Breeding ecology and conservation of yellow wagtails Motacilla flava in intensive arable farmland
Causes of population decline in the yellow wagtail Motacilla jlava jlavissima are
currently unclear. A large proportion of Britain's yellow wagtails breed on arable
farmland, and factors influencing breeding productivity in this habitat could also
influence national population trends.
Intensive autecological studies of an arable-breeding population in eastern
England revealed a number of constraints on productivity. Breeding season length
could be influenced by seasonal changes in habitat suitability. Most early nesting
attempts occurred in autumn-sown cereal fields, but cereal crops were avoided once
they reached their full height. Consequently, second broods or replacement broods
were constrained by the availability of alternative nesting habitats. In our study
region, potato crops were strongly favoured for these late broods, and yellow wagtails
appeared to abandon their early territories and perform dispersal in order to locate
these more suitable habitats.
Field-bean crops were also preferred for nesting, but nests in beans tended to
become poorly concealed following growth of the crop above them, and suffered high
predation rates. As such, bean fields may represent an ecological trap for yellow
wagtails. Preference for nesting close to tramlines in autumn-sown cereal fields also
lead to a high predation rate. Nests away from tramlines had a high success rate, but
these areas were avoided by settlers, and may represent an undervalued resource for
Soil conditions strongly influenced the breeding distribution of yellow wagtails
in our study region. On a landscape scale, higher abundance occurred on soil types
with more organic content. At a smaller scale, territory abundance within preferred
crop types was largely determined by soil penetrability. Penetrable organic-rich soils
may be more productive for insect prey, though the proximate mechanism underlying
this pattern remains unclear. Preferred foraging habitats during breeding included
wheat and potato crops, as well as field margins including ditches and tracks.
However, the availability of these habitats did not influence the condition of nestlings.
Breeding productivity was largely dependent on the habitat chosen for nesting, which
affects nest predation likelihood.
These fmdings provide an evidence-base that can be used in the design of
conservation actions for breeding yellow wagtails in arable land. Activities aimed at
enhancing late season nesting habitat availability, as well as attracting breeders away
from potential ecological traps, could be most fruitful.