Avian haemosporidian parasites in the Colorado Rocky Mountains are little studied. Within the MalAvi database, only one study (Marzal et al. 2011) reported sampling birds in Colorado, and this study was focused solely on House Sparrows (Passer domesticus). Because of the limited amount of data, the factors influencing avian haemosporidian parasite infection in the Colorado Rocky Mountains are largely unknown. A warming climate is expected to aid in the expansion of parasite distributions, and baseline knowledge and continued monitoring of the prevalence and diversity of these parasites is needed. This is especially true of high elevation resident host species that are more susceptible to infection and may be more heavily impacted by the spread of haemosporidian parasites.
In this study, we present baseline knowledge of haemosporidian parasite presence, prevalence, and diversity across a suite of avian species in the Colorado Rocky Mountains. Among the 437 birds of 24 species sampled, thirty-nine unique haemosporidian parasite haplotypes were detected, 21 species had at least 1 infected individual, and Haemoproteus parasites had a larger host-breadth and had much higher prevalence compared to Plasmodium. In addition, 6 novel haplotypes were detected among 3 different species. Using an occupancy-modelling framework to account for imperfect detection of avian blood parasites, we found that nest type is an important species-level factor influencing Haemoproteus parasitism at our study site, with open cup nesters having a higher prevalence compared to cavity and ground nesters. We also found that sex and BCI are important individual-level factors associated with Haemoproteus parasitism in some species, with males and birds with higher BCI having a higher blood parasite prevalence.
Haemosporidian lineage diversity
Diversity of haemosporidian parasites in wild birds was high, with a total of 39 lineages of Haemoproteus and Plasmodium species from 21 of the 24 avian species that were sampled (Table 2). Plasmodium lineage parasites are considered generalists in terms of host breadth, while Haemoproteus lineages are generally considered to be host-specific (Hellgren et al. 2009). However, we identified Haemoproteus in a wider range of bird species than Plasmodium and found that Haemoproteus parasites were more prevalent overall.
The small sample sizes obtained for many host species limit us from determining host specificity of the obtained lineages; however, some patterns were still apparent and most of these centered on the family Turdidae. One lineage, TURDUS2, infects many families of birds throughout Europe (Hellgren et al. 2007a), Asia (Hellgren et al. 2007b), and the United States (Oakgrove et al. 2014), yet most detections have occurred in the Turdidae (Thrush family). Accordingly, American Robins, a member of the Turdidae, at our study site had the highest proportion of TURDUS2 detections compared to other species. As this lineage was found in numerous species at our study site, American Robins may be acting as a reservoir for this parasite lineage. According to the MalAvi database, the lineage TUMIG07, has only been detected in the American Robin and Hermit Thrush (Catharus guttatus) in Alaska (Oakgrove et al. 2014). In our study, individuals from both of these species were found to be positive for the TUMIG07 lineage, along with six other species suggesting that this lineage is relatively common at our study site. The VIGIL07 lineage has only been detected in the Vireonidae family in California (Walther et al. 2016), New Mexico (Marroquin-Flores unpublished data), and Michigan (Smith et al. 2018), and, in our study, the highest proportion of individuals positive for this lineage was in the Warbling Vireo. Of the 7 detections of the TUMIG08 lineage in the MalAvi database, 4 have been from the American Robin (Oakgrove et al. 2014). Accordingly, 2 of the 4 detections of this lineage at our study site were from American Robins, with one detection in a Lincoln Sparrow, and the other in a Warbling Vireo. The POETR01 lineage has been mainly detected in the Thrush family as well (Oakgrove et al. 2014), and our one detection of this lineage was in an American Robin and therefore agrees with previous detections.
Patterns across host species
When analyzing Haemoproteus prevalence across host species, nest type and year were important variables associated with infection, with open-cup nesters in 2018 having the highest estimated prevalence of 0.38 (±0.06; Figure 3.1). Open-cup nesting has been linked to higher blood parasite prevalence in other studies (Gonzalez et al. 2014, Matthews et al. 2015, Smith et al. 2018), indicating that Haemoproteus vectors – biting midges – are more likely to come in contact with species that have open-cup nests than with ground or cavity nesters, perhaps because open-cup nesters are more vulnerable to exposure to blood feeding by the vectors.
Overall Haemoproteus prevalence also displayed marked variation between years in our study, with 2018 having a higher overall prevalence compared to 2017 (Figure 3.1). Interannual variation in avian blood parasite prevalence is common and has been found in many studies (e.g., Bensch et al. 2007, Wood et al. 2007, Lachish et al. 2011, Podmokła et al. 2014). One potential explanation for this annual variation is that the vectors responsible for Haemoproteus transmission fluctuate in abundance in response to weather variation (e.g., temperature and rainfall), which alter the habitat and microclimate they require for breeding. Higher prevalence may therefore occur in years when conditions are more favorable for vectors. Alternatively, annual variation in host demography and population dynamics could also play a role in driving this annual variation (Anderson and May 1986, Atkinson and Samuel 2010).
