Seasonal and latitudinal trends in clutch size: Thermal constraints during laying and incubation

Explaining patterns of latitudinal and seasonal trends in clutch size are two of the oldest and most fundamental endeavors in avian life history research. Underlying the majority of studies regarding any type of clutch size variation (i.e., individual, seasonal, latitudinal) of altricial birds is the premise that the primary cost of reproduction stems from feeding offspring. However, both altricial and precocial species of birds display latitudinal and seasonal variation in clutch size. Additionally, individual variation in costs of laying and incubation, recently demonstrated, indicates that understanding latitudinal and seasonal clutch size trends will require increased attention to earlier phases of reproduction. Given the strength and ubiquity of the clutch size patterns, many environmental factors, such as food supply and predation, have been proposed to account for the patterns, but temperature has been largely overlooked. Gradients in many variables may be important because the primacy of selection pressures may also vary in space and time. Furthermore, physiological systems may constrain responses to selection pressures. Thus, it is possible that intraspecific geographic and seasonal patterns in clutch size are at least partially influenced by temperature-dependent physiological processes. Therefore, we suggest that it is important to examine physiological responses of birds (e.g., embryo development, incubation energetics) directly influenced by physical properties of the environment, which exhibit predictable types of spatial and temporal variation (e.g., temperature, humidity, day length). We review two recently proposed, complementary hypotheses that are excellent candidates for this approach. By one mechanism, the thermal inertia of large clutches makes them favorable in cooler weather (the clutch-cooling hypothesis of J.M. Reid et al.). By the other, the reduction in egg viability under warm temperatures favors small clutches (the egg-viability hypothesis of S. H. Stoleson and S. R. Beissinger). Using general linear mixed models, we found that large-scale nesting patterns of Eastern Bluebirds and Red-winged Blackbirds are consistent with the egg-viability hypothesis in that females appear to initiate incubation before clutch completion when they lay large clutches at low latitudes. Although attempts have been made to overcome the logistical obstacles associated with studying large-scale phenomena through meta-analyses and multiple small-scale study sites, we demonstrate the significant potential of new technologies combined with volunteer-based studies to validate these hypotheses as we outline directions for future research.