seasonal foraging strategies of migrant and non- migrant ... - BioOne

NORTHWESTERN NATURALIST

98:82–90

AUTUMN 2017

SEASONAL FORAGING STRATEGIES OF MIGRANT AND NONMIGRANT PRONGHORN IN YELLOWSTONE NATIONAL PARK KEREY K BARNOWE-MEYER Nez Perce Tribe, Wildlife Division, Lapwai, ID 83540 USA; [email protected]

PJ WHITE, TROY L DAVIS, JOHN J TREANOR National Park Service, Yellowstone National Park, Mammoth, WY 82190 USA

JOHN A BYERS Department of Biological Sciences, University of Idaho, Moscow, ID 83844 USA

ABSTRACT—We characterized the seasonal composition and quality of migrant and non-migrant Pronghorn (Antilocapra Americana) diets in Yellowstone National Park during 2006–2007. During winter (January–April), when migrants and non-migrants occupied the same winter range, the overall percent relative density for each forage class in Pronghorn diets (n ¼ 51 composite fecal samples) was 67 þ 6% (standard error) shrubs, 17 þ 3% forbs, 13 þ 3% grasses, and 3 þ 1% other. However, spring and summer diets differed for migrants and non-migrants. Diets of migrants (n ¼ 34) to higher-elevation ranges with higher precipitation and forage quality during May–August were dominated by 68 þ 2% forbs, whereas summer diets of non-migrants (n ¼ 21) remaining on the winter range were co-dominated by 48 þ 2% forbs and 42 þ 1% shrubs. Diet quality for migrant Pronghorn, as indexed by fecal nitrogen and DAPA, was also generally higher than for nonmigrants during a period when the demands of late gestation and lactation were high. These results suggest that improved perinatal condition among fawns born to migrant females in Yellowstone National Park may be driven by higher-quality forage conditions in migrant areas, bolstering conclusions from previous studies that migration represents an adaptive strategy in this population given current conditions in the Park. Key words: Antilocapra americana, diet composition, diet quality, foraging, migration, Pronghorn, Yellowstone National Park

to 130 km down the Yellowstone River valley from higher-elevation summer ranges on and surrounding the northern Yellowstone Plateau to lower-elevation winter ranges in south-central Montana. However, Euro-American colonization and settlement reduced Pronghorn numbers and effectively eliminated their migration outside the park sometime before 1920. Portions of this migratory route have been reestablished over the past decade through efforts by the National Parks Conservation Association, working with landowners to remove and modify fences in critical bottlenecks (Skinner 1922; White and others 2007; Barnowe-Meyer and others 2013). Nonetheless, the Yellowstone Pronghorn population is now almost entirely restricted to a relatively small winter range along Yellowstone

The rolling grasslands and shrub-steppe communities occupied by Pronghorn (Antilocapra americana) are often strongly influenced by plant phenology and abiotic conditions (Yoakum 2004a). Pronghorn selectively feed on a variety of forbs and shrubs, with grasses and grass-like plants (graminoids) typically comprising a minor portion of their diet (Yoakum 2004b). Seasonal conditions force many Pronghorn populations to remain in relatively low-elevation, windswept areas during winter where snow is less deep and food is more readily available (Yoakum 2004a). The Yellowstone Pronghorn population was once numerous (1000 to 1500 animals) in the shrub-steppe and mixed-forest habitats of what is now Yellowstone National Park (Skinner 1922). During winter, individuals migrated 80 82

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YELLOWSTONE PRONGHORN FORAGING

