Seasonal variation in food availability influences the breeding strategy of White‐collared Blackbirds Turdus albocinctus on the Tibetan Plateau

Abstract

Parent birds show a continuous spectrum of breeding strategies, ranging from a low-fecundity and high-survival pattern to a high-fecundity, low-survival pattern. Investigations of parental breeding strategies under variable environmental conditions can illustrate how parents trade-off the benefits and costs of these two extreme strategies. White-collared Blackbirds Turdus albocinctus can breed twice a year on the Tibetan Plateau. We show that both life-history traits and parental feeding behaviour differ between these two breeding attempts. In the first attempt, the birds produced small clutches and fledged a small number of nestlings of high body condition. In the second attempt, they produced larger clutches and fledged more nestlings of lower body condition. Males made greater contributions to brood provisioning compared with females in the first attempt but there was no sex difference in brood provisioning in the second attempt. In the first attempt, producing smaller clutches can shorten the nestling period, and the increased male contribution to brood provisioning can protect the energy reserves of females. Thus, females can begin a second attempt sooner and produce larger clutches. During the second nesting attempt, when conditions are warmer and wetter, parents rely on a broader array of food types (both invertebrates and plant material, primarily berries) than during the first attempt, which includes only animal food such as arthropods and annelids. We suggest that this difference in breeding strategies between nesting attempts and sexes is in part influenced by marked seasonal variation in food availability.

Individual variation in parental tradeoffs between the number and size of offspring at the pre‐ and post‐natal stages

“…Nearly half of the territories had active nests in both periods, indicating that some individuals bred twice a year (first nesting cycle: from early April to late June; second nesting cycle: from early July to late August). Brown-cheeked Laughing Thrushes are extremely sensitive to the trapping procedure (two criss-crossed strings above the nest, each with eight scalable loops), which has been successfully used to capture other shrub-nesting birds such as the White-collared Blackbird (Fan et al 2017) and the Azure-winged Magpie Cyanopica cyanus (Ren et al 2016). Adults rarely returned to the nest if a trap was present.…”

“…Mean annual temperature is 7°C and precipitation is 500-700 mm (Liu et al 2011). Both temperature and precipitation vary greatly by month, with June to August having the highest temperature and rainfall (Fan et al 2017). Local vegetation is dominated by evergreen Chuan-Dian Alpine Oak forests, mixed with some deciduous trees such as Aspen Populus davidiana and Tibetan Willow.…”

“…In contrast, double-brooded parents raise two broods a year. In the high-elevation environment that has a shorter growth period for plants, the earlier the first breeding cycle ends, the more likely the second breeding cycle will succeed (Fan et al 2017). Under these conditions, a survival strategy (smaller clutch with larger eggs) in the first brood can shorten this breeding cycle so that parents can begin the second breeding cycle earlier.…”

“…Because the duration of post-fledging parental care for new fledglings is limited between two breeding attempts, particularly because of the preparation by parents for the second brood, fledglings of higher quality have a higher probability of survival before they can forage independently. In contrast, the 'brood reduction' strategy in caring for the second, larger brood allows parents to select for offspring of higher quality, given that the breeding season then ends and fledglings have limited time to learn to forage independently before winter (Fan et al 2017). Under these conditions, offspring of higher quality are more likely to survive the winter than are those of lower quality.…”

“…For example, Horned Larks Eremophila alpestris increase clutch sizes over the course of the breeding season because food supply increases, while the risk of nest predation decreases, with seasonal plant growth (Du et al 2014). In White-collared Blackbirds Turdus albocinctus, double-brooded parents produce and raise more offspring in the second than in the first brood because their diet is broadened with the maturation of berries (Fan et al 2017). It should be noted that different responses to the same environmental changes in terms of parental care patterns depend largely on individual resource-holding potential (Clutton-Brock 1991, Eggers et al 2006.…”