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<\/a>They found these nests by conducting searches within grassy glades in Mpala\u2019s whistling-thorn savanna. True to their name, social-weavers had built their ball-shaped nests in the same tree; superb starlings and gray-headed sparrows were found to have built one nest per tree, and although their nests were a similar shape, they could be told apart by the different-sized entrance holes.<\/p>\n
The study showed that out of 60 nests built by the starlings, sparrows, weavers and two unknown species, only one was built in a whistling thorn tree occupied by a less aggressive species of acacia ant, C. sjostedti.<\/p>\n
Thanks to the acacia ants, the birds and their young receive protection against predators like snakes and tree-climbing predatory mammals, including spotted genets (Genetta genetta) and slender mongooses (Herpestes sanguineus).<\/p>\n
Wilson Nderitu, a co-author of the study who has researched birds at Mpala in Kenya\u2019s central Laikipia county for 22 years, has witnessed this firsthand.<\/p>\n
If mongooses or genets attempt to raid birds\u2019 nests, the parent birds mob the attackers, the trees shake, and the ants are alerted.<\/p>\n
\u201cThat will be very helpful for the birds, because then it\u2019s going to make the ants also join in the fight, and fight whatever is coming to the trees,\u201d Nderitu says.<\/p>\n
He and others can personally testify to the effectiveness of ant bites in repelling intruders.<\/p>\n
\u201cIt\u2019s painful, you cannot tolerate them or just sit and wait for them to bite you [or] climb on you,\u201d Nderitu says. \u201cIt\u2019s not comfortable.\u201d<\/p>\n
Even locusts that land on the trees to feed on the leaves will be driven off by the ants. The birds, however, are never harmed, Nderitu says. \u201cIt\u2019s a good relationship and I think it\u2019s more advantageous to the birds.\u201d<\/p>\n The ants provide other benefits to birds beyond this frontline defense. C. nigriceps in particular is a \u201ctree architect,\u201d pruning the buds of the whistling thorns it occupies to prevent the branches from extending near the crowns of neighboring trees and allowing ants from rival colonies to cross over.<\/p>\n The pruning results in denser leaf cover that likely gives greater protection to the birds\u2019 nests against predators, the researchers say.<\/p>\n \u201cIt is really fascinating to think about how the ant-alteration of canopy connectivity may impact not only ant communities, but communities of other insects, reptiles, carnivores, and even nesting birds,\u201d says Stacy Philpott, a professor of environmental studies at the University of California, Santa Cruz.<\/p>\n Philpott, who was not part of the Kenyan study, has studied ant-bird interactions in Mexico. She says the findings from Kenya are surprising, given that ants do restrict bird foraging in other ecosystems through direct attacks, or by limiting the prey availability for birds in tree canopies.<\/p>\n \u201cBut it is not surprising if ants indeed are deterring other carnivores that prey on bird adults or nestlings,\u201d she says.<\/p>\n Philpott says she wonders if the ants could be collecting seeds or insect body parts from bird droppings, or insect or fruit remains from birds\u2019 nests.<\/p>\n Jesse Alston, a quantitative ecologist at the University of Arizona and the corresponding author of the new study, says that while he doubts the ants benefit from having the birds in their trees, he\u2019s learned to never say never when speaking about ecological systems.<\/p>\n