Conditioning for dispersal and endemism of montane forest birds on isolated volcanoes in Southern Luzon, Philippines

إنجليزي. The rugged mountainous terrain of the Philippine islands produces altitudinal gradients in its vegetation that separate the mountain avifauna into highland and lowland groups. Where even sympathetic congeners mzy occur within gradients, but for allopatric congeners to avoid competition to become restricted and specialized to their montane habitats. Generally, most montane avifauna are poor, given the constraints of a single forest stratum, no free water and harsh cold climates that also limit plant diversity. A fraction of the birds are either migrants or cosmopolitan and do not represent typical montane species. It is a fact that bird species diversity decreases with increasing altitude, but the greater restriction contributes to endemism, forming a unique taxa within a mountain range. However, on more isolated volcanic formations rarely joined only at base, and often separated by wide alluvial plains, this restricts dispersal of montane bir ds to adjacent peaks. What conditions are necessary for montane birds to persist? What are the limitations that restrict their dispersal? This paper aims to compare the distribution of birds along three isolated peaks in Southern Luzon. Observations of birds using standard methods were recorded from montane forest sites on three dormant volcanoes- Mount Banahaw, Mount Makiling and Mount Malinao. The distribution of highland birds along along isolated volcanoes in Southern Luzon are more limited by altitude and climates than by vegetation type. Tall mountain massits (c. 2,000 m) like Mt. Banahaw and Mount Isarog have extensive montane forests thus harbor more typical highland species than in the lower peaks (c. 1,000 m) like Mount Makiling. Although, Mt. makiling contains cloud forest vegetation on its peaks, only a few highland birds were present, and instead were replaced by lowland forms. Mountains of intermediate height (c. 1,500 m) like Mount Malinao have a rich montane avifauna similar to those found in higher peaks, but not as diverse. Apparently, the persistence of both highland and lowland birds on Mount Malinao indicated that the mountain was sufficiently high to meet the lower altitudinal limits required by typical montane species for dispersal. Isolation due to sea level rise contribute to subspecific endemism between related Pleistocene islands, and that mountains also serve as barriers within islands, and that mountains also serve as barriers within islands, separating populations. On isolated vocanic peaks, montane birds resticted by altitude and habitat gradients, are likely to diverge. Despite this, birds found in the Bicol peninsular appear to belong to the same race and possibly resulted from Pleistoscene colonization, only recently isolated. Most montane birds are restrictive, and only a few are able to disperse into transitional forests (c. 500 m), rarely lower. Further comparative behavioral or bioacoustic (and genetic) stu dies of isolated populations from these volcanic peaks may help determine the degree of relatedness of birds inhabiting mountains within the peninsula. variability in the distribution of montane birds were observed between mountains of Bicol (Mounts Malinao and Isarog) and Southern Tagalog (Mounts Banahaw and Makiling), wherein peninsular isolation limits dispersal of endemic Bicol races northward. Differences in tectonic plates and wide lowland isthmus between the Southern Tagalog rim and Bicol chain may contribute to this restriction, but still some persistently shared forms reveal past Pleistoscene connective within Southern Luzon.

إنجليزي. CF: UPLB Faculty, Staff and Students Professorial Chair Lecture 23 Nov 2004 College, Laguna (Philippines)