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An Occasional Review of Goshawk Research and Conservation Issues

#1: Summary of Ingraldi (1999) by Kierán Suckling

Ingraldi, M. 1999. Population biology of northern goshawks (Accipiter gentilis) in east-central Arizona. PhD dissertation, Northern Arizona University, Flagstaff, AZ.

Six-year study (1993-1998) of goshawk demographics on the entire Sitgreaves National Forest in east-central Arizona. Study site is “dominated by stands of younger age classes of ponderosa pine. Most of the ponderosa pine stands on level terrain have been logged, with steeper slopes of canyons and drainages receiving less logging impact.”

A total of 44 territories were occupied at least once during the study period on the 495,000 acre study area. The mean occupancy rate was 68 percent (48 percent - 85 percent). The mean activity rate (i.e. nesting attempt made) was 49 percent (31 percent - 69 percent). The mean nesting success rate (i.e. fledged at least one bird) was 70 percent (46 percent - 94 percent).

While the mean number of fledglings per active nest (1.26) fluctuated greatly between years (0.54 - 1.66), the number of fledglings per successful nest (1.85) remained relatively constant (1.67 - 2.0). Mean fecundity (female fledglings/breeding female) was highly variable from year to year (0.23 - 0.78, mean = 0.46). [Editor's note: Fluctuation in reproductive/occupancy parameters may be influenced by the fact that consistently active/successful nests are more likely to be first located at the beginning of the study than the end. This would create an artificial distinction between early and later years. As the effect is likely to diminish toward zero over time, it may be possible to develop a normalizing function.]

Of the 62 successful broods where sex of fledgling was determined, the male-to-female ratio (1.771) was significantly different from 11. This may indicate an environmentally stressed population that is producing more, less-energy-expensive males.

Mean adult female survivorship was 75 percent, relatively constant over the study period, within the range reported by others (70 - 86 percent), but lower than reported on the north Kaibab by Reynolds and Joy (1998). The difference may be due to the larger Kaibab sample size, better weather conditions in the early 1990s, or the presence of more areas with larger trees. [Editor's note: A comparison of equivalent years may be able to tease out the difference]. Mean fledgling female survivorship was 43 percent and variable, perhaps due to a small sample size (n=16).

Mean lambda value was 0.84, with the 95 percent CI not encompassing 1. Mean time to extinction predicted by PVA was 29 years — but only if the population is closed. This is highly unlikely, indicating that the Sitgreaves National Forest is likely a biological sink within a larger population of unknown stability. Adult survival had the greatest effect on lambda; nesting activity was the second most influential.

Occupancy rate is a poor indicator of goshawk population status to due problems with verifying non-occupancy. [Editor’s note: “Difficult” would perhaps be a better description since the problem with occupancy rates is verification, not relevancy. As occupancy affects total reproductive output, and has been correlated with suitability of habitat, it is a good indicator of population status, though it may be difficult to measure. Additionally, if habitat degradation is unequally distributed among goshawk territories, occupancy can drop dramatically within heavily degraded territories while productivity of un-degraded territories remains stable. If degradation is equally distributed, productivity would be a better index.] Given a large enough sample, success rate and fledglings per active nest are better indices of the reproductive status of a population because they are independent of survey effort. The average yearly sample size of 38 territories on the study site exceeded the minimum of 35 needed for precise estimation. Nest success rate and fledglings/active nest was lower than reported in most studies west of the 100th meridian. This may be due to “fragmentation of suitable foraging and nesting habitat, low prey populations, or dry weather conditions.” Mean fledglings per successful nest was also lower, and may be a more meaningful measure of a population's productivity as it is not as likely to be influenced by extraneous and unpredictable factors (i.e. nest tree falling down). [Editor’s note: It may be difficult to distinguish extraneous from relevant factors: the nest tree falling down, for example, could be related to management practices which favor blow down, while the probability of a nest tree being struck by lightning my increase as the total number of large trees in the area is reduced by logging.]

The nesting activity rate was negatively correlated with amount of precipitation during April and May. The number of young/successful nest was positively correlated with amount of precipitation during March and April, and negatively correlated with the mean temperature during May. The male-to-female fledgling ratio was positively correlated, and the fecundity rate was negatively correlated, with amount of precipitation during May and June.

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