MPG Spring Migration Count 2013 by RVRI

MPG Spring Migration Count 2013 by RVRI

August 8, 2013

This report summarizes Raptor View Research's spring 2013 raptor count on the MPG Ranch. During 59
days of observation from 18 March to 15 May, RVRI counted 2,238 raptors of 17 species.

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This report summarizes the spring 2013 raptor count on the MPG Ranch.

Table of Contents

Table of Figures

The Bitterroot Valley is an obvious raptor migration corridor and wintering area, with the Bitterroot-Selway wilderness to the west and the Sapphire Mountains to the east (Figure 1). The Baldy Mountain Complex (BMC) is located on the MPG Ranch, at the north end of the Bitterroot Valley. The Bitterroot River and associated cottonwood-riparian forests lie approximately 4 kilometers to the west of the BMC and the Sapphire Mountains are located approximately 9.5 kilometers to the east.

As in 2012, in 2013 we used two locations to sample the unique and inconsistent flight paths used by raptors migrating over the BMC. The primary lookout, Indian Ridge (GPS location: 46.692945°N, 114.029767°W) is located approximately four km southwest of Baldy Mountain on the western flank of the BMC at about 1100 m in elevation. We also counted from Partridge Ridge (46.688831°N, 113.991875°W), located approximately 2.2 km south of Baldy Mountain at about 1400 m in elevation (Figure 2). We counted from both sites on 42 days. On 16 days we counted only from Indian Ridge, as low cloud cover and inclement weather limited visibility from Partridge Ridge.

Of the 58 days we counted, 41 were overcast and cloudy. There was precipitation on 20 days. We were unable to count at all on 7 April and for three or less hours on 6 April and 13 April due to low visibility from heavy precipitation. The high temperature for the season was 28 °C (12 May), with an average morning temperature of 6.5 °C and an average afternoon temperature of 11 °C. Winds were variable in velocity; the median 50% of hourly records fell between 8-19 kph, with an hourly mean of 14.4 kph and an hourly median of 12.9 kph. Wind direction was also variable, though rarely eastern (35% southern, 32% northern, 25% western, 8% eastern).

We recorded 17 species. The five most abundant species included: Red-tailed Hawk (477), Sharp-shinned Hawk (318), Turkey Vulture (223), Rough-legged Hawk (198), and Cooper’s Hawk (186). (Table 1 and Appendix A).

Temporal Patterns of Raptor Migration Migrant numbers varied throughout the day, with peak numbers occurring between 1100 and 1400. We observed few raptors before 1000 and after 1600 hours (Figure 4), though we only surveyed at these times on days with good flights; our typical survey period was 1000-1700. Most raptor species hunt and feed during the cool morning hours and begin migrating in late morning and early afternoon, when warming air currents (thermals) develop. As evening approaches, they typically pause migration to search for prey and locations to roost.

In the beginning of the season we counted a high proportion of buteos and eagles. As their numbers started to diminish towards early April, we observed increases of Turkey Vultures and accipiters. As the accipiter flight began to peak in the third week of April we recorded our highest numbers of Ospreys and falcons. We also noticed the start of a secondary peak of buteo migrants in the third week in April. Northern Harriers also peaked during the close of April, though their numbers were steadier throughout the survey than other groups (Figure 5).

In this section, we present seasonal trends of migration for individual species and make comparisons between the springs of 2012 and 2013. For a few species (e.g. Golden Eagles, Rough-legged Hawks, Swainson’s Hawks, and Ospreys) we saw similar temporal patterns in both years. More species, however, showed a later peak in migration in 2013 than 2012 (e.g. Bald Eagles, Sharp-shinned Hawks, Cooper’s Hawks, Red-tailed Hawks, American Kestrels, Northern Harriers, and Turkey Vultures). We observe some species (e.g. Northern Goshawks, Ferruginous Hawks, Broad-winged Hawks, Merlins, and Peregrine and Prairie Falcons) so infrequently that we cannot make comparisons.

