Purple Martins’ use of clam shells in the 2018 nesting
Small clam shells were gathered from the nesting material in last year’s martin nests. The shells were disinfected and offered to this year’s colony, as well as the usual ground oyster shells.
6/30/2018 – Looking out an upstairs window in the early morning, I saw what appeared to be an ASY male martin passing a small object to a female. They were perched on the calcium supplement tray under their gourd rack 100 feet from the house. Looking through a 19th C wavy glass window sash at that distance, a speck of white is all I could see. But, a tray of twenty small clam shells of the genus Corbicula had been placed in the tray the previous evening. The possibility of seeing this behavior repeated up close and with a camera prompted me to set out trays of clam shells and observe from the wagon shed. I never witnessed the gifting of a shell again, but it did offer the opportunity to observe the martins’ group behavior at the shell trays. Sixteen smaller clam shells and sixteen larger clam shells were placed in separate trays under the gourd rack.
16 smaller clam shells in left tray
16 larger clam shells in right tray
7/5/2018 Forty oyster shells on left
40 clam
shells on right
7/10/2018 - 9:46 am 30 oyster shells on left
15 smaller and 15 larger clam shells on right
2 large clam, 7 small clam, 22 oyster left
The purple martin’s strong preference for clam shells over crushed oyster shells is likely because the clam shell is easily recognizable and has served the martins well for millennia, allowing evolution to take its course. The crushed oyster shells are a recent product of the poultry industry and resemble gravel. One may be as beneficial as the other. But, the clam shells are more desirable to the martins. At one point, when the shells were almost gone, 5 martins, each clutching a large clam shell in its beak, just perched on the tray for several minutes and seemed to be staring at one another. Were they showing off their prized shell? Were they waiting for one of them to make a move? Suddenly all five flew off to the gourds. I did not see which one flew first, but their reaction time is much faster than mine.
While observing the martins taking clam shells on 6/30/2018, one, then two and then three martins landed on the closer of two young persimmon trees and each began chewing off pieces of the base of a leaf. They methodically worked their way from edge to stem and appeared to have no interest in the rest of the leaf. It may be that they could only get a good perch on the leaf stem that would support their weight. This seemed to be a one time event. Persimmon leaves have antibacterial activity due to their tannins. Perhaps they also protect against parasites? Or, it may be just a case of “monkey see-monkey do” in selecting this small tree near their gourds. Once the threesome had finished, no further damage to either persimmon tree was noticed.
