Overlapping generations and care of the young has been suggested as a key innovation in social animals. It has been thought that grandmothers are key in better offspring survival. While parental care in birds is largely thought of as being short-lived and several forms of cooperative breeding, sometimes involving closely related birds, have been written about widely. Now these are direct interactions, and there can also be indirect interactions, mediated via the environment.
Emmanuel Theophilus pointing to the callus rings on a karsu oak Quercus semecarpifolia.
Modification of their environment by individuals of a species across generations has not been written about much. We do have examples of tradition, nesting sites that are used year after year by multiple generations of a species, especially in waterbirds. Whether megapode build over older nests is unclear. Some seabirds colonies effectively alter the composition of the soil with their guano, changing the kind of vegetation that can grow and thus alter the environment for future generations.
Here I describe the curious case of Himalayan trees with swollen bark rings. This is not an original discovery and the phenomenon was in fact pointed out to me by Emmanuel Theophilus of Munsiyari. It is here described briefly to encourage further observation. A rougher note can be found on my blog.
A large karsu oak with callus rings
Rhododendron with callus rings
The phenomenon of callus rings on some trees of Quercus semecarpifolia and Rhododendron arboreum in the Munsiyari region was first pointed out to S. Subramanya and me on 20 April 2016 when we took these pictures. It seems that these two species of tree respond to puncture damages made by the Rufous-bellied Woodpecker (Dendrocopos hyperythrus) which interestingly enough has been formerly called the Rufous-bellied Sapsucker. It is one of the few woodpeckers in India that is known to feed on sap released by trees. It appears that the woodpecker makes a series of punctures on a tree and revisits the tree to sip the fluid that is exuded. Many questions crop up – How long does it take for the tree to respond? What is the composition of the sap? What benefit might the tree get to release nutrient here? It appears that there is some benefit for the woodpeckers to tap where they have tapped before, and the tree itself has a physiological growth or repair response with results in these callus rings. The age of the trees, the distribution of calluses, old and new, clearly suggest that these might be form over a time scale of decades, with many generations of woodpeckers being involved.
At some spots we found that Green-tailed sunbirds (Aethopyga nipalensis) where visiting the puncture positions to drink the sap oozing, which is presumably sweet and sugary.
This is clearly a phenomenon that deserves more study.
Close up showing some fresh tapping marks.
A small postscript, the only person who has commented on this topic in literature seems to be A. E. Osmaston.
Osmaston, A E (1916): Curious habits of Wood-peckers in the Kumaon hills. J. Bombay nat. hist. soc. 24(2):363-366.