DOI: 10.5061/dryad.cfxpnvx7h
title: Microclimate and host body condition influence mite population growth in a wild bird-ectoparasite system

description: Parasite populations are never evenly distributed among the hosts they
 infect. Avian nest ectoparasites, such as mites, are no exception, as
 their distribution across the landscape is highly aggregated. It remains
 unclear if this pattern is driven by differences in transmission events
 alone, or if the environment that parasites inhabit after transmission
 also plays a role. Here, we experimentally examined the influence of the
 post-transmission microclimate, nest characteristics, and host condition
 on ectoparasite population growth in a bird-ectoparasite system. We
 infested barn swallow (Hirundo rustica erythrogaster) nests with a
 standardized number of Northern Fowl Mites (Ornithonyssus sylvarium) and
 analyzed both biotic (nestling mass, wing length, number of other
 arthropods present in the nest, and brood size) and abiotic (temperature,
 humidity, nest lining, nest dimensions, and substrate upon which the nest
 was built) predictors of mite population growth. Our results suggest that
 mite populations were most successful, in terms of growth, in nests with
 higher temperatures, lower humidity, few other arthropods, and hosts in
 good condition. We also found that nests built on wooden substrates
 support larger populations of mites than those constructed on metal or
 concrete. These findings lend insight into the factors that drive
 large-scale patterns of ectoparasite distributions.
description: Research took place at 24 b swallow breeding colonies located in
 Boulder County Colorado from May–August 2016. Adult barn swallows were
 captured using mist nets and each bird was given a USGS metal
 identification band and a unique combination of color bands. Breeding
 pairs were identified and assigned to nests through behavioral
 observations. All nests at each of the study colonies were monitored every
 other day to track lining activity, egg laying, and the hatching and
 growth of nestlings. This study was part of a larger experiment examining
 the role of genes and environment in color development in nestling barn
 swallows. For all experimental nests (n = 58), existing NFM were removed
 from nests using a heat disinfection method (Hund et al., 2015b) three
 days after clutch completion. Briefly, eggs were removed and a heat gun
 was used to heat all parts of the nests to 125 °C at least two times. The
 heating process took approximately 5 min. After the nests had cooled to
 &lt; 29 °C, the eggs were returned to the nest. After nests were
 disinfected, they were re-infected with 100 live fieldcollected northern
 fowl mites (NFM).  iButton DS1923 data loggers (Maxim
 Integrated) are miniature sensors (17.4 mm in diameter, and 6.4 mm in
 depth) that were used to track temperature and humidity throughout
 nestling development within a subset of our experimental nests (n = 38).
 iButtons were installed in the nest at the same time that NFMs were
 experimentally added to the nest (3 days after clutch completion).
 iButtons were placed in the nest cup between the feather lining and mud
 structure to capture the microclimate in which NFMs live. Nest temperature
 and humidity measurements were collected every 10 min for 14 days for a
 total of around 2024 measurements for each nest. Nest
 parasites were counted in the field when nestlings were 12 days old. We
 counted 1) how many mites were on the field assistant's hand after
 being placed in the nest for 30 s, 2) the number of mites on each
 nestling, and 3) the number of mites in the container used to hold the
 nestlings. These three counts were added together and used as a proxy for
 mite population size in each nest. Similar methods have been used in
 previous studies and correlate with mite population counts from nests
 placed in Berlese funnels (Møller, 1990; Hund et al., 2015b). Whole nests
 containing iButtons were collected 10 days (range: 7–12) after nestlings
 fledged (unless a new clutch had already been started) and placed into
 Berlese funnels for 24 h to get a more precise measure of final mite
 population size (n = 20, seven nests failed and were not included, five
 had new clutches before collection and were not included). Arthropods that
 were collected from the Berlese funnels were then sorted and counted using
 a dissecting microscope, according to procedures used previously (Hund et
 al., 2015b). The samples were separated into two categories: NFMs and
 other arthropods. Other arthropod numbers were small enough that they were
 counted individually. The mite populations were variable, but some were
 large enough that individual counting would have been unmanageable; for
 this reason, the number of mites in each sample was estimated by volume.
 To do this, 100 mites were counted and put in a micro-centrifuge tube as a
 reference. Then, mites were added into a new tube until its volume was the
 same as that of the reference tube. Once all the mites in a nest were
 accounted for, the number of complete tubes was counted, and multiplied by
 100 to get an estimate of mite population size for a given nest.