National Park’s northern boundary adjacent to Gardiner, Montana. During summer, the Yellowstone Pronghorn population is partially migratory, with .70% of the individuals migrating 15–50 km to spend May–October at higher elevations (White and others 2007). The spring migration typically occurs in April when snow begins to melt and new vegetation starts to grow at higher elevations, allowing migrants to track plant phenology across a range of elevations (BarnoweMeyer and others 2011). Despite this seasonal access, a small proportion of Yellowstone Pronghorn (,30%) remain year-round on the winter range near Gardiner, Montana. Eighty percent (n ¼ 44) of radio-collared females monitored during 1999–2005 showed fidelity across years to their migration strategy and summer-use area (White and others 2007). The adequacy of available forage to meet metabolic demands, in terms of quality and quantity, is reflected in the rates of ingestion and digestion of energy and nutrients (nutrition) and the resulting state of body condition (such as fat and protein; Harder and Kirkpatrick 1994; Parker and others 1999). In turn, body condition strongly influences the probability of breeding, overwinter survival, fawn survival and recruitment, and vulnerability to predation (Cook 2002). Higher-elevation ranges receive more precipitation and likely support higher-quality forage during the growing season (Despain 1990; Farnes and others 1999). As a result, migratory Pronghorn could have access to more-nutritious new vegetation growth at a critical time of year when the demands of lactation are high, thereby enhancing milk production and quality and, in turn, reproductive success. Previous research has found that migrant Yellowstone Pronghorn have higher reproductive success than non-migrants, despite occupying areas with abundant and diverse predator communities (Barnowe-Meyer and others 2009, 2010). We characterized the composition and quality of Yellowstone Pronghorn diets during summer and winter, and evaluated the foraging strategies of migrant and non-migrant Pronghorn across seasons. We predicted spring and summer diets would be of higher quality in the higherelevation mountain areas accessed by migrants than the lower-elevation valley winter range used by non-migrants during this period.

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METHODS Study Area Yellowstone Pronghorn occupy foothills, mountain slopes, and valley bottoms along the Gardiner, Lamar, and Yellowstone Rivers in the northern portion of Yellowstone National Park, Wyoming, and adjacent areas of Montana (Fig. 1; White and others 2007; Boccadori and others 2008). Summers are generally short and cool, and winters are long and cold, resulting in a mean annual temperature of 1.88C. Mean annual precipitation varies from about 25 cm in the Gardiner basin (approximately 1615 m elevation), where migrant and non-migrant Pronghorn share a winter range, to about 35 cm in the mountain summer ranges (approximately 2135 m elevation) used by migrants. Average snowwater equivalents (amount of water in snow) range from about 2 to 30 cm along this elevation gradient (Farnes and others 1999). However, drought conditions persisted in northwestern Wyoming during 1999–2007 (National Oceanic and Atmospheric Administration 2015). The vegetation and plant composition of the winter range for Yellowstone Pronghorn consists primarily of open grassland-sagebrush steppe with interspersed upland grasslands, wet meadows, old agricultural fields and pastures, alfalfa fields and livestock pastures on private land outside of the Park, and non-vegetated areas (Boccadori and others 2008). The vegetation and plant composition of summering areas used by migrants in the park consists of mixed grassland, shrub-steppe, and forest types, with minor patches of riparian habitat (Despain 1990; Barnowe-Meyer and others 2011). Diet Composition We estimated the botanical composition of Pronghorn diets using microscopic examination of plant fragments in fresh fecal material (Sparks and Malachek 1968) collected during January– August 2006 and January–April 2007. We observed groups of individuals, noted defecation events, then collected fecal samples once the group had left the area. We collected samples generally within 30 min of defecation from both male and female individuals, though males were likely under-sampled relative to females. We then composited samples of 7 to 10 pellets each from 3 to 4 adult Pronghorn by month and area.

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FIGURE 1. The annual range of Pronghorn in and adjacent to Yellowstone National Park, Montana and Wyoming.

We sampled the winter range and main summering areas for Yellowstone Pronghorn. The latter included: (1) the Gardiner basin and northwestern portion of Mount Everts; (2) Blacktail Deer Plateau, including Blacktail ponds and Oxbow/Geode creeks; (3) Specimen Ridge, including Crystal Bench and Little America; and (4) Lamar Valley, including the Soda Butte area (Fig. 1; White and others 2007). Staff at the Wildlife Habitat and Nutrition Laboratory, Washington State University, Pullman, Washington, oven-dried (708C) and ground (1-mm size) each composite before preparing 4 slides from each composite and examining 25 views per slide for a total of 100 views per composite sample. They identified each forage class (such as shrubs, forbs, grasses, sedge-rushes, and conifers) and all plants .5% of the diet to at least the genus level using epidermal-cell-tissue fragments. We calculated monthly means per area as percent relative density based on the average of all composites collected per month. Highly digestible species may be underestimated because we did not adjust results for

