Subtropical, dry–temperate and mixed dry–temperate coniferous forests are known as the most productive habitats harboring a higher avian assemblage of endemic and threatened species (Joshi et al., 2012). They are diverse in a topography supporting luxuriant (thick and healthy) vegetation that forms mosaics and provides vital resources for a wide array of avian species (Myers et al., 2000; Elsen, et al., 2017; Elsen et al., 2018). Ascertaining the bird population parameters, vegetation structure, and food resources is crucial for indicating the habitat suitability and productivity of a forest ecosystem (Karubian et al., 2005; Cassey et al., 2007; Amar et al., 2008; IUCN, 2011). Furthermore, the vegetation structure and composition could help our understanding the bird species diversity pattern (Vetaas and Grytnes, 2002; Rahbek, 2005; Grytnes and McCain, 2007).
Most bird species are habitat specialists, i.e., they utilize different layers of vegetation for foraging, shelter, and breeding purposes. Some avian species prefer the forest floor (e.g., common babblers, common quails, common hoopoe, magpie robins, blue whistling thrush, and pied bushchat), while others utilize the tree canopy, i.e., Indian paradise flycatchers, scarlet minivets, golden orioles, cinereous tits, yellow-footed green pigeon, speckled wood pigeon, streaked laughing thrush, and common rosefinch (Gardner et al., 2009; Malhi et al., 2014).
Notably, the food resources among the six coniferous forests are not distributed uniformly and may vary from area to area, depending on topography, microclimate, altitude, and vegetation structure and composition. Likewise, the bird relative abundance among these six forests also varies. The highest relative abundance was detected in dry–temperate coniferous forest (BG) and the lowest was detected in mix dry–temperate coniferous forest (G). This result indicated that the birds often select an area on the basis of proximate factors, i.e., landscape, terrain, substrate, microclimate, characteristics, extent of the disturbance, predation, and occurrence of food resources. These factors determine avian community parameters such as diversity, density, and distribution. Another reason could be that the floristic diversity shaping the forest structure influenced the bird population structure. Blake (2007) reported that the bird relative abundance and species composition might vary due to vegetation structure and complexity.
The “highest” bird density was estimated in the dry–temperate coniferous forest (BG) as compared to the other habitats. The higher bird density in dry–temperate coniferous forest (BG) was due to the occurrence of fruiting trees (e.g., Ficus carica, Morus alba, and M. nigra), shrubs (i.e., Zizihpus nummularia, Reptonia muscatencesea, Berberis lyceum, Gymnosporia royleana, Zanthoxylum alatum, and Myrtus communis), grasses, Solanum nigrum, and flowering trees (i.e., Acacia modesta and Ailanthus altissima) that provide adequate food sources to harbor the denser avian population. The dark–throated thrush, blue whistling thrush, and streaked laughing thrush prefer shrubs in their search for insects, arachnids, and berries. Likewise, the red–vented bulbul, Himalayan bulbul, and white–eared bulbul often utilizes fruiting trees and shrubs for berries and grasses for insects (Brooks, 2013; Old et al., 2014; Barnagaud et al., 2014; Thibault et al., 2018).
As well, the dry–temperate coniferous forest (BG) was rich in bird species. The richness was due to the complex structure and composition of the vegetation, which attracted the greatest richness of the avian species. For instance, trees that carry nuts (P. wallichiana), juicy and fleshy fruits (M. alba, M. nigra, A. modesta), shrubs that carries the berries (Z. alatum, S. nigrum,), plums (B. lyceum and M. communis), grasses bears flowers (Verbascum thapsus, Ranunculus acris, S. media) and herbs that carry cereals (P. aviculare, R. dentatus, C. montanus, and Heteropogon contortus). It was previously reported that bird species often prefer to use a complex habitat that is more diverse in vegetation composition and rich in food resources (Wu et al., 2013; Joshi and Bhatt, 2015; Saha et al., 2016). Likewise, Casas et al. (2016) stated that the vegetation structure and composition offer ideal weather conditions, adequate food resources (fruits and insects), and suitable breeding sites to harbor a wide array of avian populations to perform multiple activities.
Foraging guild results demonstrated that Indian paradise flycatchers selected pine trees for perching after sallying. During sallying, they catch insects while flying (Gokula and Vijayan, 2003; Rassussen and Anderton, 2012; Das and Adhikari, 2019), such as flies, bugs, beetles, spiders, moths, butterflies, and damselflies. The scarlet minivet prefers the tree crown for foraging and perching. It was observed that minivets flushed insects such as flies, moths, grasshoppers, crickets, cicadas, and caterpillars in foliage through gleaning in the crown and caught them on while flying. Likewise, the Eurasian golden oriole and common starlings used common fig, wax apple, and barberry to consume a wide array of fruits and berries, and prey on insects, especially caterpillars. The wax apple populated degraded habitats such as forest edges and mixed vegetation for foraging, particularly on insects (including dragonflies, honeybees, grasshoppers, moths, beetles, and crickets), through sallying, gleaning, and hovering (Somasundaram and Vjayan, 2008). They mainly utilized forest edges and cultivated areas to forage on a variety of food items, i.e., fruits, lizards, rodents, and large insects.
Notably, the black francolin, common quail, and bay–backed shrike were confined to forest edges, shrubs, and grasses, as these are a shy species with secretive behavior. Another reason may be that the forest edges provide cover from predators (Fuisz and Yosef, 1998; Pande et al., 2004; Antczak et al., 2005). Furthermore, the white–throated kingfisher selected riparian habitats and forest edges for perching and foraging (Asokan and Ali, 2010; Coulombe et al., 2011; Kesler, 2012). Moreover, other birds, for instance, the greenish warbler, preferred to use conifer trees for foraging. During foraging, they glean in the crown to prey on midges, caterpillars, beetles, leafhoppers, bugs, wasps, moths, and spiders. However, sometimes, these birds sail in dense shrubs to catch insects on their wing (Johnson and Sherry, 2001). Likewise, the bar–tailed tree creeper also selected coniferous trees for foraging. Upon reaching the top of the conifer through progressive hops, they then fly down to the base of the next tree trunk and start climbing again in a spiral fashion in their search for food (i.e., insects, caterpillars, and spiders).
Apparently, the speckled wood pigeon and rock pigeon were concentrated in fruiting trees and shrubs (i.e., common fig, wind prickly ash, yellow Himalayan raspberry, and barberry) to forage for berries and fleshy fruits. Likewise, the European turtle dove, oriental turtle dove, and spotted dove preferred grounds to feed on grains and seeds of grasses (e.g., pine woods drop seed, beard grass, milkweed, and wild sunflower). The variation in habitat selection and food utilization showed that habitat structure, land-use pattern, and food resources are the major driving factors that significantly influence the habitat use and distribution of avian species (Casas et al., 2016). It could be that the heterogeneity in vegetation composition enables the formation of complex habitats (Hu et al., 2018), which harbor a higher avian diversity (Jankowski et al., 2009; Kissling et al., 2010; Paudel and Vetaas, 2014).