We counted 51 Bald Eagles from 21 March to 9 May. We recorded 26 in March, 24 in April, and one during the first fifteen days of May. Compared to 2012, we saw fewer Bald Eagle migrants and a later peak.

Sharp-shinned Hawks were our second most counted species during this year’s survey; we counted a total of 318 from 26 March to 15 May. We observed most Sharp-shinned Hawks relatively late in the season. We saw 7 in March, 149 in April, and 162 during the first fifteen days of May. The daily high occurred on 6 May when we recorded 35 individuals. While the total number of Sharpshinned Hawks counted this spring was relatively similar to 2012, their peak occurred approximately two weeks later.

Cooper’s Hawks were our fifth most counted species this season; we recorded 186 between 22 March and 13 May. Like the Sharp-shinned Hawks, most Cooper’s Hawks migrated late in the season. We observed 13 in March, 107 in April, and 66 in May. The daily high occurred on 25 April when we saw 20. This spring’s total number of Cooper’s Hawks is approximately one-third less than last year’s total, though each year’s peak occurred at approximately the same time.

We counted 11 Northern Goshawks this season; five in March, four in April and two in May. As in 2012, observations of Northern Goshawks were irregular throughout the season.

We observed more Red-tailed Hawks than any other species in the spring counts of both 2012 and 2013. This season, we counted 209 in March, 190 in April and 78 in May, for a total of 477. Our peak day occurred on 26 March, when we counted 36. Though we saw 89 fewer Red-tailed Hawks this season than in 2012, they made up 21% of the total count, very close to the 22% of the total count in 2012. Like last spring, the peak of Red-tailed Hawk numbers occurred in the third week of March and dropped into mid-April. This year, however, we noticed a smaller, second surge in late April that lasted through the first week in May.

We counted 198 Rough-legged Hawks from 18 March through 9 May. Our highest count occurred on 28 March when we counted 17. Like last spring, the highest number of Roughlegged Hawks moved through the Bitterroot Valley in late March. Because Rough-legged Hawks breed at extremely northern latitudes, we were surprised this year to see a fairly consistent number still migrating in early May.

We observed 36 Swainson’s Hawks from mid April through 15 May. The timing of this year’s Swainson’s Hawk observations were very similar to those in 2012, though we saw more in 2013 than 2012. Swainson’s Hawks winter as far south as Argentina, further than any other Montana raptor species. This long-distance migration, combined with the fact that Montana is at the northern edge of Swainson’s Hawks’ breeding range, may explain their relatively late detection date.

We counted a total of 11 Broad-winged Hawks, compared to three in 2012. Broad-winged Hawks do not breed in Montana; the few we see each year are likely headed to the far western extent of their breeding range, in western Alberta and eastern British Columbia.

We counted 7 Ferruginous Hawks from 23 April to 10 May. Ferruginous Hawks are rarely seen at migration count sites. In Montana, Ferruginous Hawks breed in open habitats east of the Continental Divide.

We counted 156 American Kestrels from 18 March through 9 May. Unlike last season, we did not see high numbers of kestrels passing by until late April. As one of our smaller migrants, we typically see their numbers peak later than other raptor species.

We counted 9 Merlins from 19 March through 27 April. Merlins commonly migrate over coastal raptor sites, but are rarely counted at intermountain ridge sites. Falcons are built differently than other raptors, and typically use powered flight more than groups better adapted for soaring. As a result they are probably less reliant on the thermal/wind lift provided by ridge systems where intermountain counts take place. At coastal sites, however, their flightlines become more concentrated as they minimize travel over the ocean.

We counted 6 Peregrine Falcons from 11 April through 30 April. Like Merlins, Peregrine Falcons are more commonly seen at coastal count sites.

We recorded six Prairie Falcons. Prairie Falcons tend to be non-migratory or exhibit latitudinal (east-west, west-east) migration and are rarely counted at any site in North America.

