Parental Care Strategy Under Ecological Constraints In A Cooperatively Breeding Bird Tibetan Ground Tits, Pseudopodoces Humilis

Tibetan ground tit (Pseudopodoces humilis), a passerine inhabiting alpine tundra of 2500-5500 m elevation on the Tibetan plateau. The birds pair monogamously and some pairs have helpers at the nest. They live in all-purpose family territories year round and rely almost completely on soil arthropods. As is the case of many insectivorous cooperative breeders, rainfall during the breeding season is a key factor limiting food availability of the ground tits. Rainfall in these extreme environments varies greatly from year to year, and the quality of territories occupied by different ground tit pairs differs in terms of soil moisture and vegetation condition. In the unpredictability of the environment, we have a good opportunity to study in parental provisioning strategy how is evolved. In addition, we study the helping behavior how influence nestlings growth and incubating behavior of female.Field work have been carried out at Dangxiong (30°28′N,91°05′E,elevation 4300 m), northern Tibet since 2007.(1) The egg mass decrease as the incubation process continued due to the evaporation of moisture content. No significant within brood egg mass difference was detected, however, the egg mass changed with the laying order. The third egg was always the heaviest, the first and the last were relatively lighter than others. There is a significant difference with the third and the last one. The probability of abandonment by the females was greater for those lighter eggs when the brood reduction occurs. The first model indicated the fresh egg mass is unrelated to the presence of helpers, however, it is significantly influenced by the age of the breeding female, the territory quality and the interaction of the two. The yearling females had a later laying dates and a lesser egg mass than those elders with a greater body mass. The elder females with experienced foraging skill may provide them with greater nutrition reserves for eggs. The second model focused on the time and frequency of courtship feeding. The female’s foraging time have a strong influence on the courtship feeding frequency they gain from the males. When the time of foraging was controlled, the relations between the courtship feeding frequency and the time of incubation on-bout vanished. Correlation analysis shows that the foraging time decreased as incubation process carried on. The time of incubation show the same tendency, but not as evident as the former. The third model indicates the age of the females, clutch size and the interaction of foraging time with clutch size also had a strong influence on the time the female spent on-bout, as well as their own foraging time. The elder females had a lesser time of incubation on-bout, while the rates of clutch reduction among the different age groups were the same. We suggest that as the risk of predation is low, the experienced female can enhance its future survivorship and reproduction as they establish a high frequency of incubation interruptions for foraging outside the burrows. With the fourth model we found no variable include the female age was detected to have a significant effect on the nest attendances. As for incubation day no influence the nest attendance, it is consistent with the results of which nest attendance is not vary in whole incubating period. As no variable was detected to have a strong influence on nest attendance, the females may have a self-adjust mechanism for the behavioral effort they devote to their offspring. The males, which often call at the entrance of the nesting burrow to recommend their mates for a "break", may provide an assistance for this mechanism.(2) Optimal provisioning models specific to cooperatively breeding systems predict that when nestlings suffer starvation due to local food limitation, helped parents should maintain their effort to avoid productivity loss, and reduce their effort to save energy for themselves if nestlings experience no starvation. However, food shortages also increase provisioning costs so that selection might favor parents to lighten their workload. This idea was tested with the Tibetan ground tit (Pseudopodoces humilis), an insectivorous, facultatively cooperative breeder, using data from three breeding seasons over which rainfall-induced food resources varied greatly. Clutch size and egg hatchability did not depend on whether a pair was helped or not, or on food availability. However, brood reduction was lowest for group-fed broods in favorable environments and similarly high for group-fed broods in unfavorable environments and pair-fed broods in any environment. Compared to unhelped parents, helped parents invested less regardless of food availability, but where food resources were sufficient, group-fed broods received more food due to the addition of helpers’ contributions, which were independent of foraging conditions. These results would appear contrary to the traditional models and reveal that parental ground tits value their own survival at the expense of offspring productivity in the overall harsh, unpredictable environments where the birds have to trade off future fitness against current reproduction.(3) As a cooperative breeding species, the breeding pairs may gain two main fitness benefits from the helpers:load lightening, which may good for their future reproduction; and the increase of their current reproduction, such as an increase in brood size and the enhanced quality of the offspring. Our research mainly focus on the later. According to the analysis of GLMMS and a general liner model, the presence of helpers have a significant effect on the body mass, tarsus length, wing length and body condition of the offspring. For growth rate, only the increase of body mass was strongly influenced. Besides, the brood size and the male age also have an effect on the development of offspring. Thus, offspring in the families with helpers benefit from a low risk of starvation and a higher body mass as fledglings, which may enhance their survivorship in the harsh environment of the coming winter, which is consistent with our analysis which only the presence of the helper and the survivorship of the offspring are considered. In addition, the presence of the helpers also enhance the survivorship of the breeding females. No effect on brood size and the survivorship of the breeding male was detected. Further studies on the analysis with the control of other variables (e.g. the territory qualities) on the survivorship of adults are needed.(4) During the breeding season, the times of the breeders entering (leaving) burrows closely follow the sunrise (sunset) curve with small fluctuations. Fledglings gradually developed a behavioral mechanism for the changes of relative length of day and night. During the periods of incubation and pre-fledgling brood care, the females always leave the burrow later in the morning and enter the burrow earlier than its mate in the evening. After the offspring began to come out of their nest during the daylight, the males remained to be the first to came out of their nest in the morning, however, there are no significant differences at the time of night burrow entering between the males and females, but the fledglings’ evening-entering time are much earlier than the females. After offspring gain independence, the evening-entering time of females returned to be much earlier than males. The weather conditions as well as the light, have a significant influence on the bird’s burrow-entering (leaving) time as they came out earlier (later) on cloudy and rainy days. We suggest that the times and orders of burrow-entering (leaving) of the family members depend on the different roles they play during the whole breeding process. As a female-only incubation species, the energy demand are higher than males, which make them to spent more time to stay inside the burrow and take good care of the eggs and newly-born offspring. The male’s job at this period may focus mainly on territory defense and detection of the predators, so they take those risks first to keep danger out of other members of the family, and bring food to them. The offspring, as the most vulnerable and the core part of the reproduction, they were protected by both of their parents.