Lewis, Stephen B. – Breeding season diet of Northern Goshawks in southeast Alaska with a comparison of techniques used to examine raptor diet. 2001.
Chapter 1: A Video Surveillance System for Monitoring Raptor Nests in a Temperate Rainforest Environment
I used a video surveillance system to monitor northern goshawk (Accipiter gentilis) nests in the coastal temperate rainforest of southeast Alaska to gather data on their diet. I maintained five systems during the goshawk nesting seasons in 1998 and 1999, installing the cameras an average of 10 days after hatching. At these 10 nests, cameras were maintained for an average of 33 days, recording 5834 hours of nest-time. I captured an average of 69.3% of the daylight hours available from hatching to the day nests were no longer used by juvenile northern goshawks. Technical difficulties associated with maintaining video cameras in this rainforest environment included electronic malfunctions, recurrent battery failure, and problems with the recorded image. However, these video surveillance systems effectively monitored northern goshawk nests and could be adapted for most rainforest raptors that nest on open platforms. I recommend testing the systems under field conditions in which they are to be used prior to deployment.
Chapter 2: Comparison of Three Techniques FOR Assessing Raptor Diet During the Breeding Season
Video recording of prey deliveries at nests is a new technique for collecting data on diet and food habits that has not been compared with results from collections of prey remains and pellet. As part of a study of the breeding season diet of northern goshawks (Accipiter gentilis) in southeast Alaska, I compared data from these three techniques to determine the relative merits of the different methods for assessing diet. I monitored 5 nests during the northern goshawk breeding seasons of 1998 and 1999 and identified 1541 prey from deliveries, 209 prey from remains, and 209 prey from pellets. The proportions of birds and mammals varied among techniques, as did the relative proportions of prey groups and age groups. Analysis of prey deliveries gave the narrowest diet breadth of the three techniques. Prey remains and pellets gave the least similar diet descriptions. Over two-day intervals during which data was collected using all three techniques, prey deliveries gave more individual prey and prey categories than the other two techniques. I found that prey was not directly tracked through all three techniques. Analysis of prey deliveries collected by remote videography provided the most complete description of diet and I recommend that studies attempting to describe diet use this method or some other direct technique.
Chapter 3: Breeding Season Diet of Northern Goshawks in Southeast Alaska
I provided the first systematic description and quantification of the nesting season diet of northern goshawks (Accipiter gentilis) in southeast Alaska and examined how their diet varied within this island archipelago. I collected data on the diet of goshawks using three techniques. I used remote videography to record prey deliveries at nests in two spatially distinct locations of southeast Alaska to describe the diet in detail and examine spatial variation in the diet. I used prey remains and pellets collected at nests throughout southeast Alaska to describe the diet of the goshawk over a broader spatial scale. Goshawks delivered more birds than mammals overall of southeast Alaska but delivered more birds in the Prince of Wales Island area than in other parts of southeast Alaska. In northern southeast Alaska, blue grouse (Dendragapus obscurus) red squirrels (Tamiascurius hudsonicus), Stellers jays (Cyanocitta stelleri), varied thrushes (Ixoreus naevius), northwestern crows (Corvus caurina) and unknown passerine birds were the prey that contributed the most to the diet. In southern southeast Alaska, spruce grouse (Falcipennis canadensis), Stellers jay, ptarmigan species (Lagopus spp.), varied thrushes, and unknown passerine birds were the commonly eaten prey. Diet niche was narrower in the north than in the south and nests in these areas, on average, showed little overlap. The relative proportion of grouse and thrushes in the diet appeared to vary as the nesting season progressed, as did the relative proportion of different aged prey. Data from prey remains and pellets collected over all of southeast Alaska provided similar results as that from remote videography. In southeast Alaska, goshawks ate similar types of prey as seen in other locations. My data support the supposition that goshawk are generalists predators and show a certain amount of adaptability in their tolerance to varying prey bases. However, there appears to be a limit to this adaptability, which was apparent on Prince of Wales Island. In this area, an extremely restricted prey base in combination with extensive landscape alteration due to timber harvest appears to have affected goshawks ability to successfully reproduce. Goshawks in southeast Alaska rely on a few important prey species that can be affected by timber harvesting activities. Therefore, management should focus on conserving forests that structurally and functionally mimic those that historically covered this region.