Many animals use photoperiod cues to synchronize reproduction with environmental conditions and thereby improve their reproductive success. The circadian clock, which creates endogenous behavioral and physiological rhythms typically entrained to photoperiod, is well characterized at the molecular level. Recent work provided evidence for an association between Clock poly-Q length polymorphism and latitude and, within a population, an association with the date of laying and the length of the incubation period. Despite relatively high overall breeding synchrony, the timing of clutch initiation has a large impact on the fitness of swallows in the genus Tachycineta. We compared length polymorphism in the Clock poly-Q region among five populations from five different Tachycineta species that breed across a hemisphere-wide latitudinal gradient (Fig. 1). Clock poly-Q variation was not associated with latitude; however, there was an association between Clock poly-Q allele diversity and the degree of clutch size decline within breeding seasons. We did not find evidence for an association between Clock poly-Q variation and date of clutch initiation in for any of the five Tachycineta species, nor did we found a relationship between incubation duration and Clock genotype. Thus, there is no general association between latitude, breeding phenology, and Clock polymorphism in this clade of closely related birds.Figure 1Photos of Tachycineta swallows that were used in this study: A) T. bicolor from Ithaca, New York, B) T. leucorrhoa from Chascomús, Argentina, C) T. albilinea from Hill Bank, Belize, D) T. meyeni from Puerto Varas, Chile, and E) T. thalassina from Mono Lake, California, Photographers: B: Valentina Ferretti; A, C-E: David Winkler.

In a study of almost 16 000 nest records from seven swallow species across the entire Western Hemisphere, clutch sizes decline with relative laying date in each population, but the slope of this decline grows steeper with increasing distance from the equator. Late-laying birds at all latitudes lay clutches of similar sizes, suggesting that latitudinal differences may be driven primarily by earlier-laying birds. Focused comparisons of site-years in North America with qualitatively different food availability indicate that food supply significantly affects mean clutch size but not the clutch size-lay date regression. Other studies on the seasonality of swallow food also indicate that steeper clutch size-lay date declines in the North are not caused by steeper earlier food peaks there. The distribution of lay dates grows increasingly right-skewed with increasing latitude. This variation in lay-date distributions could be due to the predominance of higher quality, early-laying (and large-clutched) individuals among populations at higher latitudes, resulting from latitudinal variation in mortality rates and the intensity of sexual selection. Our results underscore the importance of studying clutch size and lay date in tandem and suggest new research into the causes of their joint geographic variation.

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We conducted a study of the breeding biology of the White‐rumped Swallow Tachycineta leucorrhoa nesting in nestboxes in a flat, farming landscape in Buenos Aires Province, Argentina. White‐rumped Swallow nesting attempts were detected from the end of September to mid December, with most clutches laid during October. Birds laid clutches of 4–6 eggs with a mode of five eggs; most broods hatched synchronously (58%), but hatching spread could last up to 4 days. Nestling growth curves adjust well to logistic functions, and at day 15 nestlings attain the asymptotic weight of 21.6 g. Clutch size in White‐rumped Swallows declined significantly as the season progressed. In addition, late‐season eggs were smaller and late‐season nestlings had a shorter nestling period and lower weight at day 15, probably leaving the nest lighter than early‐season nestlings. These data suggest that the Swallows would benefit greatly from laying early in the season, which would provide nestlings with better survival prospects. However, both major sources of nest mortality, interspecific competition for nest‐sites and nestling mortality during bad weather, decreased through the season. White‐rumped Swallows follow the pattern found for other southern species, as it has smaller clutch size, lower growth rate and remains longer at the nest than its Northern Hemisphere congener the Tree Swallow Tachycineta bicolor.

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Life‐histories and demographic parameters of southern temperate bird species have been little studied. We estimated return rates between years and sexes, and adult apparent survival and recapture probabilities with mark–recapture data on White‐rumped Swallows and found a lower return rate of unsuccessful females. There was little support for influences of sex or year on survival rates. The estimates were equivalent to the lowest value reported for a northern congener, in contrast to the prediction of geographical variation under life‐history theory.

Egg mass production is costly, but hatching from heavier eggs could be beneficial for the nestling's phenotype and fitness. Egg mass could be influenced by proximate causes, like food abundance, female condition, environmental conditions, and/or by ultimate causes, such as females depositing resources differentially within a clutch to increase the biological fitness of some eggs. Yolk mass, although poorly studied, is the source of nutrients for the embryo, so its mass should be more influential for the nestlings than total egg mass. We used a technique that allowed us to measure yolk size without destroying the eggs. We studied yolk mass in 212 eggs of White-rumped Swallows (Tachycineta leucorrhoa) and found that yolk mass was influenced by laying order, with last laid eggs having heavier yolks than first laid eggs, and the pattern was consistent with egg mass variation. Food abundance also affected yolk mass: when insect availability was high the yolks were heavier. We conclude that embryos in the last laid eggs have more resources from which to develop, and excluding food abundance, neither environmental conditions nor female's condition affected yolk mass. We encourage other researchers to study yolk mass given that multiple variables affected total egg mass.

Life history traits exhibit substantial geographical variation associated with the pace of life. Species with a slow pace are expected to invest more in their future/residual reproductive value and are more common at tropical latitudes, whereas species from high latitudes, with a faster pace, are expected to prioritize the current reproductive eff ort. Most evidence supporting this pattern comes from studies conducted in tropical and north temperate species; very little is known about patterns in southern South American species. Here, we describe the life history of a southern swallow Tachycineta leucorrhoa and use an experimental approach to test their breeding strategy over four breeding seasons. We manipulated brood size for 105 nests of white-rumped swallows to measure whether costs of reproduction were borne by adults or nestlings as alternative selection strategies towards maintaining residual or current reproductive value. Adults increased their feeding eff ort in enlarged broods, at least enough to maintain nestlings ' development/growth. In addition, adults decreased the number of visits to the nest (without having a negative eff ect on nestlings) in reduced broods. We did not detect diff erences in fl edging success among treatments, suggesting there were no diff erences in nestlings ' survival. However, enlarged broods more frequently incurred in complete nest failure, suggesting only some adults were able to cope with increased costs of reproduction. We conclude this species is characterized by a fast pace of life similar to their northern congeners and less like its tropical ones. Th is is one of the fi rst studies to use an experimental approach to test a life history hypothesis of pace of life using data from a southern South American species. We encourage researches to include southern species when evaluating latitudinal variations as we still do not have enough evidence to assume all southern subtropical species are indeed similar to tropical ones.