sex ratios we documented for two age cohorts of female Cooper's hawks in Wisconsin.