Predicting the effects of climate change on avian life-history traits

Abstract

Across North America, tree swallows have advanced their mean date of clutch initiation (lay date) by approximately 9 days over the past 30 years, apparently in response to climate change. In a sample of 2,881 nest records collected by the lay public from 1959 to 1991, we examined whether clutch size has also responded to climate change. We found that clutch size is strongly related to lay date, both within and among years, and there has been no significant temporal variation in the slopes or intercepts of the clutch-size/lay-date regressions. As a consequence, we expected increases in clutch size with advancement in lay date; however, we detected no such trend over time. The distributions of egg-laying dates were more constricted in the warmest (and earliest) years, suggesting that changes in mean clutch size might be constrained by changes in the distribution of laying dates. If spring temperatures continue to increase, we predict further reductions of variance in laying dates and relatively small increases in clutch size. Such constraints on life-history variation probably are common and need to be considered when modeling the effects of climate change on reproduction in natural populations. Predicting the long-term effects of constraints and interpreting changes in life-history traits require a better understanding of both adaptive and demographic effects of climate change.

Figure 1

An idealized representation of the relationship between clutch size and lay date under absolute and relative modes of clutch-size determination (after von Haartman, ref. 15). In the absolute mode, clutch size responds to lay date such that a change to an earlier lay date (e.g., in response to warming) would result in larger clutches being laid, with the population shifting up the same regression line (dashed portion). Under the relative mode, earlier lay dates would result in a shifting to the left of the clutch-size/lay-date regression without any shift in mean clutch size. Note that these simplified graphs assume constancy of the regression slope with changes in lay date.

Figure 2

The corrected regressions between clutch size and date of egg laying (clutch initiation) for the 17 years from the sample that had slopes significantly (P < 0.05) different from 0. These are estimates of clutch size corrected for all the other covariates in Table 1 by forcing SAS to calculate separate intercepts and slopes for each year as fixed effects. Each regression line is drawn only to the limits of the data set for each year. Note the similarity of observed slopes and the relatively minor differences among the regression intercepts across years.

Figure 3

A path analysis summarizing the main effects of all linear predictors, without interactions, on lay date and clutch size. Dashed arrows are for effects that are not significant (P > 0.05), with denser dashing for two effects with 0.10 > P > 0.05. Significant effects are designated by solid arrows, with heavy arrows for effects with P < 0.001. For significant effects, the standardized coefficients from the tests of fixed effects are presented along the arrow. For random effects (in italics), the numbers on the significant paths represent the raw covariance parameters estimated by the procedure MIXED. For simplicity of presentation, we omitted all interaction and quadratic terms from the path diagram. For further details see text.