DOI: 10.5061/dryad.dz08kprv6
title: Divergent sexual signals reflect costs of local parasites

description: Many closely related populations are distinguished by variation in sexual
 signals and this variation is hypothesized to play an important role in
 reproductive isolation and speciation. Within populations, there is
 considerable evidence that sexual signals provide information about the
 incidence and severity of parasite infections, but it remains unclear if
 variation in parasite communities across space could play a role in
 initiating or maintaining sexual trait divergence. To test for variation
 in parasite-associated selection, we compared three barn swallow
 subspecies with divergent sexual signals. We found that parasite community
 structure and host tolerance to ecologically similar parasites varied
 between subspecies. Across subspecies we also found that different
 parasites were costly in terms of male survival and reproductive success.
 For each subspecies, the preferred sexual signal(s) were associated with
 the most costly local parasite(s), indicating that divergent signals are
 providing relevant information to females about local parasite
 communities. Across subspecies, the same traits were often associated with
 different parasites, indicating that parasite-sexual signal links are
 quite flexible and may evolve relatively quickly. This study provides
 evidence for 1) variation in parasite communities and 2) different
 parasite-sexual signal links among three closely related subspecies with
 divergent sexual signal traits, suggesting that parasites may play an
 important role in initiating and/or maintaining the divergence of sexual
 signals among these closely related, yet geographically isolated
 populations.
description: Field work for <i>H.r. erythrogaster</i> was
 conducted May- September 2012 in Colorado, USA where we collected data on
 173 males from 23 breeding colonies (typically barns or bridges) in
 Boulder, Jefferson, and Weld counties. Field work for <i>H.r.
 rustica </i>took place April-August 2013 in Jihocesky, Czech
 Republic where we followed 84 males at 6 breeding colonies near the towns
 of Třeboň and Lužnice.
 Field work for <i>H.r. transitiva </i>took place January-June
 2015 in northern Israel, where we collected data on 42 males in the
 villages of Ami’ad, Hoquq, and Kahal. We had smaller sample sizes in
 Israel because barn swallows breed at lower densities there.  Breeding
 sites within each subspecies were located geographically near one another
 and there is likely gene flow between them. 
 At each breeding
 site, we attempted to capture all adults using mist nests. Each adult was
 given a numbered band and a unique combination of color bands. We measured
 morphology (wing length, mass, tail streamer length), and collected
 ventral plumage samples and blood samples (Safran et al.
 2008; Jenkins et al. 2013; Hubbard et al. 2015). Using behavioral
 observations, we identified social pairs and determined their nest. All
 nests were monitored every three days until the end of incubation, at
 which point, nests were monitored daily to determine hatch date. When
 nestlings were 12 days old, we banded them and measured morphology (wing
 length and mass), and collected blood and plumage samples (Hubbard
 <i>et al. </i>2015). For each subspecies, we measured
 parasites of adults and offspring, cost of parasites to male survival and
 reproductive success, and associations between the expression of
 ornamental traits and parasite infections. We used the same methods for
 all three subspecies to enable direct comparisons and the same people
 (AKH, JKH) led the field work and performed the measurements in all three
 countries. <i>Measurement of
 parasites</i> We quantified all ectoparasites
 infecting adults using a loupe (3.5x magnification) at the start of the
 breeding season (all ectoparasites were quantified by the same person,
 AKH, in all three subspecies) (Møller 1991). Nest parasites were
 quantified 12 days after nestlings hatched (supplement 1.1 for details).
 Haemosporidian blood parasites were detected via nested PCR targeting
 cytochrome <i>b </i>using the Hellgren et al. (2004) protocol,
 followed by Sanger sequencing of positive samples (supplement 1.2 for
 details). Given that this method cannot tell us about infection intensity,
 data for Haemosporidians for each individual was coded as present or
 absent in our analyses. We used non-lethal field sampling methods in wild
 birds and were therefore unable to measure all possible parasites (i.e.
 gut parasites, bacteria, viruses). Our parasite community is thus limited
 to the parasites that we could quantify via visual and molecular methods
 using samples collected from living animals (see supplement 1.3 for
 parasite terminology). For two subspecies, in Colorado
 and the Czech Republic, we were able to collect data on a subset of
 parasites across multiple years and thus test if parasite abundance and
 variance was relatively consistent within a population across years, as
 well as ensure that our data from one year was representative of typical
 parasite infections for that population. We compared parasite abundance
 across years within subspecies with repeated measures using general linear
 mixed models (Poisson distribution) with year as a fixed effect. We also
 compared the variance within a population between years using a
 Kruskal-Wallis test. We measured ectoparasites in Colorado in 2012
 (n=173), 2013 (n=196), and 2014 (n=103), and in the Czech Republic in 2013
 (n=84) and 2014 (n=81). <i>Measures of parasite
 costs</i> For each subspecies, we tracked the
 following measures of reproductive success: onset of breeding, where early
 onset is associated with increased offspring survival, quality, longevity,
 and seasonal reproductive success in temperate breeding birds, including
 barn swallows (Dubiec and Cichoñ 2001; Gurney et al. 2012; Saino et al.
 2012; Öberg et al. 2014), the number of nestlings successfully fledged
 from a male’s first brood, and whether that male attempted a second brood
 during that season (Møller 1990a; Saino and Møller 1995). We also measured
 male survival to the next breeding season for the Czech and Colorado
 populations, but were unable to collect these data for the Israeli
 population. Adult barn swallows have high breeding site fidelity from year
 to year, so presence the following year is considered a good measure of
 survival (Safran 2004; Schaub and Von Hirschheydt 2009).
 <i>Measures of sexual signals</i>
 Expression of all three ornamental traits (streamer length,
 throat color, and breast color) was measured in each subspecies. Unbroken
 tail streamer length was measured in millimeters from the base to the tip
 of the feather (Safran and McGraw 2004; Wilkins et al. 2016). Feather
 samples from the throat and breast region were collected and stored in
 envelopes. These feathers were later mounted on cards and analyzed using a
 spectrophotometer (Wilkins <i>et al. </i>2016). We used
 brightness and hue as measures of color expression (supplement 1.4 for
 details). We confirmed within our dataset that the expression of sexual
 traits varied between subspecies (supplement 2.1), and that the signal(s)
 used by females within each subspecies was associated with reproductive
 success (supplement 2.2). These results supported the presence of
 divergent sexual selection and corroborated previous experimental and
 comparative studies of barn swallows (Scordato and Safran 2014; Safran et
 al. 2016a; Romano et al. 2017).
description: This is a combine datasheet that contains data from both the
 sexual selection portion of the study and the repeated parasites measures
 across years (for a subset of parasites). For the Colorado 2013, 2014
 individuals and the Czech Republic 2014 individuals, we only include
 parasites information. Missing information for other individuals was due
 to our inability to collect this information in the field (i.e. nest was
 found after eggs were laid, so clutch initiation date was impossible to
 determine, or sample was lost or missing). For some subspecies certain
 parasites were not present (i.e. flies in Colorado or Haemosporidians in
 Israel), so these values are left blank for these subspecies. We were also
 unable to collect survival data for Israel.