Integrating light-level geolocation with activity tracking reveals unexpected nocturnal migration patterns of the tawny pipit

Migratory birds complete their seasonal journeys between breeding and
non-breeding sites with a series of migratory flights that are separated
by prolonged stopovers. While songbirds are the most common taxa among
migratory birds, empirical data on flight and stopover behaviour along
their entire migratory journeys are still rare. Here, we integrate
activity and barometric pressure tracking with classical light-level
geolocation to describe migration behaviour of tawny pipits Anthus
campestris breeding in Central Europe. Surprisingly, tracked pipits used,
on average, as many as 10 stopover sites during their six week,
>5000 km long autumn migration. This conforms to a typical hop-type
pattern of migration. In contrast to common knowledge which considers the
tawny pipit as a typical diurnal migrant, our data revealed that more than
2/3 of all migratory movements were carried out at night. Nocturnal
departure times were highly variable within individuals and spread across
the entire night while landing most often took place within the first few
hours after sunrise. Consequently, there was a negative relationship
between departure timing relative to sunset and flight duration. Short
flights of up to 2 h were most common and median flight duration was 4.5
h. There was a hyperbolic relationship between flight duration and maximum
flight altitude and flight altitudes during night were two times higher
compared to daytime. The overall ratio of flight vs stopover duration
during migration was on average 1:6.5. This closely matches predictions
from theoretical models. We show that multi-sensor tracking has the
potential to provide unprecedented details on migratory behaviour of
individual birds along their entire migratory journeys, and it also
improves the precision of geographical locations derived from light-level
geolocators.