 Nestling mass, right wing length, and the number of nestlings
 were measured for each nest on day 12. Nestling mass was measured in grams
 using an electronic balance ( ± 0.01 g, AWS-100). Flattened wing length
 was measured for the right wing in millimeters ( ± 0.5 mm) using a wing
 rule (AFO Banding Supplies). Nestling mass was divided by wing length to
 calculate individual body condition. Individual nestling body condition
 scores were used to calculate an average nestling body condition for each
 nest. Brood size consisted of the number of live nestlings on day
 12. On day 12 after hatching, nest lining (amount of
 feathers in the nest cup) was evaluated on a qualitative scale from zero
 to three (zero being no feather lining, three being so many feathers that
 they could barely fit in the nest cup). Nest dimensions were measured
 using a measuring tape and nest area was calculated by multiplying the
 widest point of the nest cup by the height of the nest. The substrate on
 which the nest was built was categorized as either wood, metal, concrete,
 or other.
description: In this experiment, we first disinfected all experimental nests
 with a heat gun and then added 100 live mites into the nest at the start
 of the incubation period. For the larger experiment, we had 43 nests that
 survived to day 12. For a subset of these nests (n=20) we installed
 temperature/humidity ibuttons deep within the nest cavity. We then
 collected most of these ibutton nests after nestlings had fledged and
 placed them in berlese funnels to quantify the total mite
 population.  Temperature and humidity data for nests
 with ibuttons was processed from the raw ibutton files (took readings in
 30min intervals). These raw data files are available upon
 request.  <strong>The data
 set: Nest_mite_microclimate_dryad includes the following
 columns: <br></strong><strong>Site: </strong>name of field site (breeding colony)<br><strong>Nest: </strong>number of nest within that site<br><strong>Collected: </strong>whether or not that nest was collected after nestlings fledged<br><strong>Substrate: </strong>substrate that the nest was built on<br><strong>lining: </strong>amount of nest lining (feathers, hair) in the nest cup. 1=litte, 3=lots<br><strong>n_width: </strong>width of nest in cm<br><strong>n_height:</strong> height of nest in cm<br><strong>n_area: </strong>total nest area<br><strong>avg_r_nt:</strong> average daily temperature range for nest<br><strong>avg_r_nh:</strong> average daily humidity range for nest <br><strong>mean_nt:</strong> average of daily mean nest temperature<br><strong>avg_max_nt:</strong> average of daily max nest temp<br><strong>avg_min_nt:</strong> average of daily min nest temp<br><strong>max_nt</strong>: maximum nest temp reached during the experimental period<br><strong>min_nt</strong>: minimum nest temp reached during the experimental period<br><strong>mean_nt:</strong> average of daily mean nest temperature<br><strong>avg_max_nh:</strong> average of daily max nest humidity<br><strong>avg_min_nh:</strong> average of daily min nest humidity<br><strong>max_nh</strong>: maximum nest humidity reached during the experimental period<br><strong>min_nh</strong>: minimum nest humidity reached during the experimental period<br><strong>nestings_d3:</strong> number of nestlings in the nest three days post-hatching<br><strong>nestlings_d9</strong>: number of nestlings in the nest nine days post-hatching<br><strong>nestlings_d12</strong>: number of nestlings in the nest twelve days post-hatching<br><strong>total_nestlings_mass_d9</strong>: total mass of all the nestlings on day 9<br><strong>total_nestlings_mass_d12</strong>: total mass of all the nestlings on day 12<br><strong>avg_body_condition_d9</strong>: average body condition (mass/wing length) of the nestlings on day 9<br><strong>avg_body_condition_d12</strong>: average body condition (mass/wing length) of the nestlings on day 12<br><strong>field_mites_d9</strong>: mites quantified in field count on day 9<br><strong>field_mites_d12</strong>: mites quantified in field count on day 12<br><strong>mites_bur: </strong>mites quantified from berlese funnel. When there were lots of mites they were estimated by 50 (using standard volume in tube)<br><strong>swallow bugs:</strong> swallow bugs recovered from berlese funnel <br><strong>other_invertebrates:</strong> count of other invertebrates (not mites, not swallow bugs) recovered from berlese funnel