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 Ϸ9 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-size͞ 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. O ver the past century, global surface temperature has increased by an average of 0.6°C (1). It is now widely accepted that air temperatures have been increasing as a result of increased anthropogenic carbon dioxide in the atmosphere. Air temperature could be one of the key environmental cues affecting the seasonal timing of egg laying in birds, and there is evidence that increases in mean temperatures have led to earlier lay dates in birds (2-6) and accelerated phenologies in many other taxa (7-9).One of the most general patterns in the life histories of birds is that females that lay later in a breeding season tend to lay smaller clutches (refs. 10 and 11, but see refs. 12 and 13 for exceptions in multiple-brooded species), raising the possibility that earlier breeding may be leading to larger clutches being laid. Clutch size is an important life-history trait, because it sets a hard upper limit on offspring production. It is also the easiest life-history trait to measure, and data on clutch size variation often are available when data on other life-history traits are not.There is considerable evidence that the correlation between lay date and clutch size may be the result of a strong biological coupling (7,14), yet the nature of this biological link is very poorly understood. In an early attempt to study this connection between lay date and clutch size, von Haartman (15) suggested a dichotomy in the way that birds adjust their clutch sizes to the timing of breeding. At a study site in Lemsjöholm, Finland, both pied flycatchers (Ficedula hypoleuca) and starlings (Sturnus vulgaris) laid smaller clutches as each breeding season progressed. However, wh...

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