Action: Relocate nests at harvest time to reduce nestling mortality
A replicated controlled study from Spain found that clutches that were temporarily removed from fields during harvest and then replaced had higher hatching and fledging rates than control clutches. Effects were greater on clutches that were older when moved.
If nests are likely to be destroyed by machinery during harvest or mowing, it may be possible to move them and then return them after the danger has passed. If nests are extremely likely to be destroyed during harvest or mowing then it may be best to remove the chicks and hand-rear them. Studies on the effects of this intervention are found in the chapter on captive breeding and hand-rearing.
Supporting evidence from individual studies
A replicated, controlled study from 1987-91 in five areas of cereal fields in southwest Spain (Corbacho et al. 1999) found that nestling mortality of Montagu’s harriers Circus pygarus was significantly lower, and fledging success significantly higher, for clutches that were removed from fields before harvesting, and returned within an hour, compared to control (unmoved) clutches (28% mortality and 75% of nests fledging at least one chick in 72 managed clutches vs. 67% mortality and 29% fledging success in 39 controls). Outcome was highly dependent on clutch age at time of harvest: no clutches less than ten days old at harvest fledged young, whilst nest management increased the proportion of successful clutches aged 11-20 days at harvesting from 14% to 75%. The average harvest date of barley fields was later than for wheat or oat fields, but the small number of clutches (13) in barley fields made it impossible to assess the influence of nesting habitat on unmanaged clutch success. The nature of the crop (wheat and/or oat vs. barley) did not influence breeding success in managed clutches.