在都市環境中的臺灣藍鵲相較於紅嘴藍鵲表現出較高之攻擊性

Abstract

隨著人類活動日益增加，都市化正迅速改變地景，迫使生物面對更高的人為干擾。釐清快速環境變化對生物造成的衝擊、和生物是否能因應衝擊展現適當的行為反應，將有助我們在都市化的效應下，營造對生物友善的環境。儘管都市化對生物帶來諸多挑戰，前人研究發現部分生物(如都市適應型物種)，能透過調整自身行為，有效利用都市資源，並藉此適應新環境。在繁殖的季節，面對外界的干擾，生物常藉由展現攻擊行為保護牠們的子代，攻擊行為也因此和生物的生存與繁殖表現有相當大的關聯。雖然具較高攻擊性的個體通常能更有效地保護巢中的子代，並獲取較多資源，進而提升其繁殖成功率；頻繁且高強度的攻擊行為，也可能減少牠們分配於其他重要活動(如育雛、覓食)的能量與時間，反而不利於繁殖表現。都市環境中高度的人為干擾會如何影響生物的攻擊性，進而影響攻擊性與繁殖表現間的權衡仍有待研究。基於上述攻擊性與繁殖表現之間的權衡，本研究提出第一個假說：動物於繁殖期的攻擊性展現，是提升子代存活率和遭受受傷風險下的權衡結果。另一方面，本研究也想探討在面對相似都市環境時，不同種的生物是否也採用相似的行為策略，權衡攻擊性與繁殖表現。一般而言，生物的生態特徵與行為展現，多同時受親緣與環境因子調控。環境居住在相似環境且親緣關係相近的物種常被認為會具有較相似的。因此，本研究提出第二個假說：物種間的行為相似性和親緣距離及物種所處的環境相似度呈正相關。 為驗證上述假設，我於2024年三月至六月繁殖季期間，以兩種分布於高度都市化環境、且親緣關係接近的鴉科鳥類—紅嘴藍鵲(Urocissa erythroryncha)與臺灣藍鵲(U. caerulea)作為研究對象。本研究共調查8個分布於香港地區的紅嘴藍鵲繁殖群、和19個分布於台北市和新北市地區的臺灣藍鵲繁殖群。我比較兩個鳥種在觀察者等速接近巢位時，發出警戒聲、接近觀察者、和出現俯衝攻擊的行為的傾向，並分析這些行為和巢位環境之間的相關性。此外，我也記錄了兩鳥種的雛鳥離巢成功率。種間比較的結果顯示，相較於紅嘴藍鵲，臺灣藍鵲有較高機率接近觀察者，且每次觀察中，會展現較多次且頻繁的接近行為。但是，兩物種發出警戒聲和進行俯衝攻擊的機率及頻度上均無顯著差異。在巢位環境特徵方面，臺灣藍鵲巢位附近的人類活動較紅嘴藍鵲低。整體而言，兩鳥種的雛鳥離巢成功率無顯著差異。在種内攻擊性表現和巢位環境的相關性結果發現，巢位較靠近道路的臺灣藍鵲，於育雛階段有較高機會出現警戒聲和俯衝攻擊行為，同時也展現較多次的警戒聲。相較之下，紅嘴藍鵲則在攻擊性和巢位環境特徵間則無明顯趨勢。綜觀上述，本研究結果不能完全支持「動物於繁殖期的攻擊性展現，是提升子代存活率和遭受受傷風險下的權衡結果」之假說，亦没有明確證據支持第二項假說。本研究結果解釋，高度都市化的環境將成為強烈的選擇壓力，藉由行為間相互權衡的機制，降低動物行為的多樣性。

Although urbanization presents significant challenges to wildlife by rapidly transforming landscapes and introducing high levels of human disturbance to wildlife, some species (e.g., urban adapters) have demonstrated the ability to exploit urban resources and adapt to this novel environmental condition. Understanding the mechanisms that shape species' responses to urbanization is crucial for developing urban planning strategies that balance the needs of both wildlife and human society. Aggression is a common nest defense behavior in animals that profoundly affects their survival and reproductive performance. Individuals with higher levels of aggressiveness are more likely to obtain necessary resources for reproduction and protect their offspring effectively, enhancing their reproductive outcome. However, frequent and intense aggression can also reduce the time and energy available for individuals to engage in other fitness-related activities, suppressing their reproductive performance. It is thus considered that animals would have achieved a balance between aggressiveness and reproductive performance. Under urbanization, currently, it remains unclear how the high level of anthropogenic disturbance modulates animal aggressiveness through this behavioral trade- off. Here, I examined the first hypothesis that the aggressive behavior displayed by animals during the breeding season is shaped by a trade-off between enhancing offspring survival and the increased risk of injury. On the other hand, this study also aimed to explore whether different species adopt similar behavioral strategies when facing similar urban environments. Generally, the ecological traits and behavior of organisms are simultaneously influenced by both phylogenetic and environmental factors. Species that inhabit similar environments and share close phylogenetic relationships are often considered to exhibit greater similarities. Therefore, I proposed the second hypothesis: behavioral similarity between species is positively correlated with both phylogenetic relatedness and similarity of environmental conditions, respectively. I conducted nest surveys and behavioral experiments during the breeding season of the two closely related blue magpie species, from March to June 2024, that inhabit urban environments: the Red-billed Blue Magpie (Urocissa erythroryncha) in Hong Kong and the Taiwan Blue Magpie (U. caerulea) in Taipei and New Taipei City. Aggressiveness was assessed based on the presence of attacks, approaches, and alarm calls observed during each experimental trial. I compared the aggressiveness, nesting environment, and fledging success of the two species and analyzed the correlations between the aggressiveness and nesting environment of each species. The results were derived by the observation on a total of 8 Red- billed Blue Magpie nests and 19 Taiwan Blue Magpie nests. Between species, I revealed that Taiwan Blue Magpies were more likely to approach investigators during the experiment compared to the Red-billed Blue Magpies. However, there was no significant difference in the attack or alarm call behaviors of the two species. Regarding the nesting environment, I found that the nests of Taiwan blue magpie were distributed in areas with a lower level of human activity compared to Red-billed Blue Magpies. Within species, I found that Taiwan Blue Magpies that built nests closer to roads were more likely to emit alarm calls and attack investigators during the nestling stage, and they also produced alarm calls more frequently. In contrast, there was no clear association between aggressiveness and nesting characteristics in Red-billed Blue Magpies. In conclusion, these findings weakly support the first hypothesis that the expression of aggressive behavior in animals is shaped by a cost–benefit trade-off. Moreover, there is no sufficient evidence to support the second hypothesis. This study highlights that urbanization can act as a strong selective pressure that limits the diversity of animal behavioral strategies through behavioral trade-offs.