differential digestibility (Sparks and Malachek 1968; Striby and others 1987). Forage Quality During January–April 2007, we collected and froze (–178C) 2 to 3 replicate samples per month of edible portions of major forage plants used by Yellowstone Pronghorn on their winter range (Boccadori 2002). Sampled plants included the following forage types and genera: shrubs (Artemisia, Chrysothamnus, Eriogonum, Krascheninnikovia, Salix, Sarcobatus); forbs (Alyssum, Atriplex, Phlox); grasses (Agropyron, Bromus, Oryzopsis, Poa); sedges-rushes (Carex); conifers (Juniperus); and lichens. Replicate samples (1 g oven-dried at 708C) were sent to the Wildlife Habitat and Nutrition Laboratory for freeze drying, grinding, and estimation of percent crude protein (% nitrogen times 6.25), percent in vitro dry-matter digestibility, and gross energy (cals g1). To estimate the quality of winter diets, we calculated the sum of products of forage-quality values (percent crude protein, %CP; percent in

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TABLE 1. Percent relative density of each forage class in Yellowstone Pronghorn diets during January–April in 2006 and 2007, based on microscopic examination of 51 composite fecal samples. Plant/Forage

Jan 2006

Feb 2006

Mar 2006

Jan 2007

Feb 2007

Mar 2007

Apr 2007

TOTAL SHRUBS Artemisia tridentata Artemisia frigida Chrysothamnus Krascheninnikovia Salix Minor shrubs TOTAL FORBS Alyssum Atriplex Descurainia Phlox/Leptodactylon Minor forbs Unknown forb TOTAL GRASSES Agropyron Bromus Koeleria Oryzopsis Poa Minor grasses TOTAL SEDGE/RUSH TOTAL CONIFERS Juniperus Pseudotsuga Minor conifers TOTAL LICHEN/MOSS/THORN

74.4 44.4 9.7 8.8 10.1 0.0 1.3 12.7 1.7 5.9 0.0 2.8 2.3 0.0 0.5 1.9 1.9 0.0 0.0 4.7 1.9 0.2 2.3 1.3 0.0 1.0 0.0

73.5 65.6 3.1 2.0 0.3 0.3 2.2 16.3 7.4 4.5 0.0 1.6 2.0 0.7 7.2 0.6 1.0 0.0 0.0 1.8 3.7 0.2 1.7 1.1 0.0 0.6 1.2

66.4 60.2 1.7 2.2 0.0 0.0 2.3 14.1 5.9 4.9 0.0 0.7 1.9 0.9 15.7 3.3 3.2 0.0 0.5 5.5 3.2 0.0 2.9 1.1 1.3 0.5 0.9

75.1 61.6 5.4 5.2 0.9 0.0 1.9 16.4 0.3 11.9 0.0 1.7 1.9 0.5 2.2 0.0 0.0 0.0 0.0 0.0 2.2 0.1 5.8 4.8 0.0 0.9 0.6

75.4 58.3 10.1 4.6 1.6 0.0 0.9 12.0 0.0 5.8 0.0 1.2 3.9 1.1 8.1 2.1 0.5 0.0 0.0 1.8 3.7 0.1 3.7 2.6 0.0 1.0 0.8

68.9 56.2 5.2 7.4 0.0 0.0 0.2 11.4 6.9 0.0 0.0 0.0 3.5 1.0 17.9 7.5 3.8 0.0 0.0 4.6 2.2 0.4 1.0 0.0 0.0 1.0 0.6

34.1 21.0 9.7 1.7 0.0 0.0 1.6 35.5 21.8 0.0 6.5 0.0 5.5 1.7 29.3 10.7 3.8 1.1 0.0 9.8 3.9 0.3 0.0 0.0 0.0 0.0 0.8

vitro dry-matter digestibility, %IVDMD; and gross energy, GE) of individual forage types and their relative composition in diets. This approach incorporates biases from fecal diet composition estimates and likely underestimates variance due to analyzing the product of diet composition and forage quality (Hobbs and others 1979; Parker and others 2005). In addition, staff from the Wildlife Habitat and Nutrition Laboratory measured fecal nitrogen (%FN) and 2, 6 diaminopimelic acid (DAPA; mg g1) from composite samples of pellets collected during January–August 2006 and January–April 2007. Fecal nitrogen reflects dietary nitrogen and should lead to an increase in fecal DAPA, which is an amino acid residue in the cell walls of rumen bacteria. As digestible energy in the rumen increases, bacteria loads increase, causing more DAPA to be excreted in feces (Mould and Robbins 1981; Davitt and Nelson 1984). RESULTS Winter Diets The overall percent relative density for each forage class in Pronghorn diets during January–

