Bed-Site Characteristics of Pronghorn Fawns

1. What they did

Earlier studies disagree about what makes good fawning habitat for pronghorn: some found fewer fawns died in open country, where mothers and fawns can see predators coming, and others found more fawns survived where cover was dense. The authors set out to describe the bed sites that pronghorn fawns choose in the Trans-Pecos Region of Texas, and to test whether the choice of bed site affected whether fawns survived.

• Marking fawns (1990 to 1992). Over three fawning seasons, from April to June, they caught 101 fawns within 5 days of birth and fitted each with a small ear-tag radio transmitter (32 g) that signalled if the fawn died. To limit disturbance, they wore surgical gloves, masked their scent with creosote leaves, took no body measurements, and handled each fawn for only 1 to 2 minutes.
• Finding bed sites. They radio tracked each fawn once a day and watched it from 20 to 40 m away. Each bed site was flagged and mapped. Capture sites were used only if an undisturbed fawn had chosen them.
• Measuring bed sites. Late in each fawning season, they measured shrub, grass, forb and rock cover, shrub density, plant heights, terrain slope, the slope of the bed itself, and the distance to the nearest concealing cover (a plant or object a fawn could hide against). They also measured visibility with a 240 cm pole: how much could be seen at 25 m and 50 m from the heights of a bedded fawn (0.1 m), a standing fawn (0.5 m) and a standing adult (1.0 m), and whether sight lines were open at about 150 m.
• Comparing. They measured 225 random sites the same way, and compared bed sites with random sites, bed sites of fawns that survived with those of fawns that died, and bed sites of young (5 days or less), middle (6 to 14 days) and old (over 14 days) fawns.

2. What they found

Survival

• Only 12 of the 101 fawns (12%) survived to mid to late August. Of the 89 that died, 83 were killed by predators, 5 went missing and 1 died of other causes.
• They measured 616 bed sites: 61% on slopes, 29% in bottoms and 10% on ridges.

Bed sites compared with random sites

• Fawns bedded where there was less shrub cover (2.8% versus 3.5%) and fewer shrubs (1,299 versus 1,631 per hectare), especially in bottoms and on ridges.
• Bed sites had slightly more grass cover (16.1% versus 15.0%) and less rock cover (11.8% versus 15.4%).
• Fawns bedded much closer to concealing cover: on average 20.3 cm away, against 36.8 cm at random sites. Almost all of this cover was plants: grasses (45%), shrubs and forbs (34%), and cacti and yucca (20%). Fawns tended to bed with their backs to it.
• Beds were flatter than random sites (0.6% versus 1.2% slope), although the surrounding terrain was just as steep.
• From a bedded fawn’s height, bed sites were harder to see into at 25 m and 50 m than random sites. From standing heights, bed sites gave a better view at 50 m and at long range (about 150 m).

Fawns that survived compared with fawns that died

• Survivors’ bed sites had less rock cover (8.8% versus 12.6%), gentler terrain (4.3% versus 5.3%) and flatter beds (0.4% versus 0.6%).
• Their shrub cover, shrub density and grass cover were closer to random sites than to the bed sites of fawns that died.
• Survivors tended to bed nearer concealing cover (15.7 versus 21.5 cm; not quite significant).
• Visibility was the same at both groups’ bed sites.

Fawns of different ages

• Young fawns bedded among more shrubs and less grass than old fawns, and on more rock and steeper beds.
• Middle-aged fawns chose the most concealed beds: visibility from a bedded fawn’s height was lowest at their sites at all three distances.
• Old fawns bedded in the most open sites, and distance to concealing cover increased with age (not significantly).

3. What they concluded

• Fawns chose bed sites that were mostly different from what was available at random. They chose places that hid them while bedded but gave a good view of the surroundings before and after bedding.
• The preference for long-range visibility did not translate directly into better survival, because visibility was the same at the bed sites of survivors and of fawns that died.
• Bed-site choice is tied to avoiding predators and changes as fawns grow. Very young fawns may rely more on their mothers’ choices; middle-aged fawns, still vulnerable but more active, may actively seek better hiding places; older fawns that can outrun predators rely more on seeing them coming.
• The most important features were shrub cover, shrub density, grass cover, nearest concealing cover and bed slope, and to a lesser extent rock cover, terrain slope and plant heights.
• Bed sites vary greatly between regions, so fawns may learn or imprint on the best local combination of features.

4. How tracking was or was not used

The study did not use tracks and signs. Fawns were found with radio telemetry and watched from a distance, and their bed sites were then flagged and measured. The fawn age classes were based on observations made during capture and radio tracking.

6. Other technologies, skills or innovations

• Mortality-sensing ear-tag radio transmitters on newborn fawns, with steps to limit human scent and handling.
• A visibility method using a 240 cm pole read from three heights (bedded fawn, standing fawn, standing adult) at 25 m, 50 m and about 150 m, in four directions.
• Random “artificial bed sites” measured in the same way, to test which features fawns actually selected.

7. Implications for conservation (from the paper)

• For pronghorn fawning areas in the Texas Trans-Pecos, the authors suggest about 3% shrub and cactus cover, about 1,500 shrubs and cacti per hectare, 3% forb cover, 16% grass basal cover, overstory around 60 cm and understory around 14 cm, clumped and taller understory plants to give fawns concealing cover, and slopes of 4 to 5%.
• Livestock stocking rates that allow more grass and forb growth can give fawns cover and mothers forage, without blocking long-range visibility.