冠羽畫眉雛鳥競爭機制

Abstract

競爭是動物群體間無可避免的現象，對晚熟性雛鳥而言，親鳥所能提供的食物資源有限，加上鳥巢空間狹小，導致雛鳥之間常會發生激烈的競爭。冠羽畫眉(Yuhina brunneiceps)具特殊之參與式合作生殖行為，由多對無親緣關係的成鳥共用一巢哺育雛鳥。由於巢中雛鳥間的親緣關係較低，因而雛鳥之間產生利益衝突的可能性更高。本研究目的為探討冠羽畫眉雛鳥競爭的機制，並測試搶奪競爭模式(scramble competition model)或誠實訊息模式(honest signalling model)在雛鳥乞食行為的應用。搶奪競爭模式的預測：(1)雛鳥為了獲得足夠的食物，在競爭個體數越多的環境中，乞食表現會越激烈，因此當巢中雛鳥數越多，隨著競爭個體數增加，乞食時間越久; (2)當孵化順序不同步時，巢中雛鳥競爭能力差距較大，所以整體的乞食時間較同步孵化時短，且乞食時間差距在巢中雛鳥數增加時更大; (3)當孵化順序不同步時，較早孵化的個體體型較大且體力較佳，在乞食競爭上比晚孵化者占優勢，乞食時間較短，獲得食物比較多；(4)為提高乞食的投資報酬率，早孵化雛鳥會占據離親鳥回巢餵食最近的區域，以在短時間內獲取較多的食物量，減少體力的消耗。誠實訊息模式的預測：(1)雛鳥乞食表現受到飢餓程度的影響，獲得食物的時間間隔越長，雛鳥處於越飢餓的狀態，因此多由時間間隔較長的雛鳥獲得食物；(2) 雛鳥乞食表現受到飢餓程度的影響，飢餓雛鳥往親鳥餵食方向靠近，非由體型大的雛鳥獨占。本研究資料來自2005-2007年每年3-8月冠羽畫眉繁殖季，於巢邊架設錄影設備，從影片中記錄雛鳥的乞食持續時間、獲得食物的間隔、每小時獲得食物次數及乞食距離作為雛鳥競爭程度的指標。依巢中雛鳥數分為小(2-3隻)及大(5-6隻)兩組，從影片中監測雛鳥孵化同步性為同步或不同步(12小時以內孵化者為同步)，並將育雛時期分為前期(1-6天)及後期(7-12天)。研究樣本共計22巢，約5549筆紀錄，考量資料來源為22巢觀測值中重複取樣進行量化，因此採用廣義線性混合模式法(generalized linear mixed models, GLMM)進行分析。分析結果發現：雛鳥的乞食時間會隨巢中雛鳥數增加而增加，且不同步孵化的雛鳥其整體乞食時間較短。加入孵化同步性後，同步孵化的雛鳥其乞食時間會隨巢中雛鳥數增加而增加；不同步孵化的雛鳥雖然在大組的乞食時間也有增加的趨勢，但增加幅度較小。將雛鳥依孵化順序分為早孵化及晚孵化組，早孵化雛鳥的乞食時間顯著較短，獲得食物間隔較短，每小時獲得食物的次數較多，乞食距離也較近。本研究結果支持搶奪競爭模式假說下的四項預測，顯示不同步孵化是影響冠羽畫眉子代競爭能力的重要機制，而早孵化雛鳥有獨占食物的生存優勢。

Competition is widespread in animals. In altricial birds, nestlings highly rely on parent provision food. When food is limited and the nest is a narrow environment, it may cause the nestling competition. Taiwan yuhina (Yuhina brunneiceps) is an endemic species in Taiwan. They use join-nesting cooperative breeding system, almost formed non-kin group. Because the low relatedness between nestlings, it is more possible to induce the competitive conflicts of benefits. The object in this research is to investigate the mechanisms of nestling competition in Taiwan Yuhina and test between scramble competition model and honest signaling model. Under the scramble competition model, I predict that (1) to get more food, when brood size enlarges; the begging duration may get longer. (2) When asynchronous hatching, the competitive abilities would be large difference between nestlings, so that the whole begging duration would be shorter than synchronous hatching brood. (3) When asynchronous hatching, the early-hatched chicks have the dominance than late-hatched chicks such as obtaining more food and the shorter begging duration because the larger body and the better competitive ability. (4) To increase the rewards of begging, early-hatched chicks would jockey the nearest position. Under the honest signaling model, I predict that begging behavior reflects nestlings’ true need which is influenced by the hungry level so that (1) there is no difference between early-hatched and late-hatched chicks in eating interval. (2) When nestling become hungry, they would get closer to the feeding position of parents. It is equal for all nestlings to occupy this ‘good’ begging position. Data were collected in breeding season from March to August in 2005 to 2007. I used the micro infrared camera recoding the begging duration, eating interval, food obtained times and begging distance to indicate the level of nestling competition. The brood size was divided in small (2-3 nestlings) and large (5-6 nestlings) and monitored the hatching pattern (if all nestlings born in 12 hours I defined it as hatching synchrony). The nestling stage was divided in early stage (1-6 days) and later stage (7-12 days). All data was came from 22 nests about 5549 records and analyzed with generalized linear mixed models (GLMM). According to the results, the begging duration increase with brood size increase and it decrease in asynchronous brood. To consider the hatching pattern and the brood size, I found that the begging duration extent of asynchronous brood is smaller than synchronous one. Also the early-hatched chicks have shorter begging duration and eating interval, obtain more food per hour and have nearer begging distance. I suggest that asynchronous hatching play an important role in nestling competition in Taiwan Yuhina and early-hatched nestlings have dominance in food monopoly. It provides clear evidences to fit the scramble competition model.