April in 2006 and 2007 based on microscopic examination of 51 composite fecal samples was 67 þ 6% (SE) shrubs, 17 þ 3% forbs, 13 þ 3% grasses, and 3 þ 1% other (sedge, lichen, juniper; Table 1). Pronghorn diets during January through March were dominated by sagebrush (Artemisia; ¼ 64 þ 3%), with Big Sagebrush (Artemisia tridentata) making up an average of 58 þ 3% of diets. Coinciding with new vegetation growth in spring, however, the proportion of shrubs in April diets decreased to 34%, with sagebrush making up 31% of April diets. Forbs increased to 36% of April diets, with grasses accounting for another 30%. Mean percent crude protein was similar during January and February for shrubs (¼ 10.0 þ 0.5%) and forbs (¼ 10.2 þ 0.3%), but increased substantially by April to 16.7% for shrubs and 15.1% for forbs (Table 2). Mean percent in vitro dry-matter digestibility increased from January to April for shrubs (38.0 to 53.2), forbs (44.0 to 65.6), and grasses (42.5 to 56.4; Table 2). Mean gross energy (cals g1) was similar from January to April for shrubs (¼ 4902 þ 18.9 cals g1), forbs (¼ 4554 þ 32.5 cals g1), and grasses (¼ 4576 þ 47.5 cals g1; Table 2).


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TABLE 2. Crude protein, in vitro dry matter digestibility, and gross energy of potential forage plants on the winter range for Yellowstone Pronghorn during January–April 2007. Crude protein (%) Plant/Forage

Jan

Feb Mar Apr

AVERAGE SHRUBS Artemisia tridentata Artemisia frigida Chrysothamnus Krascheninnikovia Sarcobatus* Salix* Eriogonum* AVERAGE FORBS Alyssum Atriplex Phlox/Leptodactylon AVERAGE GRASSES Agropyron Bromus Oryzopsis Poa AVERAGE SEDGE/RUSHES AVERAGE CONIFER (JUNIPERUS) AVERAGE LICHEN/MOSS/THORN

9.5 11.1 10.6 8.7 11.1 13.7 6.5 5.1 10.5 7.7 13.1 10.8 9.9 4.2 17.3

12.2 14.1 13.4 8.8 21.5 14.6 7.3 5.9 14.5 19.8 12.8 10.8 8.7 6.5 18.5 3.8 5.8

10.5 12.2 9.1 9.8 17.3 14.5 7.2 3.4 9.9 11.8 9.5 8.4 8.9 6.1 19.5 4.3 8.1 5.7 11.6 8.6 9.1 7.0 9.1

16.7 20.4 15.8 9.2 29.4 25.7 8.1 8.4 15.1 16.2 19.0 10.1 16.2 13.7 18.9 13.2 19.0 16.6 8.9 9.4 7.5 6.7

Dry-matter digestibility (%)

Gross energy (cals g1)

Jan

Feb

Mar

Apr

Jan

Feb

Mar

Apr

38.0 49.9 39.1 44.5 35.4 42.4 28.9 25.6 44.0 51.3 45.0 35.6 42.5 37.8 48.8 31.6 51.9 42.6 46.8 33.4

42.0 57.1 37.8 49.5 39.8 42.2 40.6 27.3 44.7 49.6 44.9 39.5 38.4 43.1 38.1 28.1 44.1 45.7 49.8 34.3

45.1 62.5 45.1 51.3 51.5 41.6 38.0 25.9 54.7 52.8 59.1 52.0 43.8 48.6 50.4 39.0 37.3 52.0 43.9 40.9

53.2 77.5 48.7 51.1 63.9 61.6 37.6 32.4 65.6 72.2 68.5 56.2 56.4 60.2 52.7 48.9 64.1 78.9 49.2 36.4

4913 5137 4920 5464 4796 4857 4917 4298 4651

4921 5185 4908 5454 4943 4810 5035 4112 4513

4927 5221 4727 5485 4758 4855 5016 4430 4524

4846 5123 4635 5429 4596 4693 4915 4528 4527

4719 4584 4648 4648

4592 4434 4576 4554

4375 4673 4639 4639

4457 4597 4442 4468 4264 4595

4597 5830 4211

5747 4301

5875 4297

5725 4262

* Only trace amounts were detected (Table 1).

