DOI: 10.5061/dryad.59zw3r23c
title: Differences in perceived predation risk associated with variation in relative size of extra-pair and within-pair offspring

description: Extra-pair paternity (EPP) is a widespread phenomenon in birds.
 Researchers have long hypothesized that EPP must confer a fitness
 advantage to extra-pair offspring (EPO), but empirical support for this
 hypothesis is definitively mixed. This could be because genetic benefits
 of EPP only exist in a subset of environmental contexts to which a
 population is exposed. From 2013-2015, we manipulated perceived predator
 density in a population of tree swallows (Tachycineta bicolor) breeding in
 New York to see whether fitness outcomes of extra-pair and within-pair
 offspring (WPO) varied with predation risk. In nests that had been exposed
 to predators, EPO were larger, longer-winged, and heavier than WPO. In
 non-predator nests, WPO tended to be larger, longer-winged, and heavier
 than EPO, though the effect was non-significant. We found no differences
 in age, morphology, or stress physiology between extra-pair and
 within-pair sires from the same nest, suggesting that additive genetic
 benefits cannot fully explain the differences in nestling size that we
 observed. The lack of an effect of predator exposure on survival or
 glucocorticoid stress physiology of EPO and WPO further suggests that
 observed size differences do not reflect more general variation in
 intrinsic genetic quality. Instead, we suggest that size differences may
 have arisen through differential investment into EPO and WPO by females,
 perhaps because EPO and WPO represent different reproductive strategies,
 with each type of nestling conferring a fitness advantage in specific
 ecological contexts.
description: We presented model predators (snake, cat, or raptor) or
 non-predators (turtle, rabbit, or duck) within a population of tree
 swallows (<em>Tachycineta bicolor</em>) breeding in artificial
 nest boxes in Ithaca, New York over the course of three years (2013-2015).
 We collected red blood cell and plasma samples from incubating adult
 females, provisioning adult females and males, and Day 12 nestlings. We
 also collected three morphological measurements (head-bill length, wing
 length, and mass) from each individual in the study. Red blood cells were
 used to determine parentage of nestling tree swallows using nine
 microsatellite loci. We used plasma samples to determine circulating
 baseline and stress-induced corticosterone concentrations using
 radioimmunoassay. We measured provisioning behavior between Nestling Days
 8 and 13 by observing the number of times that either adult entered its
 nest box during a 30-minute monitoring period. Because this study was
 specifically interested in comparing the relative performance of
 half-siblings of differing paternity status across treatment groups,
 archived data examining nestling survival to fledging
 ("All.Nestlings.Mixed.Survival.txt"), size, and corticosterone
 concentrations include only those nestlings arising from broods of mixed
 paternity. Nestling size and corticosterone were further compared only for
 those half-siblings that survived to fledging
 ("All.FL.Nestlings.Mixed.Size.CORT.txt"). Prior to analysis, we
 condensed three nestling size attributes taken at two time points (Day 6-7
 and Day 12-13 post-hatching) using principal component analysis. Original
 size measures are presented in one data file
 ("All.FL.Nestlings.Mixed.Size.PCA.txt"), while the first two
 principal components arising from this analysis are presented in a second
 data file ("All.FL.Nestlings.Mixed.Size.CORT.txt"). We also
 compared attributes of social males vs. extra-pair sires, and this data
 set ("Sires.txt") provides pairwise differences in age, size,
 and corticosterone concentrations for each social male compared to the
 average value of all extra-pair sires at his nest. Adult female and male
 corticosterone concentrations and pair provisioning rates were compared
 across treatment groups
 ("Female.CORT.txt", "Male.CORT.txt",
 "Provisioning.txt"). We examined timing of breeding and patterns
 of settlement across treatment groups by way of reference to clutch
 initiation dates in each box ("Lay.Dates.txt"). We also compared
 rates of extra-pair paternity across treatment groups, sides of our study
 area, and years of study ("EP.Rates.txt"). All of these data
 files are utilized in statistical analyses carried out in R
 ("R.Code.Hallinger.et.al.JEB.R")
description: Missing values are represented by value "NA". Columns
 are coded for each data file as follows:
 "All.FL.Nestlings.Mixed.Size.CORT.txt": Band = unique
