Some vegetative cover may be of such density that the open spaces are not large enough for a midsize mammalian predator to slip through and of such stiffness that a predator could not easily bend the plants out of the way. In such cases, a foraging mammal may avoid these areas because the energetic cost of moving through them may exceed the potential gain of searching them for food. I have seen some stands of phragmites that were thick enough to discourage raccoons from searching them thoroughly for food. Likewise, nests surrounded by thorny vegetation, such as blackberries or cacti, may gain some protection from these plants because of the pain they can inflict on a passing predator (Figure 13.1). For instance, red-legged partridge (

Alectoris rufa

) prefer to nest in stands of stinging nettle (

Urtica dioica

), but it is not known if nettle deters nest predators (Rands 1986). In the tropics, some canopy-nesting birds (for example, the Baya weaver) prefer to nest in trees that have thorns on their trunks and branches. Birds that nest in these trees suffer less nest predation than those that nest in other trees (Quader

2006). Once an olfactory predator has detected an odor plume from an incubating bird, it

will probably endure much discomfort and pain to reach the nest and obtain food. Hence, briar patches may only be effective against a foraging predator that has not detected a nest’s odor plume. The closer a predator can get to a nesting bird without having to enter the briar patch, the more likely it is to detect its odor plume. Hence, wide patches of briars or cacti should provide more protection from olfactory predators than narrow barriers (Figure 13.2). Although this prediction seems reasonable, I am unaware of any studies that have tested it.