Songbirds in seasonal environments often adjust their breeding strategy according to spatial or temporal changes in breeding conditions. Here we investigate how horned larks Eremophila alpestris, a multi‐brooded songbird on the Tibetan Plateau, responded to the changing risk of nest predation and food availability across breeding attempts. We showed that both nest concealment and food supply increased with plant growth, and horned larks adjusted their breeding strategies accordingly. First they selected nest‐sites where predator density was low, which enhanced nest survival. Second, clutch size increased with improving breeding conditions. They did not adopt an ‘egg‐size’ strategy as egg size did not change with laying sequence or breeding attempt. Instead, they adopted the ‘brood survival (feeding later‐hatched nestlings more)’ and ‘brood reduction (feeding early‐hatched nestlings more)’ strategies during early and later attempts. Moreover, nestlings’ growth varied with breeding attempt: more energy was invested into the growth of body mass during the first attempt but more energy was expended on the growth of linear structures during later attempts. This difference in energy allocation reflected changing food availability. We suggest that temporal changes of environmental factors are also the important force driving the evolution of avian breeding strategies.
Altricial birds may vary their relative reproductive investment at the egg versus nestling stages as an adaptive strategy to deal with variable or unpredictable food availability between stages. We investigated the parental investment strategies of Giant Babax (Babax waddelli) breeding in a harsh, high-altitude environment on the Tibetan plateau. In the modal clutch size of three, egg size declined strongly with laying order, resulting in an intra-brood size hierarchy throughout the nestling period. However, provisioners counteracted the initial handicapping of smaller, last-hatching young by two means: (1) preventing siblings from competing over a predictable feeding site; and (2) preferentially feeding the smallest nestling. Consequently, nestling growth curves differed near significantly by hatching order. We suggest this contrasting investment during the egg and nestling stages may be a strategy to maximize reproductive success in a harsh and unpredictable, high-altitude environment. The handicapping of last-laid eggs provides the opportunity for more efficient brood reduction in unfavorable conditions, but can be counteracted by preferential feeding under favorable conditions.Keywords Babax waddelli Á Parental investment strategy Á Brood reduction Á Brood survival Á Size hierarchy Zusammenfassung Riesenbabaxe (Babax waddelli) vermischen Strategien zum Brutverkleinern und Brutü berlebenNesthocker variieren ihre relativen Fortpflanzungs-Investitionen während der Ei-und Nestlings-Phasen als eine Anpassungsstrategie, um das wechselnde, oder nicht vorhersagbare, Nahrungsangebot in den unterschiedlichen Stadien in den Griff zu bekommen. Wir untersuchten die elterlichen Investitionsstrategien des in einer rauen Umgebung auf dem Tibetischen Hochplateau brütenden Riesenbabax. Bei einer modalen Gelegegröße von drei Eiern nahm die Größe der Eier stark mit der Reihenfolge, in der sie gelegt wurden, ab, was in einer brutinternen Größen-Hierarchie während der gesamten Nestlingszeit resultierte. Die Fütternden wirkten diesem anfänglichen Handicap der kleineren, später geschlüpften Jungen aber auf zwei Arten entgegen, indem sie: (1) die Jungen davon abhielten, an einem Futterplatz miteinander zu konkurrieren und (2) die kleineren Nestlinge beim Füttern bevorzugten. Die Wachstumskurven der Nestlinge unterschieden sich beinahe signifikant entsprechend der Reihenfolge des Schlüpfens. Unserer Meinung nach könnte dieses sich voneinander abhebende Investieren während der Ei-und Schlüpf-Stadien eine Strategie zur Maximierung des Fortpflanzungserfolgs in einer rauen, unberechenbaren und hoch gelegenen Umwelt sein. Diese Benachteiligung der später gelegten Eier bietet eine Gelegenheit für eine effizientere Brutverkleinerung bei ungünstigen Bedingungen, der aber bei günstigen Bedingungen durch eine Bevorzugung beim Füttern entgegengesteuert werden kann. Communicated by T.
