Data and code from: Fitness consequences of parasitism in a changing world: A case study with bird blow flies and sagebrush songbirds

Host-parasite interactions increasingly are influenced by human-induced
rapid environmental change (HIREC), and the fitness effects of parasitism
may be compounded or exacerbated by host traits and/or exposure to
additional extrinsic stressors associated with HIREC. Potential
interactions between parasitism and different stressors associated with
environmental change, however, remain poorly understood for most systems.
We examined how parasitism by bird blow flies (Trypocalliphora braueri),
ambient weather conditions, and habitat disturbance jointly affected
offspring traits and juvenile mortality for two declining species of
sagebrush songbirds (Brewer’s Sparrow, Spizella breweri; and Sage
Thrasher, Oreoscoptes montanus) in Wyoming, USA. We evaluated two
alternative hypotheses: that parasitism could act (i) in an independent
and additive manner with temperature and habitat alteration (Multiple
Stressors Hypothesis), or (ii) synergistically to exacerbate the effects
of temperature and habitat alteration (Parasitism-HIREC Interaction
Hypothesis) on offspring traits and juvenile mortality. We assessed
morphometric traits of nestlings and survival of fledglings in relation to
parasite loads, temperature, and habitat disturbance associated with
natural gas development using liner mixed models and Cox
proportional-hazard models. Higher parasite loads and colder temperatures
were associated with different effects for nestlings of each host species,
reducing tarsus and wing chord length for Brewer’s Sparrow and increasing
mass for Sage Thrasher. Despite differences in the effect of parasitism on
nestling traits, post-fledging mortality risk for both species increased
with higher parasite loads. The effects of parasitism and temperature
mainly were additive, with limited evidence that weather exacerbated the
effects of parasitism. Habitat disturbance had a weak positive effect on
nestling tarsus length and post-fledging survival probability for Brewer’s
Sparrow. Although parasitism rarely results in direct mortality of hosts,
parasites can nonetheless exert considerable fitness consequences,
especially when combined with extrinsic stressors associated with
human-induced environmental changes.