Winter diets contained mean proportions of 11.7 þ 0.7% crude protein and 49.8 þ 1.3% digestible dry matter from January through March. Both crude protein and dry-matter digestibility varied significantly by month, with April values significantly greater than in January, February, and March (pairwise comparisons following one-way ANOVA, P 0.018 and , 0.001, respectively). Coinciding with new vegetation growth in spring, these proportions increased in April to 16.8% crude protein and 65.6% digestible dry matter. Gross energy in Pronghorn diets stayed relatively constant from January through March (5086–5030 cals g1), but then decreased substantially to 4674 cals g1 during April.

Composite fecal samples contained mean proportions of 1.74 þ 0.02% nitrogen and 0.51 þ 0.01 mg/g DAPA during January–March 2006 and 2007 (Table 3; Fig. 2). Fecal nitrogen did not vary by month during the winters of 2006 or 2007 (one-way MANOVA, P 0.146). DAPA did not vary by month during the winter of 2006 (one-way MANOVA, P ¼ 0.332), but did vary during the winter of 2007 (one-way MANOVA, P ¼ 0.005), rising from 0.46 mg g1 during January to 0.53–0.55 mg g1 during February– March. Coinciding with new vegetation growth in the spring of 2007, overall values increased during April to 2.18 þ 0.16% fecal nitrogen and 0.59 þ 0.07 mg g1 DAPA (Fig. 2).

TABLE 3. Fecal nitrogen (%) and 2,6 diaminopimelic acid (DAPA; mg g1) from composite samples of pellets collected from Yellowstone Pronghorn during January–August 2006 and January–April 2007. 2006 Nutrition Index FECAL NITROGEN Mean SE DAPA Mean SE n

2007

Jan

Feb

Mar

Jun

Jul

Aug

Jan

Feb

Mar

Apr

1.71 0.07

1.80 0.05

1.77 0.06

3.56 0.25

2.62 0.06

2.69 0.09

1.65 0.05

1.73 0.03

1.87 0.11

2.18 0.16

0.55 0.04 5

0.51 0.02 13

0.55 0.03 6

0.73 0.05 5

0.52 0.01 35

0.46 0.02 11

0.46 0.01 13

0.55 0.02 6

0.53 0.00 2

0.59 0.07 6

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FIGURE 2. Variation in fecal nitrogen (%) and 2,6 diaminopimelic acid (DAPA; mg g1) by month and migratory strategy from composite samples of pellets collected from migratory and non-migratory Yellowstone Pronghorn in 2006 and 2007.

Summer Diets Based on microscopic examination of 55 composite fecal samples, the overall percent relative density for each forage class in Pronghorn diets during May–August in 2006 was 58 þ 5% forbs, 31 þ 5% shrubs, 8 þ 0.8% grasses, and 3 þ 0.4% other (sedge, lichen; Table 4). Diets of non-migratory Pronghorn that remained on the Gardiner basin winter range through the summer were composed primarily of 42 þ 1% shrubs

and 48 þ 2% forbs, with 8 þ 1% grasses, and 2 þ 0.2% other (Table 4). In contrast, diets of migratory Pronghorn were dominated by 68 þ 2% forbs, with 20 þ 3% shrubs, 8 þ 1% grasses, and 4 þ 0.5% other. There was wide variation in diet composition for migrant Pronghorn using different summering areas (Table 4). Composite fecal samples contained mean proportions of 2.8 þ 0.07% nitrogen and 54 þ 0.02 mg g1 DAPA during May-June through

TABLE 4. Percent relative density of each forage class in migrant and non-migrant Yellowstone Pronghorn diets in each of the main summering areas during May–August 2006. Results are based on microscopic examination of 55 composite fecal samples. Main summering areas (Gardiner Basin [also the winter range], Blacktail Deer Plateau, Specimen Ridge, and Lamar Valley) are described in detail in the text and appear in Figure 1. Non-migrant diets

