Birth and Fawn Bed Site Selection by Pronghorns in a Sagebrush-Steppe Community

1. What they did

Pronghorns are a “hider” species: for their first three weeks, fawns spend up to 90% of their time lying hidden while their mothers are away. The authors asked what kind of shrub cover mothers choose for giving birth and fawns choose for bedding, in a sagebrush-steppe community in southcentral Wyoming.

• Birth and bed sites versus low use areas (June 1982 and 1983). They measured shrub canopy cover and height at 13 birth sites and 50 fawn bed sites, and at 47 random points in areas 10 to 30 km away that few mothers used. At each site they measured every shrub crossing five 10-m lines spreading out from the site. The fawns were not marked, so some bed sites may have belonged to the same fawn.
• Bed sites versus each fawn’s own home range (June 1984). They caught 7 newborn fawns (4 males and 3 females, including 2 siblings) with a hand-held net and fitted each with a solar-powered radio transmitter on an ear tag. They found the fawns each day for 3 weeks, without flushing them. One to 5 days after finding a bedded fawn, they returned to the exact bed and measured cover along four 5-m lines, metre by metre, comparing 72 bed sites with 228 random points inside the fawns’ home ranges.

2. What they found

Birth and bed sites compared with low use areas

• Birth sites and fawn bed sites had similar shrub cover (12.4% and 13.1% on the 10-m lines), and both had more cover than random points in low use areas (9.8%).
• The difference came from big sagebrush. Douglas rabbitbrush and winterfat did not differ.
• Plant heights did not differ between birth sites, bed sites and random points. Big sagebrush was the tallest plant measured, averaging 14.1 cm.

Bed sites compared with the fawns’ home ranges

• Fawns had more shrub cover within the first metre of their beds than at random points in their home range, for 6 of the 7 fawns.
• All fawns chose similar cover within that first metre. Further out, cover differed between fawns, which the authors think reflects the surrounding vegetation rather than the fawns’ choice.
• The same held for cover taller than 10 cm, the authors’ estimate of the height of a bedded fawn.

Behaviour

• Mothers giving birth were almost always alert. They never used the patches of dense, tall (over 1.5 m) big sagebrush and rubber rabbitbrush, which grew mostly in draws and washes, places predators often visit.
• No mother was seen choosing a bed site for her fawn. After nursing, fawns moved away from their mothers and chose their own beds.
• The radio-tagged fawns never used the same bed twice.

3. What they concluded

• Mothers giving birth, who rely on sight and speed to defend against predators, seem to choose places where cover hides the newborn but does not block their view.
• Fawns chose beds with more shrub cover than was generally around them, but they too avoided the tallest, densest stands.
• Birth and bed sites may be similar because fawns “imprint” on the vegetation where they were born, because mothers influence the general area, or because fawns seek concealment and thermal cover while keeping a view of their surroundings.
• Mothers probably influence only the general area where they leave their fawns; the fawn picks the actual bed.
• Choosing the right cover helps a small newborn hide from predators until they are big enough to run from them.

4. How tracking was or was not used

The radio-tagged fawns were found by radio and seen each day. To measure each bed, the researchers came back 1 to 5 days later and found the exact spot from marked locations, compass bearings and the depression the fawn had left in the sandy soil. The paper does not say how the birth sites and bed sites of the unmarked fawns in 1982 and 1983 were found, apart from mothers being watched while giving birth.

5. Tracker or observer certification

Not reported. The paper does not say who relocated the beds or how the bed depressions were recognised.

6. Other technologies, skills or innovations

• Solar-powered radio transmitters (32 g) fixed to an ear tag.
• Line intercept method for shrub canopy cover, measured in 1-m steps outward from the bed.
• Multi-Response Permutation Procedure to compare bed sites with random points.