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.

We presented model predators (snake, cat, or raptor) or non-predators (turtle, rabbit, or duck) within a population of tree swallows (Tachycineta bicolor) 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")

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