Influence of intraclutch egg-mass variation and hatching asynchrony on relative offspring performance within broods of an altricial bird

Abstract

Rates of growth and size of nestlings at fledging have important consequences for future survival and reproductive success in many passerine birds. Within broods of altricial species, these characteristics are often influenced by size hierarchies established early in the nesting period due to hatching asynchrony and within-clutch variation in egg mass, but the concurrent effect of these factors is poorly understood. We investigated the relative influence of these variables on nestling performance within broods of tree swallows, Tachycineta bicolor (Vieillot, 1808). Nestlings that hatched earlier within nests were heavier and larger than their later-hatched siblings at 4 days of age. Similarly, earlier-hatched nestlings grew their ninth primary flight feathers faster and had longer ninth primaries just prior to fledging than did later-hatched siblings. Differences in egg mass of siblings also contributed to mass and size hierarchies at 4 days of age, but did not affect any other difference in sibling performance. We conclude that within-clutch variation in performance of offspring is determined primarily by size hierarchies resulting from asynchronous hatching. Intraclutch egg-mass variation appears to have little effect on performance of siblings and may be best explained by proximate constraints on females.Résumé : Les taux de croissance et la taille des petits au nid au moment de l'envol ont des conséquences importantes sur la survie future et le succès reproductif chez de nombreux passereaux. Dans les portées des espèces nidicoles, ces caractér-istiques sont souvent influencées par les hiérarchies de tailles établies tôt durant la période au nid à cause de l'asymétrie de l'éclosion et de la variation de la masse des oeufs dans une même portée; cependant, les effets concourants de ces facteurs restent mal connus. Nous avons étudié les influences relatives de ces variables sur le succès au nid chez des portées d'hirondelles bicolores, Tachycineta bicolor (Vieillot, 1808). Les petits qui éclosent plus tôt au nid sont à l'âge de 4 jours plus lourds et plus grands que les oisillons de même fratrie éclos plus tard. De même, les petits éclos plus précocement dé-veloppent leurs neuvièmes rémiges primaires plus rapidement et possèdent des neuvièmes rémiges primaires plus longues juste avant le moment de l'envol que les autres oiseaux de la fratrie qui ont éclos plus tard. Les différences de masses des oeufs dans une même portée contribuent aussi aux hiérarchies de masses et de tailles à l'âge de 4 jours, mais n'affectent pas les autres performances de la portée. Nous concluons que la variation de la performance des petits au sein de la portée est déterminée principalement par les hiérarchies de tailles résultant de l'asynchronisme de l'éclosion. La variation de la masse des oeufs dans la portée semble avoir peu d'effets sur la performance des petits d'une même portée et peut s'expliquer le mieux par des contraintes proximales chez les femelles.[Traduit par la Rédaction]

Trade-offs in the early development of tree swallows (Tachycineta bicolor): offspring characteristics, offspring quality, and parental care

“…Within clutches of tree swallows in my study, egg mass increased with laying order, which is in agreement with many previous studies of tree swallows (Zach 1982;Wiggins 1990;Ardia et al 2006;Bitton et al 2006; but see Whittingham et al 2007) and some other altricial species (Howe 1976;Ojanen 1983;Mead & Morton 1985;Hillstrom 1999;Saino et al 2004;Rosivall et al 2005;Liljesthrom et al 2012;Newbrey et al 2015). Like egg mass, yolk mass also increased with order of laying within clutches, as observed in previous studies of tree swallows (Ardia et al 2006), white-rumped swallows (Tachycineta leucorrhoa; Barrionuevo et al 2014), and red-winged blackbirds (Agelaius phoeniceus; Newbrey et al 2015), but in contrast to Chilean swallows (Tachycineta meyeni) for which size of yolk did not vary with laying order (Liljesthrom et al 2012).…”

“…In addition, these nestlings could possibly benefit from having a greater immune response than their older siblings by having a greater amount of carotenoids (Bortolotti et al 2003) and immunoglobulins (Graczyk et al 1994) from their larger yolks. This suggests that female tree swallows in my study population follow a "brood survival" strategy (Slagsvold et al 1984), as has been indicated for other tree swallow populations (Wiggins 1990;Ardia et al 2006;Bitton et al 2006). …”

“…Two related potential qualityindicating traits are egg mass and yolk mass. Both egg and yolk mass have been reported to increase with laying order (Zach 1982;Wiggins 1990;Ardia et al 2006;Bitton et al 2006;Newbrey et al 2015; but see Whittingham et al 2007), which indicates that female birds may allocate more egg components, such as albumen proteins and minerals and/or yolk lipids and carotenoids, to eggs they lay later in the laying order (Ardia et al 2006;Ferrari et al 2006; but see Newbrey et al 2015). One would expect that nestlings hatching from larger eggs laid later in the laying sequence would benefit from these extra resources and show improved performance, but experimental findings do not always support this expectation.…”

“…Although the last-laid egg is generally heaviest in tree swallows, these eggs also tend to hatch last (Zach 1982, Wiggins 1990, Bitton et al 2006. First-laid eggs, in turn, tend to hatch first (Clotfelter et al 2000).…”

“…First-laid eggs, in turn, tend to hatch first (Clotfelter et al 2000). First-hatched nestlings grow faster and are heavier than their later-hatched siblings at fledging, as well as having longer ninth primary flight feathers at fledging (Zach 1982, Bitton et al 2006. Early hatched offspring also are more likely to fledge successfully than their younger siblings (Zach 1982).…”