Traffic is often acknowledged as a threat to biodiversity, but its effects have been mostly
studied on roads subjected to high traffic intensity. The impact of lower traffic intensity such
as those affecting protected areas is generally neglected, but conservation-oriented activities
entailing motorized traffic could paradoxically transform suitable habitats into ecological
traps. Here we questioned whether roadside-nesting bee-eaters Merops apiaster perceived
low traffic intensity as a stressor eliciting risk-avoidance behaviors (alarm calls and flock
flushes) and reducing parental care. Comparisons were established within Doñana
National Park (Spain), between birds exposed to either negligible traffic (ca. 0±10 vehicles
per day) or low traffic intensity (ca. 10±90 vehicles per day) associated to management and
research activities. The frequencies of alarm calls and flock flushes were greater in areas of
higher traffic intensity, which resulted in direct mortality at moderate vehicle speeds ( 40
km/h). Parental feeding rates paralleled changes in traffic intensity, but contrary to our predictions.
Indeed, feeding rates were highest in traffic-exposed nests, during working days
and traffic rush-hours. Traffic-avoidance responses were systematic and likely involved
costs (energy expenditure and mortality), but vehicle transit positively influenced the reproductive
performance of bee-eaters through an increase of nestling feeding rates. Because
the expected outcome of traffic on individual performance can be opposed when responses
are monitored during mating (i.e. negative effect by increase of alarm calls and flock
flushes) or nestling-feeding period (i.e. at least short-term positive effect by increase of nestling
feeding rates), caution should be taken before inferring fitness consequences only
from isolated behaviors or specific life history stages.Peer reviewe