Habitat change and timing of dusk flight in the Eurasian woodcock: a trade-off between feeding and predator avoidance?

Eurasian woodcock (Scolopax rusticola) over-wintering in northern Spain follows a pattern of daily movement from diurnal sites in woodlands to nighttime sites in grazed fields, congruent with previous reports on other European populations. Fields have a much higher abundance and biomass of earthworms, the main prey of woodcock, than woodlands, but are only used by birds at night, when they emerge about 30 minutes after sunset. Emergence time was quite uniform among fortnightly periods except for the second half of November when it was earlier, probably reflecting extended activity resulting from the higher energy requirements of migrating birds. Woodcock emergence time relative to sunset was negatively related to illumination, so that birds moved earlier when the sky was overcast. Also, birds tended to move earlier at low air temperatures, a pattern which is to be predicted since woodcock have higher energy requirements at low temperatures because of the increased cost of thermoregulation. Emerging late, at low light intensity, avoids exposure to raptors but results in reduced energy gains and therefore the timing of moving to feeding fields may be modified by energy considerations. We discuss the idea that there is a divergent spatio-temporal gradient of food and safety between woodlands and fields and, therefore, that predation risk is an important determinant of the timing of evening emergence of over-wintering woodcock. Thus, their daily activity cycle would reflect a trade-off between feeding and predator avoidance. Special features of the visual fields of the woodcock, allowing detection of predators at long distances, and chemoreceptors in the bill enabling detection and capture of prey by probing using non visual cues, comprise adaptations that facilitate successful feeding at night with reduced predation risk.