In many species, nestling demands vary continuously during early development and both parents have different parental care strategies at each nestling age. Sexual conflict arises when each parent expects its partner investing more in parental care. It is largely unknown how the two parents respond to the dynamics of nestling demands and resolve the sexual conflict during nestling period, especially on Qinghai-Tibetan Plateau. To address this question, we monitored parental care behaviors of horned larks (Eremophila alpestris) using video-recording systems. We found that male horned larks invested less in parental care, but had a larger body size than females, which is consistent with the parental investment hypothesis. Only the female brooded nestlings, but both parents contributed to feeding efforts. Feeding rates of males and females were negatively correlated, indicating that they used evolutionarily stable strategies. Strategies of parental care via nestling age were sex-specific. Females continuously adjusted care behaviors to follow the dynamics of nestling demands as nestling age increased, such as decreasing brood attentiveness and increasing feeding rate. By contrast, male feeding rate showed no significant correlation with nestling age, but increased with the synchrony feeding rate. We suggest the synchrony feeding behavior may act as a control measure for females to promote and assess the males' contribution. We consider low mating opportunities drive males to act as assistants for females, and correspondingly cause males to pay less attention to nestling demands than females.
Life-history theory assumes that selection favors parents that can maximize their reproductive success via behavioral strategies. As brood size determines the reproductive value of each nestling, parents may adjust their food-allocation patterns according to brood size. We test this assumption in the Horned Lark (Eremophila alpestris (L., 1758)). Our findings revealed that nestling begging forms varied with brood size, by gaping in one-chick broods and postural activity in two-and three-chick broods. Accordingly, parental food-allocation patterns differed in different-sized broods. In one-chick broods, parents increased feeding rates with the gaping duration of nestling. In two-chick broods, parents did not change food-allocation patterns according to nestlings' begging. In three-chick broods, however, they fed later-hatched nestlings more even when early-hatched nestlings begged more intensely. Horned Larks exhibited obvious sexual differences in parenting style and ability, which resulted in nestlings from two-and three-chick broods changing their begging intensity according to the sex of the provisioning adult. Furthermore, nestling growth pattern diverged with brood sizes, with body mass growing faster in one-chick broods than in two-and three-chick broods. Growth rate of beak gape and tarsus length did not differ significantly among brood sizes, but beak gape was larger and tarsus length was shorter in one-chick broods than in larger broods at fledging. Our results thus support the idea that parents may use food allocation to regulate sibling rivalry, which in turn cause nestlings to beg food in different forms and grow in different patterns so that their reproductive success can be enhanced.Résumé : La théorie du cycle biologique repose sur l'hypothèse voulant que la sélection favorise les parents qui peuvent maximiser leur succès de reproduction par des stratégies comportementales. Comme la taille de la couvée détermine la valeur de reproduction de chaque oisillon, les parents pourraient ajuster leurs habitudes d'attribution de nourriture selon la taille de la couvée. Nous avons vérifié cette hypothèse chez l'alouette hausse-col (Eremophila alpestris (L., 1758)). Nos constatations révèlent que les formes de demande des oisillons varient selon la taille de la couvée, allant du bâillement pour les couvées d'un oisillon à une activité posturale dans les couvées à deux ou trois oisillons. Par conséquent, les habitudes d'attribution de nourriture par les parents varient selon la taille de la couvée. Dans les couvées à un oisillon, plus la durée du bâillement de l'oisillon est grande, plus le taux d'approvisionnement par les parents est élevé. Dans les couvées à deux oisillons, les parents ne modifient pas leurs habitudes d'attribution de nourriture selon la demande des oisillons. Dans les couvées à trois oisillons, cependant, ils nourrissent plus les oisillons éclos plus tard, et ce, même quand les demandes des oisillons plus précoces sont plus intenses. Les alouettes hausse-col présentent des différences sexue...
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