Hatching asynchrony and food allocation within broods of Pied-billed Grebes, Podilymbus podiceps

Abstract

FORBES, M. R. L., and ANKNEY, C. D. 1987. Hatching asynchrony and food allocation within broods of Pied-billed Grebes, Podilymbuspodiceps. Can. J. Zool. 65: 2872-2877. Broods of Pied-billed Grebes, Podilymbus podiceps, breeding near Minnedosa, Manitoba (55'16' N, 99'50' W), in the summer of 1984, hatched from five to eight eggs over 3-7 days. We found a negative correlation (partial r = -0.56, df = 1 1, p < 0.05) between the length of the hatching interval and the number of chicks surviving to 35 days of age, after controlling for clutch size. The mean number of sibling aggressions ( i = 1.6, n = 20, 2 SE = 0.4) was higher during foraging bouts with below-average rates of food delivery by parents, as opposed to bouts (.i = 0.5 aggressions, n = 18, 2 SE = 0.2) with above-average rates. Also, larger chicks initiated and won more disputes over positions near adults (Fisher's exact test, p < 0.05, n = 40) than did their smaller sibs. Thus, size hierarchies among siblings influenced food allocation within Pied-billed Grebe broods. Food distribution, however, was nonrandom for 12 of 18 foraging bouts in which there were no apparent size differences between attendant chicks. Signals of hunger status from begging chicks, and parental regulation of which chicks maintained positions near them, also affected food allocation among siblings. Finally, handling times of prey, captured by adults and fed to chicks, depended on brood age (partial r = -0.21, df = 183, p < 0.05) after controlling for prey size. For Pied-billed Grebes, noticeable size hierarchies among chicks remained throughout most of brood development and may have been advantageous during times of low food availability, or may have enabled parents to forage more effectively on a wide range of prey types and sizes. FORBES, M. R. L., et ANKNEY, C. D. 1987. Hatching asynchrony and food allocation within broods of Pied-billed Grebes, Podilymbus podiceps. Can. J. Zool. 65 : 2872-2877. Des portCes de Grkbes a bec bigarrC, Podilymbuspodiceps, des environs de Minnedosa au Manitoba (55'16' N, 99'50' W) ont donne cinq a huit oeufs eclos en 3-7 jours durant 1' CtC 1984. Aprks avoir apporte les corrections nkcessaires pour compenser lenombre d'oeufs par portee, il ressort une correlation negative (r partiel = -0,56, dl = 11, p < 0,05) entre la longueur de l'intervalle d'Cclosion et le nombre de petits qui survivent jusqu'a 1'8ge de 35 jours. Le nombre moyen d'agressions au sein d'une portie (i = 1,6, n = 20, 2 E.T. = 0,4) est plus ClevC lorsque les parents apportent une quantite de nourriture infkrieure a la moyenne que lorsque cette quantitC est superieure a la moyenne (i = 0,5, n = 18,2 E.T. = 0,2). De plus, les oisillons de grande taille initient et gagnent plus de disputes pour maintenir une position favorable pres des parents (test exact de Fisher, p < 0,05, n = 40) que ne le font les oisillons de la mime portCe qui sont de plus petite taille. Ainsi, une hierarchic basCe sur la taille influence la distribution de la nourriture chez les oisillons de la portee. Cependant, ...

Environment quality predicts parental provisioning decisions

“…However, many ¢eld studies suggest that parents do combine cues (e.g. Forbes & Ankney 1987;McRae et al 1993;Malacarne et al 1994;Kacelnik et al 1995;Price & Ydenberg 1995;Leonard & Horn 1996;Ostreiher 1997;Ko« lliker et al 1998), for example, feeding according to nest position, which, as larger chicks are better able to shove their smaller siblings out of the way, and may be di¡erentially motivated to do so by the amount of food in their stomachs, is some combination of hunger and size. We therefore also tested a set of cue-combining strategies.…”

“…Empirical studies in birds have uncovered a variety of parental feeding patterns, including preferentially feeding the smallest chicks, largest or oldest chicks, chicks begging the most, and feeding in random order (e.g. Ryden & Bengtsson 1980;Reed 1981;Stamps et al 1985;Forbes & Ankney 1987;Smith & Montgomerie 1991;Redondo & Castro 1992;Martins & Wright 1993;Malacarne et al 1994;Kacelnik et al 1995;Mondloch 1995;Price & Ydenberg 1995;Leonard & Horn 1996;Clark et al 1997). Despite the large amount of published data on this topic, no proposal has yet been made for why such a variety of strategies should exist.…”

Prey Size Influences Female Competitive Dominance in Nestling American Kestrels (Falco Sparverius)

“…Thus, moderate competitive disparities among nestlings may enhance parental fitness (e.g., Mock and Ploger 1987). However, excessively disadvantaged nestlings grow poorly or die even when food supplies are sufficient to support broods with more equally matched nestlings (Forbes and Ankney 1987, Skagen 1987, Amundsen and Stokland 1988 and references therein). The sex difference in competitive ability in kestrels could impose such excessive handicaps on sons, and lead to maladaptive brood reduction that would not occur in broods with less sexual size disparity.…”

Prolonged incubation of unhatchable eggs in Red-necked Grebes (Podiceps grisegena)

“…(1) additional costs of incubation (Ricklefs 1993); (2) decreased renesting chances if the entire clutch is unviable (Holcomb 1970); (3) reduced parental care for previously hatched offspring during the first days after hatching, the period of highest chick mortality (Wobus 1964); (4) decreased chances of later chicks surviving; older siblings often monopolize parental attention with increasing developmental disparities within broods (Forbes & Ankney 1987); (5) increased risk of brood parasitism, a phenomenon occurring in Red-necked Grebes (Cramp & Simmons 1977). Provided that it is adaptive, reproductive advantages conferred by prolonged incubation must outweigh these risks.…”