Migrant diets

Diets in main summering areas

Plant/Forage

May

July

Aug

Jun

July Aug Gardiner Blacktail Specimen Lamar

TOTAL SHRUBS Artemisia tridentata Artemisia frigida Chrysothamnus Other shrubs each ,5% TOTAL FORBS Achillea Alyssum Erodium/Geranium Erysimum/Lithospermum Helianthus/Rudbeckia/Solidago Lomatium Lupinus Other forbs each ,5% TOTAL GRAMINOIDS TOTAL OTHER

39.9 17.7 20.7 0.0 1.5 52.8 1.5 6.5 0.0 4.5 0.0 6.4 1.5 32.4 7.0 0.3

43.1 3.9 35.0 1.2 3.0 48.4 0.5 1.1 1.9 2.8 1.5 6.4 1.0 33.2 7.2 1.3

43.1 2.7 31.5 5.5 3.4 44.3 1.1 0.0 0.0 2.4 2.5 4.9 12.5 20.9 11.4 1.2

14.8 10.7 2.3 0.9 0.9 71.4 9.7 0.0 6.2 1.5 1.2 2.7 5.9 44.2 13.7 0.1

23.6 4.1 13.0 1.8 4.7 67.5 0.3 0.0 15.7 3.8 2.6 3.2 4.9 37.0 7.7 1.2

22.9 10.7 4.7 3.9 3.6 65.8 3.4 0.0 5.5 0.0 8.4 4.8 3.9 39.8 10.6 0.7

43.1 3.9 35.0 1.2 3.0 48.4 0.5 1.1 1.9 2.8 1.5 6.4 1.0 33.2 7.2 1.3

17.0 3.1 9.4 2.6 1.9 74.2 0.5 0.0 18.3 1.3 2.6 1.3 4.4 45.8 8.2 0.6

28.4 4.6 16.2 1.5 6.1 63.0 0.3 0.0 12.7 5.5 2.1 3.9 5.4 33.1 7.0 1.6

9.8 3.2 3.0 1.5 2.1 77.4 0.0 0.0 26.8 0.0 6.6 4.5 2.7 36.8 9.6 3.2


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TABLE 5. Fecal nitrogen (%) and 2,6 diaminopimelic acid (DAPA; mg g1) from composite samples of pellets collected from migrant and non-migrant Yellowstone Pronghorn during May–August 2006. Non-migrants Nutrition Index FECAL NITROGEN Mean SE DAPA Mean SE n

Migrants

May

July

August

June

July

August

2.84 0.15

2.37 0.06

2.53 0.10

4.14 0.16

2.80 0.06

2.75 0.11

0.65 0.07 4

0.56 0.01 14

0.43 0.06 3

0.80 0.05 5

0.49 0.02 21

0.47 0.02 8

August, with these indices of diet quality peaking in May–June (Table 3; Fig. 2). Fecal nitrogen varied by month for non-migrants and migrants in 2006 (one-way MANOVA, P 0.005), peaking in May and June, respectively, in both groups (though May was not sampled for migrants, and June was not sampled for nonmigrants; Table 5). Values of DAPA followed a similar pattern (one-way MANOVA, P 0.006). Composite fecal samples from non-migratory Pronghorn from May–June through August contained mean proportions of 2.48 þ 0.06% fecal nitrogen and 0.56 þ 0.02 mg g1 DAPA. Diets of migratory Pronghorn over this period contained mean proportions of 2.98 þ 0.10% fecal nitrogen and 0.53 þ 0.02 mg g1 DAPA. Migrants had statistically higher values of percent fecal nitrogen (one-way ANOVA, P , 0.001), but not DAPA (one-way ANOVA, P ¼ 0.117), than nonmigrants in May–June; higher values of fecal nitrogen in July (one-way ANOVA, P , 0.001); and similar values of fecal nitrogen and DAPA in August (FN: one-way ANOVA, P ¼ 0.319; DAPA: one-way ANOVA, P ¼ 0.373; Table 3; Fig. 2). Interestingly, non-migrants had statistically higher values of DAPA than migrants in July (one-way ANOVA, P ¼ 0.009). DISCUSSION The composition and quality of Yellowstone Pronghorn diets varied seasonally in patterns similar to other locales, with a predominant focus on shrubs in winter, forbs in summer, and a mixture of forbs and shrubs in spring and autumn (see Yoakum 2004b). In autumn and winter, Pronghorn in shrub-steppe habitats often browse on shrubs because they are higher in protein and more available than most forbs or grasses (Yoakum 2004b). Winter is the leanest period of the year, and both migrants and non-