Age, sex, BCI, and migration were not considered important variables across species. These variables have been linked to higher haemosporidian parasite prevalence in other studies (e.g., Hatchwell et al. 2001, Deviche et al. 2005, Garvin et al. 2006, Calero-Riestra and Garcia 2016), thus site and broader geographic variation are important factors to consider when describing relationships between patterns of prevalence and host life history traits, and these relationships (or lack thereof) should be interpreted with caution. Further studies are needed to address the influence of host traits on patterns of avian haemosporidian parasite infection and to determine whether such patterns exist and persist at large spatial scales and across a wider host-parasite community.
When analyzing Plasmodium prevalence among host species, no associations were found between prevalence and species-level traits, likely due to the low number of individuals that were positive for the parasite. Low abundance of Plasmodium parasites and vectors in the host community can limit the transmission of blood parasites and may explain low prevalence. Elevation governs the distribution of parasites belonging to different genera, with Plasmodium parasites being more prevalent at lower altitudes and Haemoproteus parasite prevalence increasing with elevation (Rooyen et al. 2013). Accordingly, Eisen et al. (2008) found that Culex spp. mosquitoes, the main vectors of Plasmodium parasites, had not yet established in areas in and around Rocky Mountain National Park. Associations between exposure to mosquitoes and Plasmodium prevalence across host species has been demonstrated (Medeiros et al. 2015), supporting the idea that Plasmodium vectors may be absent, or in low numbers at our study site.
Patterns of Haemoproteus infection within individual species
Sex was associated with Haemoproteus infection in the Ruby-crowned Kinglet, White-crowned Sparrow, and Wilson’s Warbler (Figure 3.4). Sex-related differences in haemosporidian parasite prevalence are often observed in nature, however, sex-bias in parasitism is not universal and consistent, and often varies between and within host-parasite systems (McCurdy et al. 1998). Contrary to our prediction, our study demonstrated a strong male-biased parasite prevalence in the three species mentioned above, with Ruby-crowned Kinglet having the largest difference between sexes (0.53 in males vs. 0.01 in females). Although the greater stress of reproduction in females might translate to weakened immune responses (Møller et al. 1999), there is overwhelming evidence that sex-associated hormones can directly influence the differential susceptibility of each sex to infections (Loye and Zuk 1991). For example, testosterone has immunosuppressive effects in many species, leading to higher susceptibility of males to parasite infections (Zuk 1996, Zuk and McKean 1996, Hughes and Randolph 2001). This is not the case for every host-parasite relationship, as was illustrated by our failure to find an association between parasite prevalence and sex in the other seven species that we analyzed.
BCI was positively associated with Haemoproteus infection in the Red-breasted Nuthatch and the Ruby-crowned Kinglet when species were analyzed individually (Figure 3.5). Similar results have been found in other species such as the American Kestrel (Falco sparverius), the Yellow-rumped Warbler (Setophaga coronate), and the Great Tit (Parus major; Dawson and Bortolotti 2000, Cozzarolo et al. 2018, Ots and Horak 1998). This positive correlation may be due to infected individuals with lower body condition having lower capture probability. If infected individuals with low body condition are less active and are less likely to fly into mist nets, that leaves only infected individuals with greater body condition to be caught. Similarly, if individuals with low body condition are unable to survive the acute stage of Haemoproteus infection, then this may leave more infected individuals with higher body condition. The eight other species analyzed in our study did not show an apparent relationship between prevalence and BCI, which is a common result in wildlife studies given that host condition and its responsiveness to infection could change in response to foraging resources that fluctuate in space and time (Schultz et al. 2010, Sanchez et al. 2018). Some parasites cause minimal or no effects on condition in certain host taxa (Sanchez et al. 2018), and some infections might only exert negative fitness effects during stressful periods or under resource limitation (Khan and Fallis 1970, Applegate 1971).
We found no relationship between Haemoproteus prevalence and individual-level traits (sex, age, BCI) in the American Robin, Mountain Chickadee (Poecile gambeli), Pine Siskin (Carduelis pinus), or Dark-eyed Junco (Junco hyemalis), contrary to our hypotheses. Our results suggest that the individual-level traits examined in this study may not be important predictors of Haemoproteus infection for all species. However, we did identify a pattern in nest type indicating that aspects of avian life history and ecology shape, to a limited extent, their parasite community and the proportion of individuals infected by blood parasites.
PCR replicate was an important variable associated with detection probability for Haemoproteus infection for three individual species (Lincoln Sparrow, Warbling Vireo, and White-Crowned Sparrow) as well as for the all-species analysis, with PCR results varying among the three PCR runs (Figures 3.2 and 3.3). Nested PCR assays for haemosporidian parasites are known to be vulnerable to false negative results for parasite intensities at very low samples (Ishtiaq et al. 2017) and is most likely responsible for the variation we found between PCR replicates.
Detection probability for Plasmodium parasites was estimated at 0.30, which could be due to a decreased detection for Plasmodium based on blood samples (Svensson-Coelho et al. 2016) because Plasmodium enters latent, exoerythrocytic phases during chronic infection and may even be absent in the blood stream (Valkiunas 2004). Thus, sampling peripheral blood may not allow for detection of all true infections with Plasmodium, leading to underestimates of prevalence.