In the case of avian divorce, our analysis is informed by a recent comparative analysis that found support for 5 of 19 predictors gathered from the literature [29]. The well-supported predictors of divorce are continuity of partnerships (i.e., whether partners remain together year-round or separate during the non-breeding season), sexual dichromatism, visual ornamentation (scale from 0 = not ornamented, to 7 = very ornamented; data from [28] when available, missing species measured using identical protocols by JF, see acknowledgements, and CAB), coloniality, and adult mortality. Divorce rates are expected to be higher in species with stronger sexual selection (i.e., more visually ornamented or more sexually dichromatic) because of stronger competition for high quality mates [28]. Similarly, divorce is expected to be more common in species with high mortality rates because delaying breeding in order to wait for partners that may never come back can be prohibitively costly (also a likely explanation for the effect of separating during the non-breeding season); or because in short- lived species, it is more likely that higher quality mates will become available through the death of their own partners (see [29]). In addition to these five predictors, we include in this model two-way interactions that explore the potentially different effects of environmental variability on species that establish continuous versus temporary partnerships. Such interactions are biologically meaningful because individuals that stay together year-round are constantly aware of each other’s status and do not incur searching or waiting costs at the onset of new breeding opportunities [26]. 2. Climatic Variables From each local weather station included in this study we obtained monthly averages of precipitation and temperature retaining only years with complete records from 1800 to 2009. Because this timeframe captures only fairly recent events in evolutionary history, the climate variables described below reflect the ecological conditions (or ‘climatic tolerances’) to which the species in this study are currently adapted (just as our metrics of infidelity and divorce reflect current levels of secondary mate choice in this group). Raw climate data were used to compute the following descriptive measures of the local variation in precipita- tion and temperature at each site: (1) within-year variance (i.e., variance in the monthly means for each year then averaged across years); (2) predictability of annual cycles (see below); (3) mean for the breeding season (see below); and (4) among-year variance in the mean of the breeding season. The predictability of annual climate cycles was quantified via Colwell’s P [47] using standard bin sizes of 50 mm for precipitation and 0.5 degrees Celsius for temperature. Colwell’s P is an information-theory-based index that can be interpreted as a measure of the extent to which the onset, duration, and intensity of local climate cycles differ among years. Sites with highly repeatable annual cycles yield P<1 whereas those in which climate cycles show considerable variation among years yield P<0. Because of a lack of data on the phenology of most populations included in this study, we estimated indirectly the timing and duration of the breeding season at each locality through le Houerou’s [48] mean growing season formula. This formula identifies the months of the year in which environmental conditions allow significant plant productivity, considered here to be a proxy for suitable breeding conditions. Although the timing and duration of the breeding season are also likely to depend on species-specific factors, le Houerou’s formula is appropriate for an analysis at this scale because it provides a realistic, albeit coarse approximation of the months of the year in which reproduction is more likely to occur in different parts of the World. Specifically, the growing season months at a given site are defined as those in Fluctuating Selection and Reproductive Flexibility which the average daily temperature is greater than or equal to a critical threshold (0uC for arctic and boreal regions [49], 6uC for temperate and subtropical regions [50], and 12uC for the tropics [51]) and the total precipitation in millimeters is at least twice the average temperature in degrees centigrade. The mean growing season formula has been used and validated in comparative studies at a global scale in the field of evolutionary anthropology [e.g., 50]. 3. Statistics We used phylogenetic generalized least squares (PGLS) regres- sion models with Pagel’s l [52] to account for the potential non- independence of data from species that vary in levels of relatedness [53]. Higher l values (estimated from the data via restricted maximum likelihood [54]) indicate stronger similarities between closely related taxa [52]. Pagel’s l models are mathematically equivalent to ordinary least squares regression analysis (i.e., no phylogenetic correction) when l = 0, and to PGLS regression analysis with a Brownian model of evolution when l = 1. Logarithmic or Arcsine transformations of raw variables were applied as needed to meet the assumptions of additive linear relationships and multivariate normality in PGLS. All continuous predictors were then standardized prior to analysis to facilitate comparison of their relative effects. Models were run using the MATLAB routine REGRESSIONv2.m described in [54]. We began each analysis with a fully parameterized model and proceeded to drop each non-significant term one at a time, starting with the interactions when available. Finally, we computed variance inflation factors (VIF) for each significant predictor in the reduced final model to confirm that there were no major multicollinearity issues (VIF.5 are considered evidence of multicollinearity, see [55]). We report the R2 (computed as in equation 2.3.16, p. 32, from [56]), the estimated Pagel’s lambda, and the b coefficients and test statistics for each significant term in our final models. The phylogenetic hypotheses used in our comparative analyses (Figs. S1 and S2) are based on species-level molecular phylogenies generated from both nuclear and mitochondrial DNA sequences (see Table S1). When molecular data were not available for a species that was the only member of its genus in our sample (Infidelity dataset: N = 3/122; divorce dataset: N = 3/86), we used instead sequences from available congeners. DNA sequences were downloaded from GenBank (http://www.ncbi.nlm.nih.gov/ Genbank), aligned using MUSCLE [57], and concatenated using Phyutility [58]. Phylogenies were estimated in RAxML 7.0.4 [59] using maximum likelihood, a General Time Reversible model of nucleotide substitution with a C model of rate heterogeneity, and topological constraints from Hackett et al. [60]. The resulting trees were ultrametrisized using Sanderson’s [61] algorithm for molecular dating with penalized likelihood. Results Our fully parameterized model of avian infidelity includes coloniality, adult mortality, and climate variables (see Methods) as potential predictors. After removing all non-significant terms from this model (final PGLS regression model: R2 = 0.08, l = 0.55; VIF = 1.22, see [55]), we find that extra-pair broods are more common in species that experience higher adult mortality (b = 0.050, t = 2.056, df = 119, P = 0.042; Fig. 1A), and breed in environments with greater within-year variance in temperature (b = 0.042, t = 2.068, df = 119, P = 0.041; Fig. 1B). Our fully parameterized model of avian divorce includes coloniality, mortality, continuity of partnerships, sexual dichro- matism, visual ornamentation, climate variables, and the two-way interactions between partnership continuity and the climate PLoS ONE | www.plosone.org 3 February 2012 | Volume 7 | Issue 2 | e32311