Banner Year for Satellite Telemetry Deployments

By Dr. Jeff Smith, HWI Science Director, and Mark Vekasy, HWI Field Studies Coordinator

During fall 2002, HWI deployed 36 satellite transmitters on 13 Northern Goshawks, 13 Red-tailed Hawks, and 10 Golden Eagles. At Chelan Ridge, Jim Watson of the Washington Department of Fish and Wildlife also deployed two of his units on Cooper's Hawks. The magnitude of this success is apparent when noting that between 1999 and 2001 HWI deployed only 32 units.

Mixed Success Meeting Deployment Targets

By outfitting 6 eagles in the Goshute Mountains, we exceeded our target for the Intermountain Flyway (5). We also met our target of 4 eagles in the Manzano Mountains within the Rocky Mountain Flyway. We did not fare as well in the Pacific Northwest, however, with a target of 2 eagles each at Bonney Butte and Chelan Ridge but no captures at either site. Somewhat the opposite scenario applied to Northern Goshawks. We fully met our targets for this species at Bonney Butte and Chelan Ridge (4 each), whereas goshawks of sufficient size and condition proved to be relatively scarce in the Goshutes and Manzanos, likely due to drought effects. We outfitted only 3 goshawks in the Goshutes and 2 in the Manzanos, whereas the target was 13 total for the two sites. We met our target of 4 adult Red-tailed Hawks in the Manzanos and 3 adults at Bonney Butte, but fell one short of our targets in the Goshutes (4 of 5, all adults) and at Chelan Ridge (2 of 3, including one first-year bird). Tracking summaries and maps for all of our telemetry birds can be viewed on our website at www.hawkwatch.org.

Special thanks to on-site telemetry specialists Bob Davies, Rick Gerhardt, and Leigh Greenwood for coordinating deployments at Chelan Ridge, Bonney Butte, and the Manzanos, respectively.

Red-tail Tracking Yields Greatest Success

To date, we achieved our greatest success tracking Red-tailed Hawks. In September 2002, HWI Science Director, Jeff Smith, presented a paper at the Third North American Ornithological Conference in New Orleans which summarized the results of our red-tail tracking, and he will present a similar paper this spring at the Hawk Migration Association of North America conference in Corpus Christi. The remainder of this article represents a brief summary of some of the material presented in these talks. We will develop similar summaries for Golden Eagles and Northern Goshawks in future issues of RaptorWatch as our data for these species improves.

Identifying New Flyway Dimensions

HWI began outfitting raptors with satellite transmitters during fall 1999. With our fourth year of tracking underway, some significant findings can be identified. One of the most revealing has been frequent movements of adult Red-tailed Hawks from the Intermountain Flyway into southern California and especially Baja California for the winter. After 20+ years of banding in the Goshute Mountains, we had never received a band return from Baja for any species. Moreover, although 4 of 33 red-tail band returns amassed as of summer 2002 came from southern California, all such birds were banded and recovered during their first year. We often assume that young birds wander away from traditional flight paths, so recovery of these birds outside of the Intermountain area was not particularly surprising. However, of 11 adult red-tails outfitted with transmitters in the Intermountain Flyway, two wintered in southern California and three in Baja. Thus, it would seem that southern California and the Baja Peninsula are targeted destinations for a larger number of Intermountain red-tails than we previously suspected.

Our initial trackings of red-tails within the Pacific Coast Flyway also quickly confirmed a new flyway dimension. Most of the birds outfitted at Chelan Ridge have returned north to summer territories in central and northern British Columbia, another area from which we have never received a band return, undoubtedly due to low human population density. Otherwise, all of our Oregon and Washington red-tails have remained within the expected boundaries of the Pacific Coast Flyway; that is, generally within or west of the Cascade and Sierra Nevada ranges from British Columbia through southern California.