We counted 149 Northern Harriers from 23 March through 13 May. We saw fairly steady numbers throughout the season, with a slight peak in early May. In contrast, in 2012 we observed a major push over just two days; we counted 66 on 10-11 April. In total we counted 55 fewer Northern Harriers in spring 2013 than in 2012.

We counted 85 Ospreys from 2 April to 15 May. This year’s Osprey migration was very similar to last, with high counts centered around a peak in late April. The three adult Ospreys outfitted with GPS transmitters last breeding season arrived to the Bitterroot Valley just before the highest number of migrants moved through; the male from the ranch entrance nest, female from the ranch entrance nest, and male from the north center pivot nest arrived 11 April, 19 April, and 22 April respectively.

The Turkey Vulture was the third most counted species; we observed 223 Turkey Vultures from 18 March to 15 May. Though we saw a peak of vulture migration in early April in both years, the peak was far less pronounced this season than in 2012.

As in 2012, only one other western count site was active during the spring 2013 migration: Dinosaur Ridge, a foothills site on the outskirts of Denver, Colorado. This year, Dinosaur Ridge’s observers counted 1,100 raptors in 371 hours of observation, for an average of 2.96 raptors/hour. In both 2012 and 2013, the Dinosaur Ridge site showed some similarity in species composition to the MPG Ranch count (e.g., relatively high numbers of Redtailed Hawks, Turkey Vultures, and American Kestrels), but lacked other species that we detected in high numbers (e.g., Northern Harrier and Rough-legged Hawk) (Figure 6). In both seasons, our count had a much higher average number of raptors/hour. Totals from Dinosaur Ridge vary widely from year to year, and they have observed over 2,000 raptors in a single season in the past (S. Hoffman, pers. com.).

We observed approximately 15% fewer migrating raptors in 2013 as in 2012 (Figure 7). This pattern is largely attributable to a lack of very high daily counts this year. In 2012 we counted over 150 raptors on three separate days (including a daily high of 236 on 11 April), compared to this season’s daily high of 127. This year’s peak was relatively late compared to last; we saw numbers climb during the last week of April and stay high through the first week of May, whereas 2012’s flight had mostly subsided by April’s end. Because weather conditions were similar between years, it is difficult to determine what caused the later peak.

In general, species composition was extremely similar in the spring 2012 and 2013 counts. Proportions of accipiters, buteos, eagles, falcons and Northern Harriers were remarkably similar, while the proportion of Ospreys and Turkey Vultures switched (Figure 8).

In the past two years, we have counted more migrating raptors in spring than fall. Because spring flights are typically less temporally and geographically concentrated than those in the fall, there are few sites suitable for surveys. The fact that the MPG Ranch sees such high numbers in both spring and fall is remarkable; to our knowledge it is the only site in the West where both spring and fall counts are conducted. Red-tailed Hawks and Sharp-shinned Hawks are the most abundant species in both fall and spring counts. So far, we have observed many more Turkey Vultures and Ospreys in the spring, while American Kestrel and Redtailed Hawk numbers are higher in the fall (Figure 9).

As a group, buteos and accipiters dominate raptor counts in both fall and spring. While the proportion of accipiters is similar in both seasons, we see higher proportions of Ospreys, Turkey Vultures, and eagles and lower proportions of buteos and falcons in the spring (Figure 10).

As a group, buteos and accipiters dominate raptor counts in both fall and spring. While the proportion of accipiters is similar in both seasons, we see higher proportions of Ospreys, Turkey Vultures, and eagles and lower proportions of buteos and falcons in the spring (Figure 10).

