Are urbanization and brood parasitism associated with differences in telomere lengths in song sparrows?

Urbanization reflects a major form of environmental change impacting wild
birds globally. Whereas urban habitats may provide increased availability
of water, some food items, and reduced predation levels compared to rural,
they can also present novel stressors including increased light at night,
ambient noise, and reduced nutrient availability. Urbanization can also
alter levels of brood parasitism, with some host species experiencing
elevated levels of brood parasitism in urban areas compared to rural
areas. Though the demographic and behavioral consequences of urbanization
and brood parasitism have received considerable attention, their
consequences for cellular-level processes are less understood. Telomeres
provide an opportunity to understand the cellular consequences of
different environments as they are a well-established metric of biological
state that can be associated with residual lifespan, disease risk, and
behaviour, and are known to be sensitive to environmental conditions. Here
we examine the relationships between urbanization, brood parasitism, and
blood telomere lengths in adult and nestling song sparrows (Melospiza
melodia). Song sparrows are a North American songbird found in both urban
and rural habitats that experience high rates of brood parasitism by
brown-headed cowbirds (Molothrus ater) in the urban, but not the rural,
sites in our study system. Among adults and nestlings from non-parasitized
nests, we found no differences in relative telomere lengths between urban
and rural habitats. However, among urban nestlings, the presence of a
brood parasite in the nest was associated with significantly shorter
relative telomere lengths compared to when a brood parasite was absent.
Our results suggest a novel, indirect, impact of urbanization on nestling
songbirds through the physiological impacts of brood parasitism.