Similarly, all of our telemetered Manzano Mountains red-tails have remained within the expected boundaries of the Rocky Mountain Flyway; that is, generally within or east of the Rocky Mountains and east of the Sierra Madre Occidental in Mexico (10 band recoveries show the same pattern). Some mixing of birds from the Intermountain and Rocky Mountain flyways has occurred, especially in north-central Mexico, but no red-tails originating from the Rocky Mountain Flyway have crossed to the west side of the Sierra Madre Occidental. Mixing of birds from the two flyways in the southern U.S. and northern Mexico appears to result from a relative lack of significant geographical barriers. This contrasts with the northern part of these flyways, where both the Rocky Mountains and Cascade-Sierra Nevada ranges appear to represent distinct biogeographic boundaries that generally separate different migratory populations.

Quantifying Migration Route Fidelity

The primary objective of our telemetry studies is to quantify migration route fidelity; that is, to learn if individual birds tend to use the same pathways year after year. The answer to this question will help us determine whether the yearly variation we see at our migration sites reflects primarily variation in the population sampled each year or actual changes in the status and trends of source populations. So far, our telemetered red-tails have generally shown high fidelity to regional flyways and general migration routes, but potentially significant pathway variation with regard to specific locations like our monitoring sites.

We outfitted 17 red-tails before fall 2002, but due to technical failures or premature hawk mortality, only 6 of these transmitters lasted across multiple fall migrations. One of these birds, after migrating through the Manzanos to a location in Durango, Mexico, never returned north again, suggesting that it was actually a Mexican resident that simply decided to wander north one year.

The other 5 birds stayed within the broad confines of the defined regional flyways and, with one exception, followed largely similar pathways each successive fall. The exception was a Goshute bird that took a path around the east side of Great Salt Lake during two fall seasons after we outfitted it west of the lake. The remaining 4 birds followed roughly similar pathways in successive seasons, but due to limitations of the data, we cannot be sure that any of these birds passed back through the original banding locations again. For 2 birds, mapping of their subsequent fall pathways appeared to track directly back over the banding locations at Chelan Ridge and the Goshutes, whereas for the other 2 the mapped pathways diverged west or east of the Goshutes by up to several hundred kilometers. In each case, however, three limitations of satellite telemetry data preclude answering this question with certainty: 1) the accuracy of the location data can be limited, potentially varying by as much as several kilometers from the true location; 2) unexpected gaps in the transmissions sometimes occur reducing the detail obtained; and 3) if a bird is moving fast, the frequency of data acquisition (generally every 2 days during migration seasons) may not be sufficient to outline a pathway that is detailed enough to confirm passage over a specific location.

Despite the mixing of birds in the southern U.S. and northern Mexico during winter, come spring it appears that most red-tails return to their respective flyways and tend to follow a path similar to the previous fall migration. We obtained paired fall and spring migration data for 12 of the 17 birds outfitted between 1999 and 2001 and, with two notable exceptions, all used similar pathways in both seasons. One Goshute bird used the same pathway during 2 fall migrations and the same pathway during 2 spring migrations; however, the fall and spring pathways differed (a loop route around the Great Salt Lake and Great Salt Lake Desert complex). The second was another Goshute bird that traveled southwest to its wintering grounds in Baja California, but then returned north to southern British Columbia by traveling along the eastern Sierra Nevada range.

Summarizing Preliminary Results

At this point, we can confidently state that the majority of our red-tails have shown a fairly high degree of route fidelity across years and seasons, but with the caveat that the scale of inference is generally insufficient to confirm repeat passage at highly specific locations like count sites. It also appears that route fidelity is generally higher within seasons than between fall and spring seasons. Another key finding is that all adult red-tails for which we have obtained repeat data have shown high fidelity to summer and winter territories. This also suggests that, since the destinations generally remain the same, the pathways followed in between should remain reasonably consistent. With 13 new red-tails outfitted in fall 2002, we anxiously await data from the 2003 migration seasons to see if these patterns are maintained and shed even more light on this crucial question of migratory fidelity.