Daniel Isaksson’s scientific contributions

Summary • Predators hunting by sight often search for prey from elevated perches or hovering positions above the prey habitat. Theory suggests that prey visibility depends strongly on predator perch height and distance, but their quantitative effects have not been experimentally tested in natural habitats. • We estimate for the first time how prey visibility depends on predator perch height, distance and vegetation height in an open natural habitat, based on visibility measurements of two targets: a mounted bird and a graduated plate, from five perch heights (0·2–8 m) and six distances (5–120 m). • For both targets, their proportion visible increases strongly with observer perch height and proximity. From the lowest perch, visibility of the target bird declines to • Models of predator search suggest that hunting success and predation rate depend strongly on the prey detection rate, which is expected to decline with distance r approximately as r−d. However, d, the distance decay parameter, has not previously been empirically estimated in natural predator habitats. For distance – prey visibility relationships similar to those observed here, we find a realistic estimate of d to be 2·1–2·4. • The results demonstrate the crucial role of relative perch and vegetation height for prey visibility, which is of relevance for habitat management. The strong increase of prey visibility with predator search height suggests that removal of predator perches can improve the survival of endangered prey populations in open habitats. Conversely, perch preservation or addition can improve habitat suitability for some predator species where perches are rare or lacking.

Ground-nesting birds have declined world-wide, probably partly due to high nest predation. A non-lethal method for decreasing predation uses protective cages at nests. Tests have mainly looked at the effect of such nest exclosures on hatching success and adult predation, but several additional aspects need to be explored for a comprehensive evaluation of this conservation technique. Here, we test the effect of nest exclosures in two common European shorebirds: northern lapwing (Vanellus vanellus) and redshank (Tringa totanus), measuring hatching success, incubation length, hatching synchrony, hatchability, partial clutch loss, chick condition, and adult predation. In both species, protected nests had higher hatching success than unprotected nests. Taking into account incubation time, nest abandonment, hatchability and partial clutch loss, protected nests still hatched more young than unprotected controls. In lapwings, but not in redshanks, protected nests were incubated longer, but this did not impair the condition of lapwing chicks. Protected redshanks suffered increased predation on incubating adults, which often sit on the nest until a predator is close by. Our results emphasize the need for caution in the use of nest exclosures, particularly in redshanks and other species with similar incubation behaviour. Exclosures can, however, be a useful management tool in shorebirds that leave their nest early, when an approaching predator is still far away.

Coastal pastures and other wet grasslands are important but decreasing breeding habitats for many waders (Charadrii). Since loss of suitable habitat is a major reason for population declines, protection and restoration of these habitats is crucial. Reduction of the often high rate of nest predation is a potentially important tool in future conservation work. Here, we focus on predators’ use of raised structures in the landscape when searching for prey. Hooded crows (Corvus corone cornix) use man-made structures such as stone walls and barbed wire fences when foraging on coastal pastures in SW Sweden. However, few studies have examined wader breeding success in relation to man-made structures, and the extent to which such structures are used by searching nest predators. We measured the spatial distribution and rate of predation on wader nests in relation to such structures. Crows spent more time at or near man-made structures than expected by chance, but we found no significant difference in nest predation relative to distance from man-made structures. However, wader nests were placed farther away from man-made structures than expected by chance in two out of three years. Waders thus tend to avoid breeding close to man-made structures, which therefore reduce the suitable breeding area and probably also the local wader population size.