Cave Life Adaptation Mechanism Of Himalayan Swiftlets(Aerodraus Brevirostris)

The Himalayan Swiftlets (Aerodramus brevirostris), subordinate to Apodidae family and Apodiformes order, comprises three subspecies and can be found in China. The Sichuan subspecies (A.b. innominata) is distributed in Hupingshan National Nature Reserve (NNR) in Shimen County, where it is by far the only distribution area known in Hunan. And the species is also distributed in Shanghai, Sichuan, Guizhou, Hubei, Guangxi and Hongkong SAR in China. The study is a tough one due to certain features of the species, such as small population size, secluded and narrow habitat environment, and particular life habit including nesting in dark caves, rapid flight speed, strong flight performance, as well as wide sphere of activities. Thus domestic and international researches are rare, especially the research on the adaptation mechanism of its cave life and breeding ecology.The study started in1994and ended in2012, choosing the Shenjing Post for work strongholds, which is located in the core area of Hupingshan NNR. Diverse research methods were adopted, such as day by day consecutive point observation, field survey, morphological structure anatomy, sound wave characteristics analysis, bird banding mark, film and video, infrared unattended monitoring, etc. A certain level of systematic studies have been conducted, mainly on the species selection mechanism of nesting site, echolocation mechanism, breeding ecology behavior, migration behavior, day and night activity rule, and molecular system evolution, etc, with the findings shown as follows:1. The species has a small population and is only distributed in a very narrow region. There are only two distribution areas found in karst caves within the study area, among which there are about1000nests with2000individuals in Shenjing karst cave, while approximately60nests and120individuals in Yingzijian karst cave. The bird banding and individual mark indicates this species is monogamous with a sex ratio of about1:1. The nest and its location are exclusive with no sign of fights and invasion. The species in Shenging Cave has been separated into two "families", which settle in different sections of the cave in terms of spatial distribution.2. Based on the relationship of its chirping and the behavior, the chirping of Himalayan Swiftlets can be categorized into:positioning, alarming, playing, and nestling’s chirping. In June2012in Shenjing Cave, the free-flying echolocation calls were recorded with Avisoft-UltraSoundGate116(e), and the results were analyzed by certain sound software (BatSound software, release1, Pettersson Elektronik AB, Sweden). The results suggested that the echolocation sounds of Himalayan swiftlets are broadband double FM clicks, separated by a short pause, and the inter-pulse intervals between double clicks (99.3±3.86ms) were longer than those within double clicks (6.6±0.42ms)(t-test:t=-23.888, P<0.01). Except the peak frequency (6.2±0.08kHz,6.2±0.10kHz, respectively; t=0.572, P>0.05) and pulse duration (2.9±0.12ms,3.2±0.17ms; t=-1.550, P>0.05), others (maximum frequency, minimum frequency, frequency bandwidth, and power) were significantly different between the first and second click. The maximum frequency of the first pulse (20.1±1.10kHz) was higher than that of second (15.4±0.98kHz)(t=3.239, P<0.01), while the minimum frequency of the first pulse (3.7±0.12kHz) was lower than that of second (4.0±0.09kHz)(t=-2.316, P<0.05); resulting in the frequency bandwidth of the first pulse (16.5±1.17kHz) was longer than that of second (11.4±1.01kHz)(t=3.303, P<0.01). The power of the first pulse (-32.5±0.60dB) was higher than that of second (-35.2±0.94dB)(t=-2.463, P<0.05). More important, we found that Himalayan swiftlets emitted echolocation pulses including ultrasonic sound, that the maximum frequency reached33.2kHz.3. Analysis of nesting cave selection mechanism. The clustering analysis concerning habitat selection impact factors in Shenging Cave has been conducted by SPSS11.5. Findings indicate that four safety conditions are required for the nesting selection. The first is the height of the nest, which requires3m higher above the ground (the actual measurement is at least2.5m), often beyond man or other animals’reach so as to keep the nest from being damaged. The second is the "islet condition", referring to the passageway between the nest and the ground to prevent other animal from climbing to the nest. The third is the light condition, for Himalayan Swiftlets can move flexibly by echolocation capability while other animals’ movement may be confined in dark caves. And the Fourth is the humidity, which means sufficient humidity may help maintain good stickiness of saliva and thus keep stable structure of the nest.4. Breeding ecology behavior of Himalayan Swiftlets. The courtship process may be fulfilled in wintering ground on the basis of monogamy, with mating conducted in the air or in the nest. The old nest can be fully reused, only mended before hatching. For those with no nest or the nest destroyed, they have to build new ones, which are usually on secure cave walls suitable for living in groups. The nests are primarily composed of saliva and mosses, mixed with a little feather and grasses. The nest weights17.84±4.18; in terms of nest measurement, the external and internal diameter as well as the depth are (88±7.78)×(78.75±8.43),(71.83±7.95)×(66.91±8.40) and25.25±8.10respectively. Nest construction can take up to60days. The eggs are oblong, white, unspotted, with a size of21.3×12.1mm and a weight of2.7g. The earliest egg laying starts on31May, while the latest on around20July. If nest building is ongoing, the egg laying is often delayed, with the earliest date known on26June. In most cases, two eggs can be found for each couple each reproductive season, yet one egg per couple is also occasionally observed, thus the average amount for each couple is1.7eggs. The egg laying usually goes on continually, occasionally ceasing for just one day or two. And egg hatching starts immediately after the laying of the first egg. Both males and females are involved in hatching but females invest more time in it. The hatching temperature is37.28±1.02℃, the duration is27.85±3.39days, and the hatching rate is71.43%. The brooding sequence conforms to the hatching sequence, difference in1-3days. Nestlings usually weigh1.68±0.08g, with little body totally pink, covered with no feather and eyes yet not open when just getting out of shells. Nestlings are altricial brooding, and are fed by parents for about27days. After that, they spend some extra3or4days learning flying in the cave before turning into fledglings. Fledglings gain substantial weight before leaving parents’nests which could be even more than that of an adult. They will then lose some weight before getting fully mature.It has also been found that the natural enemy of Himalayan Swiftlets in caves is mainly the Rattus edwards, which occasionally threatens the eggs, nestlings and even adult birds, however with minor danger to Himalayan Swiftlets; while the natural enemies outside caves primarily are raptorial birds like Accipiter virgatus offinis and Buteo buteo burmanicu, etc.Day and night activity rules. Certain monitoring indexes, such as morning-evening leaving and homing status, frequency of homing at day, rate of staying at nest, and birds clustering behavior etc, have indicated that they are sufficiently in line with seasonal change and species’ breeding behavior rules. Basically the species repeat their behavior model, which can be called "with the lamb, and stopped work at sunset". That is, at early days of breeding, the rate of staying at nest at day and the rate of homing at evening are increasing, which generally reach the peak at the middle period of breeding. And the birds clustering often occurs at the early and the late period of breeding.5. The morphological characteristics:The Himalayan Swiftlets weighs19.29±1.41g and the body length is130±3.69mm. Its beak is flat and of short width, with the width of beak base almost equal to the length of culmem. The upper part of its body is infuscate, with light-colored waist feather decorated with crineous scapus texture; the lower part of the body is taupe with distinct scapus texture. The length of the wing is larger than or equal to the body length, exceeding tail end when folding. The scape of the tail feather is of high resilience; four toes onward suitable for perching on cave wall; and the tarsometatarsus covered with feather, whose rear side has a blanket of thin callus due to the long-time supporting itself against the cliff. Other morphological features include:the front toe and the four toe tips have well developed claws; the flexible tail feather helps it rest on cave walls; small body type and swift flight make it appropriate for pass through narrow and zigzag caves freely.The internal structure adapts it to dark cave life. In regard of alimentary system, the salivary gland is capable of secreting viscous saliva, and the elastic esophagus without craw can reserve food bolus made by captured insects for nestlings. Additionally, the species has a well functioning digestion system with gastric emptying time for about6hours. As to its circulating system, the cardiac is more powerful than the common birds, with the weight accounting for2%of its body weight. When discussing the skeletal system of Himalayan Swiftlets, the skeleton of the posterior limbs is particularly suitable for cave behavior, for the tail tip of the pubis is1/3longer than the ischium, which can better protect its pelvic cavity; and the far end of metatarsal bones is formed into a typical "roller" joint mode, composed of the2nd,3rd and4th trochlea metatarsus at the same level. Data show that the length of tibiotarsus of the posterior limb bone is the longest, with the femora longer than the metatarsal bones, and the length ratio among the femora, tibiotarsus and metatarsal bones is1:1.57:0.75, similar to other species of Apodiformes order while different from other arboreal species. All those morphological structure features have reflected that the structure and functions of Himalayan Swiftlets conform well to its cave life behavior.The experimental results of molecular system evolution analysis indicate that the Himalayan Swiftlets may have a relatively close genetic relationship with Edible-nest Swiftlet (Aerodramus fuciphagus), both of which have the capability of echolocation.6. The Himalayan Swiftlets is the summer residents, coming at early April and leaving at early November. The way of its emigration is one-time clustering, while the way of immigration is divided into several groups with more than once activities, with resident period for209.4±1.4(207-213,n=9) days. The migration activity is closely linked to climate and food factors.