A Study On Parental Feeding Strategy And Nestlings Begging Pattern In Plain Laughing Thrush Garrulax Davidi

Parent-offspring conflicts in altricial birds provide us an ideal model to explore the evolution of social relationships in animal societies. In a subspecies of Plain Laughing Thrushes (Garrulax davidi) that breed in a harsh, high-altitude environment on the northeastern margin of the Tibetan plateau, I collected data concerning the reproductive biology and behavioral ecology, based on which I investigated the relationship between parental investment strategy and sibling rivalry pattern. Furthermore, I tried to develop a model applicable to a type of birds characterized by asynchronous hatching that was represented by Plain Laughing Thrushes.Plain Laughing Thrushes in large nests hatched asynchronously but fledged synchronously. Both parents participated in incubation and provision. According to the hatching order, I classified nestlings into core (hatched in the first day) and marginal offspring (hatched in the second day or later). Brood time was staged into dependent period (2days after hatching), interim growth period, and independent period (2days before fledging). Results showed that1) fresh weight of eggs had a mean value of5.17±0.36g. Initial body weight of hatchlings and fledglings were4.27±0.41g and32.85±3.16g on average, respectively.2) Brood reduction occurred at the pre-hatching stage, for the clutch sizes (3.00±0.53) were significantly larger than brood sizes (2.64±0.73), whereas there was no significant difference between brood size and the number of fledglings (2.50±0.69).3) Laying order had no effect on eggs’ fresh weight. However, clutch size significantly affect egg size, with three-egg clutches containing larger eggs (5.30±0.34g) than two-(4.98±0.03g) and four-egg clutches (4.95±0.31g).4) Initial body weight of new hatchlings was correlated neither with brood size nor the hatching order. Fledgling body weight did not vary across broods, but it exhibited significant intra-brood variation in relation to hatching order, with the first hatched nestlings being the largest at fledging (33.98±2.66g) and the fourth hatched nestlings the smallest (26.24±3.07g).5) Nestlings with different hatching order differed significantly in growth rates of body weigh, with the fourth-hatched nestlings growing slowest. By comparing the slopes of three linear measurements to the body weight, the fourth-hatched nestlings, however, grew significantly faster in wing length, beak length and tarsus length than their older siblings.6) Nestlings in the dependent period moved weakly and they competed with siblings by"display competition". Parents delivered food to the brood by a "tentatively re-feeding strategy"."Nestling with highest begging intensity were probably to be feed earlier than those with lower begging intensity because they became the first food-receivers in half of the feeding bout.At the nestlings’ dependent period, provisioning parent delivered more than one insect in each feeding bout, so that they could allocate food more times and feed more than one nestling each time. If food-receivers were not satiated, the food would be re-allocated to other nestlings.7) With the nestling age, the disposing capacity of Plain Laughing Thrush nestlings increased, and the form of the sibling competition changed from "display competition" to "Scramble completion". In the in-dependent period, nestlings begged initiatively, and parents delivered food by a "direct-single assignment strategy". Under the condition, nestling with highest begging intensity often became the first food-receivers and obtained more food than others.8) In total, provisioning parents adopted two feeding strategies, the "tentatively re-feeding strategy" and the "direct-single assignment strategy" at different nestling period to weaken sibling competition. In the dependent period, nestlings were easily to be satiated, and parents had enough time to deliver more food. Therefore, the feeding position has no effect on food-allocation of provisioning parents. So, the former tactic played the key role in weakening sibling rivalry, the CV in body weight of within-brood nestlingswas low. Meanwhile, the CV value did not increase with brood sizes. However, in the independent period, nestlings need more food to be satiated. Parents cannot deliver enough food to satiate one nestling in each feeding bout. Then, both the begging intensity and competitive behaviors of nestlings limited the food distribution within the brood. At this stage, it was the"provider changing delivery food orientation", but not the "tentatively re-feeding strategy", that played an important role in realizing balanced food-allocation among nestlings. The CV in body-weight of within-brood nestlings was same to that in dependent period in nests with small brood sizes (n=2,3). But, in nests with larger brood size (n=4), CV in body-weight of within-brood nestlings was much higher than that in the dependent period.Based on this study on parental feeding strategy and sibling competition of Plain Laughing Thrushes, I developed a new model to describe how provisioning adults distribute food within brood where nestlings compete over varying amount of resources. It indicates that in the dependent period, both the nestling begging instinct and parental activeness could affect food-distribution pattern. In contrast, in the independent period of nestling, it’s the nestlings’ activeness that influences food-distribution with the brood.