migrants in Yellowstone National Park have similar food supplies because they share a winter range (Boccadori and others 2008). Not surprisingly, the winter diets of these Pronghorn were dominated by shrubs. Though the digestibility of shrubs is relatively low, they have high crude-protein content and are readily available (not covered by snow). Conversely, the availability of forbs, which are fairly high in crude protein and digestibility, is lower in winter due to plant phenology and snow cover. Likewise, grasses are a poor choice during winter because the relatively small rumens in Pronghorn are not well-adapted to handle bulky fibrous grasses. Selective browsing by Yellowstone Pronghorn in winter likely contributed to their over-winter survival because the crude protein content of diets was well above the minimum level (5 to 7%) required for maintenance (Yoakum 2004b). Also, fecal nitrogen and DAPA were similar or higher to levels reported for other Pronghorn populations (Yoakum 2004b). Tracking the protein content and digestibility of new plant growth in spring and summer is a common strategy of wild ruminants (Fryxell and others 1988). As a small ruminant, Pronghorn can benefit from being more selective during the growing season because emerging forbs and grasses are highly nutritious, digestible, and abundant. As expected, Yellowstone Pronghorn transitioned from a shrub-focused diet in winter to a diet focused on young grasses and forbs in spring and summer. Barnowe-Meyer and others (2011) investigated a range of body condition and reproductive metrics for this population from 1999–2001, and noted higher indexed condition at birth and age at death for fawns born to migrant females. This study was partly intended to investigate possible drivers of these patterns. We were not able to

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differentiate the diets of migratory and nonmigratory individuals during winter. Nutritionally, however, late May and June coincide with high reproductive demands during late gestation and lactation. Diets during this period can have a strong influence on the condition and survival of newborn young (Pettorelli and others 2005, 2007). By migrating to higher-elevation areas, migrant females have access to potentially more and higher-quality forage for approximately 6 wk prior to parturition in June (BarnoweMeyer and others 2011). In the spring and summer months of 2006, migrants grazed primarily on forbs, whereas non-migrants selected more evenly among forbs and shrubs. Diet quality for migrant Pronghorn, as indexed by fecal nitrogen, appears to have been substantially higher than for non-migrants during May–June and July, though we urge caution in the interpretation of these results because of sampling limitations during May and June. These results are particularly noteworthy because higher fawn survival rates in migrant areas (Barnowe-Meyer and others 2011) likely correspond to a higher relative abundance of lactating females in these areas, with correspondingly lower fecal nitrogen levels (Monteith and others 2014). DAPA levels for migrants in July were lower than for non-migrants, an unexpected result requiring additional investigation. Nonetheless, DAPA levels for migrants exceeded those for non-migrants in August (though not significantly). Collectively, our results suggest an overall pattern of higher diet quality for migrant Pronghorn relative to nonmigrants. In conjunction with previous research on this population, our results provide further evidence of an adaptive advantage of migration for this population, given current conditions in Yellowstone National Park. Migratory individuals gain access to and make use of high-quality foraging areas within the park, which likely leads to improved perinatal condition among fawns born to females in these areas (Barnowe-Meyer and others 2011). This enhanced condition of migrant fawns appears to contribute, in turn, to improved rates of summer survival and recruitment among migrants and, over time, an increasing proportion of migrants in the Yellowstone Pronghorn population (White and others 2007; Barnowe-Meyer and others 2010). The extent to which migration represents an adap-

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tive strategy in this population is therefore sensitive to changes in the relative quality of spring and summer forage between migrant and non-migrant areas as well as other biotic and abiotic determinants of reproductive success in Yellowstone Pronghorn. ACKNOWLEDGMENTS This project received financial support from the Bernice Barbour Foundation, Montana State University, National Park Service, University of Idaho, and Yellowstone Park Foundation. We thank the Rocky Mountains Cooperative Ecosystem Studies Unit, College of Forestry and Conservation, University of Montana, for facilitating agreements.

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Submitted 22 February 2016, accepted 30 September 2016. Corresponding Editor: Denim Jochimsen.