冠羽畫眉生蛋期競爭策略

Abstract

一般認為造成孵化不同步(hatching asynchrony)的原因是親鳥在蛋未完全生完前即開始孵蛋(incubation)的行為，而孵化不同步的現象會造成幼鳥發育時間點差異，導致幼鳥群形成體型大小的差異，晚孵化的幼鳥因為牠們的發育晚、體型小而處於競爭的劣勢，在競爭食物上較不利，導致離巢時較其他個體來的輕且死亡率較高。冠羽畫眉(Yuhina brunneiceps)屬於一夫一妻多對共用一巢制合作生殖，群體中的個體均有繁殖機會，群成員共同分擔築巢、孵蛋及育雛等工作，但群內成員大多無親緣關係，且繁殖群中具有不同的位階關係。本研究於2004-2007年繁殖季於南投地區的台大梅峰農場進行，藉由每日行為觀察及錄影紀錄來了解冠羽畫眉親鳥面對群內不同位階的繁殖競爭及衝突下，如何調整其生蛋行為，以提升自己子代的競爭力並維護子代數量的優勢。結果顯示冠羽畫眉母鳥以夜孵當天清晨的打鬥時間最長且激烈，顯示在夜孵當天清晨生蛋的重要性。母鳥開始夜孵時巢中蛋的數目與先孵化的幼鳥數目呈顯著正相關，先孵化的幼鳥數目與體重較重的幼鳥數目亦呈顯著正相關，即先開始孵的蛋可能先孵化，先孵化的幼鳥具有體重較重的優勢。72%的幼鳥第八天體重類別均與第二天相同，表示先孵化會造成幼鳥初期體重優勢，這優勢會一直持續至幼鳥後期。同步孵化在生蛋期的平均雨量接近顯著多於不同步孵化的巢，推測雨量可能是造成冠羽畫眉母鳥選擇策略主因，降雨時母鳥不再額外消耗自身能量，生較少的蛋並降低打鬥強度，無法有效阻止其他母鳥生蛋導致同步孵化，幼鳥存活率較高，體重較重；而競爭導致孵化不同步時，雖然幼鳥死亡率較高卻可以產生數量較多的幼鳥。

Hatching asynchrony is usually caused by parents starting incubation before clutch completion. By producing age-size hierarchy between nestlings, later-hatched nestlings may suffer from delayed development compared with early-hatched nestlings. Furthermore, being disadvantaged due to food resource competition which is caused by their smaller size may results in them fledging with lighter body mass as well as higher mortality. Each group member in joint-nesting system has similar reproductive probability. Under this situation, females of different hierarchy may adopt different laying strategies to increase individual fitness. I examined how female adjust their laying behavior in the cooperative breeding birds, joint nesting Taiwan Yuhinas (Yuhina brunneiceps). I found that the number of eggs before nocturnal incubation was correlated with the number of early-hatched nestlings, and the number of early-hatched nestlings was also positively correlated with the number of heavy nestlings. It suggested that eggs which incubated earlier would hatch earlier, and these nestlings could be advantageous in terms of body mass is concerned. 72% of nestlings retain the advantage of body mass until they fledge. The average precipitation in the egg-laying period of the nests that nocturnal incubation start after egg completion was higher than nests that nocturnal incubation start before egg completion. The precipitation may be the key factor that influence female’s decision. Female didn’t pay extra energy during raining days, and they generally laid less eggs and have reduced competing intensity that can’t successfully prevent other female laying eggs, which causes hatching synchronously. As a result, they produced less nestlings with heavier body mass, and higher survival rate than nestlings that hatched asynchronously. Competition behavior causes hatching asynchrony (HA), even though the mortality of nestlings was higher in HA but can produce more nestlings than in hatching synchrony (HS).