Giant Babaxes mix brood reduction and brood survival strategies

Abstract

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.

Azure‐winged Magpies Cyanopica cyanus trade off reproductive success and parental care by establishing a size hierarchy among nestlings

“…Two strategies used by parent birds, egg-size variation and asynchronous hatching, have differing importance in causing offspring size hierarchies in different species (Clark & Wilson 1981, Slagsvold 1984. For example, in Tree Swallows Tachycineta bicolor and Common Blackbirds Turdus merula, the influence of asynchronous hatching outweighs that of eggsize variation in establishing size hierarchy among nestlings (Zach 1982, Magrath 1992, but in the Giant Babax Babax waddelli, egg-size variation is the main cause of size hierarchy among nestlings (Du et al 2012). In the Azure-winged Magpie, parents adopt the egg-size strategy to reduce partially the size hierarchy among nestlings that is caused by hatching asynchrony.…”

Variation in egg and clutch size of the Black Redstart (Phoenicurus ochruros) at the northeastern edge of the Qinghai-Tibetan Plateau

“…Large eggs appear to have an advantage in harsh environments (Smith et al 1995;Fox and Czesak 2000). On the Tibetan Plateau, elevation affects the breeding ecology of birds (Lu et al 2007(Lu et al , 2008(Lu et al , 2010Du et al 2012Du et al , 2014 through the strong effects of temperature and rainfall, which also affect plant distribution and food availability. In our study area, temperature and rainfall were lower in 2008 than in 2009, suggesting that the climate in 2008 was harsher than in 2009 (Table 3).…”

“…The other strategy is the "brood-survival strategy", in which birds produce a relatively large final egg, which could increase the probability that the offspring hatched from such an egg will be competitive with its elder siblings and fledge successfully. However, some birds, such as the Giant Babax (Babax waddelli), could mix brood reduction and brood survival strategies to maximize reproductive success depending on food availability (Du et al 2012).…”

“…Over the past half century, studies of the impact of weather on the population biology of birds represent a major field of study for ornithologists (Crick 2004). Climate not only affects the metabolism of birds but also directly or indirectly affects the reproduction of birds (Zhao et al 2002b;Crick 2004;Lu et al 2007Lu et al , 2008Lu et al , 2010Du et al 2012Du et al , 2014. On the other hand, birds possess several mechanisms by which they can adjust the magnitude and pattern of their breeding effort in relation to environmental conditions and to their own breeding condition (Slagsvold et al 1984).…”

“…In many bird species, egg size is often considered an important index of egg quality and is correlated with the survival of their offspring (Murton et al 1974;Amat et al 2001). Egg size can be adjusted in females with changes such as laying date (Hill 1984), female age (Desrochers and Magrath 1993), seasonal change (Coulson 1963;Du et al 2012), laying order (Murphy 1994;Zhao et al 2002a) or the condition of the female body (Hőrak et al 1995), but no consistent conclusions about the effect of any of these factors have been drawn until now. Christians (2002) noted that although each of the above-referenced factors could explain part of the variation in egg size, these factors are not completely independent; therefore, it is possible…”