Environmental Characteristics and Functional Significance of Pronghorn Fawn Bedding Sites in Alberta

1. What they did

By 1969, pronghorn numbers in Alberta had fallen to a 15-year low of about 7,000 animals, and few young were being added to the population even though birth rates were high. For their first 3 weeks, pronghorn fawns spend about 90% of their time lying bedded, apart from their mothers. The author asked what kinds of places newborn fawns bed in across the grassland range of southeastern Alberta, and whether fawns in some kinds of places survived better than others.

• Finding and catching fawns (1971 to 1976). Most fawns (78%) were found by a systematic “blind search” around places where mothers had been seen. The rest were watched with their mother and allowed to bed down before the search began. After finding one fawn, the team searched for a twin. Fawns were caught in their beds with a long-handled net, aged by size, behaviour, coat and umbilicus, sexed, weighed, measured and had their rectal temperature taken.
• Describing the bed sites. At each bed the team recorded the topography, the vegetation (sagebrush density along a 60-m line), the substrate, the direction the slope faced, the fawn’s position next to any landmark, and whether the exact spot gave more concealment than a random spot nearby would have. They measured the distance to the nearest sagebrush plant, to heavy cover and to water, and took temperatures and wind speeds at the bed and 150 cm above it.
• Following survival. Before release at their beds, 370 fawns were fitted with colour-coded expandable neck collars and 62 with radio transmitters. Over the 6 years, more than 1,500 person-days were spent in the field, with more than 50 days a year spent looking for marked animals; aircraft were used extensively. A fawn seen alive 2 weeks or more after marking counted as “relocated”, which the author used as an index of survival.

2. What they found

The fawns

• 465 fawns were caught, with an estimated mean age of 2.8 days (4 hours to 10 days). Males made up 57%. Mean weight was 4.2 kg.
• Twins bedded a mean of 48 m apart: within 5 m of each other 26% of the time, and more than 80 m apart 19% of the time. When close together, they usually faced different directions.
• About 75% of mothers gave birth between 23 and 30 May, and more than 90% had given birth by 1 June.

Where fawns bedded

• Most fawns bedded on slopes or the crests of hills; 31% were on flat ground. There was no favoured direction of exposure.
• 28 of 167 beds were in small depressions, 1 to 3 m across and 10 to 30 cm deep, made by water, wind or livestock. These were often deep enough to hide a bedded fawn from side view.
• Most beds were in native grassland with sparse or light sagebrush, the commonest habitat in the area. 14% were on cultivated land. The mean distance to the nearest sagebrush plant was 102 m, to heavy cover 326 m, and to water 586 m.
• Only 30% of fawns were concealed and 3% partly concealed; 67% were not. Concealed fawns were mostly in sagebrush (55%), then depressions (18%) and tall grass or forbs (13%); others lay beside rocks, tyre ruts, cactus clumps or cow dung. For most fawns, their colouring was their main camouflage. Fawns often bedded with their backs against an object, and faced downhill on slopes.
• Grass was the main bed surface in 62% of cases and bare soil in 36%.

Microclimate

• Mean temperature at ground level in the bed was 20.3 °C, 18.8% higher than at 150 cm above it.
• Mean wind speed at fawn height (10 cm) was 6.4 km/hour, 59.5% less than at 150 cm (15.8 km/hour).
• A fawn’s body temperature rose with air temperature. In two cases, short spells of rain or standing in a slough lowered a fawn’s body temperature by about 2 to 4 °C (1.9 to 4.0 °C).

Bed sites and survival

• 127 marked fawns were seen alive at least once after their first 14 days, a minimum of 29.4% of marked fawns.
• Concealed or partly concealed fawns were relocated 33.3% of the time, compared with 19.3% for fawns in the open.
• Fawns first found in small depressions were relocated more often (46%) than average, and those on the middle third of slopes less often (7%).
• Fawns first found on native range were relocated 2.36 times as often as those on cultivated land (12%). Relocation was highest in native range with heavier cover (33%).
• Topography in general, exposure and bed surface made no difference.
• There was no evidence of traditional fawning areas: mothers’ distribution in late May shifted from year to year.

3. What they concluded

• Unlike fawns in sagebrush country in Montana and Idaho, fawns in Alberta’s grassland did not depend on sagebrush for bedding. Many seemed instead to choose spots with a vertical feature and a small hollow, the same cues described for gazelle fawns, using depressions, bare patches, wheel ruts, rocks, cow dung or small clumps of plants.
• Beds were warmer and much less windy than the air above them. Cold, wet weather probably kills some fawns in Alberta, but this was not confirmed.
• The hypothesis that bed-site features do not affect survival was rejected: fawns in depressions, in native range and in heavier cover survived better, since undisturbed fawns usually bed with their heads up, and depressions or heavy cover may be especially good at hiding them from predators.
• The lack of traditional fawning areas may reflect plenty of suitable habitat and low pronghorn density.
• More research on fawn bed sites and fawn mortality in different vegetation types is needed across the pronghorn range.

4. How tracking was or was not used

Tracks were not used to find fawns: most were found by systematic searches around mothers, and the rest by watching a mother until her fawn lay down. The study is built on reading the bed itself: where it lay in the landscape, what it was made of (grass or bare soil), the size and origin of the depressions fawns used, which objects fawns lay against, and which way they faced. Marked fawns were later found again by sight, radio and aircraft, not by trailing.

5. Tracker or observer certification

Not reported. Project personnel judged whether each bed gave more concealment than a random spot would have; how they were trained or checked is not described.

6. Other technologies, skills or innovations

• Colour-coded expandable neck collars (7.5 cm wide) and radio transmitters on fawns.
• Aircraft used extensively to relocate marked animals.
• Hand-held thermometer and anemometer for temperature and wind at the bed, 10 cm above it and 150 cm above it.
• Analysis on an Amdahl 470 V/6 computer with the SPSS statistical package.

7. Implications for conservation (from the paper)

• Sagebrush rangeland, although limited in the area, gave the highest fawn survival and should be kept wherever possible.
• Large cultivated tracts on important pronghorn range should be avoided.
• Small depressions, stands of grasses or forbs over 25 cm tall and small areas of bare ground are important bedding cover and should be kept and included in habitat development or mitigation programmes.
• Preferred fawning grounds should be judged by their habitat features, not only by geographic area.