Which Bengaluru trees do birds prefer? Study has interesting answers

The birds’ interactions varies with the trees’ origin and species

A new study of interactions between mammals and birds with native and alien trees in Bengaluru has thrown up nuanced insights, with researchers calling for a need to understand such “biological encounters” to maintain a “sustainable ecosystem”.

Between December 2019 and March 2020, Purabi Deshpande, Rohan Sharma, Aleksi Lehikoinen, and Rose Thorogood studied vertebrate pollinators’ interactions with native and alien flora.

They observed 54 focal trees across 16 (eight native and eight alien) species for 125 hours and 40 minutes and tracked the interaction of 32 species of birds and two species of mammals (the Indian palm squirrel and the bonnet macaque, which was spotted only once) with these trees.

The four researchers found that both alien and native trees provide resources to birds and mammals, with limited potential for invasion. The findings were published in the journal 'Science of the Total Environment'.

Native species, including Bombax ceiba (red silk-cotton tree), Saraca asoca (Ashoka tree), Butea monosperma (flame of the forest), and alien species, including Tabebuia aurea (silver trumpet tree), Colvillea racemosa (Colville’s Glory), were observed.

The interactions were classified into pollination, nectar-robbing (taking nectar through an incision on the flower, without pollinating it), and flower depredation (consumption).

The researchers found that while the birds and mammals used both alien and native trees, the diversity of bird communities was greater in alien trees than in native trees of the same height. Native trees with a large number of flowers hosted more bird species than alien trees with a similar number of flowers.

Birds pollinated native trees more often – expectedly, due to their shared evolutionary histories – and are more likely to cheat in interactions with alien trees. With squirrels, pollination and depredation happened in alien trees while native trees are less likely to be pollinated.

Purabi, a PhD researcher at the University of Helsinki, said the key takeaway was that plantation drives in urban spaces need to factor in patterns that mark the biodiversity.

“As we lose green cover due to urbanisation, it becomes crucial that we are not planting merely to meet some quota. Plants provide more than just oxygen and what they provide to humans and animals can be different. To maintain healthy and sustainable ecosystems in the cities, biological interactions need to be understood," said Purabi.

Varied patterns

The birds’ interactions varied with the trees’ origin and species. This validated a need to study tree species individually, to understand their ability to support Bengaluru’s fauna.

The researchers observed that the tree occupancy for birds increased with the trees’ height but did not vary according to their origin. They noted that in the case of birds on trees taller than 12-18 m, alien species were occupied preferentially over native species.

The study did not confirm any preference for native trees in most of the interactions, indicating that alien trees are now a part of Bengaluru’s ecosystem. Literature on alien species also indicated that larger trees are less likely to be invasive, Purabi said.

“Unlike smaller shrubs like Lantana camara, which is a known invasive, the eight tree species in this study had positive interactions with the fauna. The results do not indicate anything about insects or bats. Further studies on these will give us a bigger picture,” she said.