 identifier for each nestling, Year = 2013, 2014, or 2015, Box.ID = unique
 identifier for each nest, Side.of.Site = Near or Far, Treatment = NP
 (non-predator) or P (predator), Julian.Lay = clutch intiation date using 1
 May as a reference point (i.e. 1 May = 1), Fledge = Y (fledged) or N (not
 fledged), EP.Status = EPO (extra-pair) or WPO (within-pair), ln.Base =
 ln-transformed baseline corticosterone concentration (in ng/mL), ln.Str =
 ln-transformed stress-induced corticosterone level (in ng/mL), PC1 = value
 for principal component loading positively on all measures of nestling
 size (size), PC2 = value for principal component loading negatively on Day
 6 nestling size metrics and positively on Day 12 nestling size metrics
 (growth)
 "All.FL.Nestlings.Mixed.Size.PCA.txt": Band = unique
 identifier for each nestling, HeadBill.1 = head-bill length (in mm) on Day
 6 or 7 post-hatching, Wing.1 = wing length (in mm) on Day 6 or 7
 post-hatching, Mass.1 = mass (in g) on Day 6 or 7 post-hatching,
 HeadBill.2 = head-bill length (in mm) on Day 12 or 13 post-hatching,
 Wing.2 = wing length (in mm) on Day 12 or 13 post-hatching, Mass.2 = mass
 (in g) on Day 12 or 13 post-hatching
 "All.Nestlings.Mixed.Survival.txt": Band = unique
 idenifier for each nestling, Year = 2013, 2014, or 2015, Box.ID = unique
 identifier for each nest, Side.of.Site = Near or Far, Treatment = NP
 (non-predator) or P (predator), Julian.Lay = clutch initiation date using
 1 May as a reference point (i.e. 1 May = 1), Fledge = Y (fledged) or N
 (not fledged), EP,Status = EPO (extra-pair) or WPO (within-pair)
 "EP.Rates.txt": Box.ID = unique identifier for each
 nest, Treatment = NP (non-predator) or P (predator), Side.of.Site = Near
 or Far, Year = 2013, 2014, or 2015, EP.Rate = proportion of offspring in
 each nest of extra-pair origin, Julian.Lay = clutch initiation date using
 1 May as a reference point (i.e. 1 May = 1)
 "Female.CORT.txt": Band = unique identifier for each
 female, Year = 2013, 2014, or 2015, Box.ID = unique identifier for each
 nest, Julian.Lay = clutch initiation date using 1 May as a reference point
 (i.e. 1 May = 1), Treatment = NP (non-predator) or P (predator),
 Side.of.Site = Near or Far, ln.Base = ln-transformed baseline
 corticosterone concentration (in ng/mL), ln.Str = ln-transformed
 stress-induced corticosterone concentration (in ng/mL), Stage = Inc
 (incubation) or Nest (provisioning)
 "Lay.Dates.txt": Box = box number within study area,
 Year = 2013, 2014, or 2015, Side.of.Site = Near or Far, Treatment = NP
 (non-predator) or P (predator), Julian.Lay = clutch initiation date using
 1 May as a reference point (i.e. 1 May = 1), Nest.Pre = number of days
 prior to start of predator manipulation during which nesting material was
 detected in box, Lay.Post = number of days following the start of predator
 manipulation in each year before eggs appeared in box
 "Male.CORT.txt": Band = unique identifier for each
 male, Year = 2013, 2014, or 2015, Box.ID = unique identifier for each
 nest, Julian.Lay = clutch initiation date using 1 May as a reference point
 (i.e. 1 May = 1), Treatment = NP (non-predator) or P (predator),
 Side.of.Site = Near or Far, ln.Base = ln-transformed baseline
 corticosterone concentration (in ng/mL), ln.Str = stress-induced
 corticosterone concentration (in ng/mL)
 "Provisioning.txt": Total.Visits = the number of times
 either the male or the female entered the nest box during a 30-minute
 observation period, Year = 2013 or 2014, Box = box number within study
 area, Treatment = NP (non-predator) or P (predator), Side.of.Site = Near
 or Far, Nestling.Age = age of nestlings (in days post-hatching) when
 provisioning was observed, Brood.Size = number of nestlings present in
 nest when provisioning was observed, Julian.Lay = clutch initiation date
 using 1 May as a reference point (i.e. 1 May = 1)
 "Sires.txt": Treatment = NP (non-predator) or P
 (predator), Side.of.Site = Near or Far, Year = 2013, 2014, or 2015, Box.ID
 = unique identifier for each nest, Diff.Age = age (in years) of extra-pair
 male(s) minus age (in years) of social male, Diff.HB = head-bill length
 (in mm) of extra-pair male(s) minus head-bill length (in mm) of social
 male, Diff.Wing = wing length (in mm) of extra-pair male(s) minus wing
 length (in mm) of social male, Diff.Mass = mass (in g) of extra-pair
 male(s) minus mass (in g) of social male, Diff.Base = baseline
 corticosterone concentration (in ng/mL) of extra-pair male(s) minus
 baseline corticosterone concentration (in ng/mL) of social male, Diff.Str
 = stress-induced corticosterone concentration (in ng/mL) of extra-pair
 male(s) minus stress-induced corticosterone concentration (in ng/mL) of
 social male