We counted 2,238 raptors of 17 species between 18 March and 15 May. Our five most abundant species included: Red-tailed Hawk (477), Sharp-shinned Hawk (318), Turkey Vulture (223), Rough-legged Hawk (198), and Cooper’s Hawk (186). Our peak count day was 25 April, though we observed high day-to-day variation in raptor numbers throughout the season. We also observed obvious temporal trends in migration by species group, with a high proportion of eagles and buteos migrating early in the season, and a high proportion of accipiters migrating late in the season. Most raptors passed by our count sites between 1100 and 1400. Proportionally, we saw similar numbers of eagles, buteos, falcons, and Northern Harriers as in the spring of 2012, but fewer Osprey and more Turkey Vultures. Our total was over twice the number observed at Dinosaur Ridge, Colorado, the only other known spring migration count conducted in the West. As in years past, we observed more raptors migrating during the spring of 2013 than in the fall of 2012. In both seasons, Red-Tailed Hawks and Sharp-shinned Hawks were the top two species counted. Our trapping efforts proved less productive than we hoped. Though we were able to band three raptors, the percentage of passing migrants we attracted to our banding station was too small to justify extended efforts.

Appendix A: Daily count summaries for 2011 fall migration at the MPG Ranch.

Appendix A: Daily count summaries for 2011 fall migration at the MPG Ranch.

Appendix A (cont): Turkey Vulture (TV), Osprey (OS), ), Bald Eagle (BE), Northern Harrier (NH), Sharp-shinned Hawk (SS), Cooper’s Hawk (CH), Northern Goshawk (NG), Broad-winged Hawk (BW), Swainson’s Hawk (SH), Red-tailed Hawk (RT), Ferruginous Hawk (FH), Rough-legged Hawk (RL), Golden Eagle (GE), American Kestrel (AK), Merlin (ML), Peregrine Falcon (PG), Prairie Falcon (PR), Unidentified Accipiter (UA), Unidentified Buteo (UB), Unidentified Eagle (UE), Unidentified Falcon (UF), Unidentified Raptor spp. (UU). Date Hours TV OS BE NH SS CH NG BW SH RT FH RL GE AK ML PG PR UA UB UE UF UU Total 5/1/2013 7 8 5 0 2 11 4 0 0 2 8 0 5 7 6 0 0 0 1 1 0 0 2 62 5/2/2013 7.5 6 4 0 9 15 8 0 0 3 10 0 8 1 9 0 0 0 2 0 0 0 1 76 5/3/2013 5.75 0 2 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0 1 10 5/4/2013 6 2 1 0 2 2 1 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 10 5/5/2013 6 5 0 0 2 4 1 0 1 0 4 1 1 2 1 0 0 0 0 0 0 0 0 22 5/6/2013 6.75 9 3 0 6 35 7 0 3 2 12 0 2 2 11 0 0 0 2 3 0 0 2 99 5/7/2013 6.75 5 0 0 10 17 6 0 2 3 9 0 1 1 7 0 0 0 7 4 0 0 1 73 5/8/2013 6.5 5 3 0 4 21 10 0 1 0 10 0 0 2 4 0 0 0 4 1 0 0 0 65 5/9/2013 6.75 10 0 1 2 21 9 0 0 2 8 0 1 0 2 0 0 0 7 0 0 1 0 64 5/10/2013 6.3 0 3 0 2 5 5 0 0 0 0 1 0 0 0 0 0 0 3 1 0 0 1 21 5/11/2013 7 2 2 0 4 14 5 1 0 2 3 0 0 0 0 0 0 0 3 1 0 0 0 37 5/12/2013 6.75 9 2 0 1 3 3 1 0 4 4 0 0 0 0 0 0 0 7 1 0 0 0 35 5/13/2013 6.5 5 0 0 3 5 6 0 0 0 7 0 0 0 0 0 0 0 1 1 0 0 0 28 5/14/2013 6.5 1 0 0 0 3 0 0 0 0 2 0 0 0 0 0 0 0 1 0 0 0 0 7 5/15/2013 5.5 7 1 0 0 2 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 12 May Sub. 97.6 74 26 1 47 162 66 2 7 21 78 2 19 15 41 0 0 0 38 13 0 1 8 621 Season Totals TV OS BE NH SS CH NG BW SH RT FH RL GE AK ML PG PR UA UB UE UF UU Total 223 85 51 149 318 186 11 11 36 477 7 198 126 156 9 6 6 72 58 4 2 47 2238

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