Every purple martin season I offer both egg shells and oyster shells to the martins in my colony. The female martins in particular eat the egg/oyster shells prior to egg laying for the calcium which is needed for egg development. Both male and female martins feed the shells to the young in great quantities up until fledging time. The nestlings need the calcium, too for bone development.
In my colony, the martins have a definite preference for egg shells over oyster shells and will go into a “feeding frenzy” when there are babies in the nests. But I just can’t save enough egg shells for the hundreds of martins in my colony so I also provide oyster shells. The adult martins will eat the oyster shells and feed them to their young. I use pullet size oyster shells which can be bought at most feed stores; I buy a 50 pound bag.
During the early part of the season the female martins are nearly always raped by the males when the females fly down to eat the egg/oyster shells for themselves. The males KNOW that when females are looking for egg/oyster shells the females need to be fertilized and the poor females are often buried alive on the ground by gangs of sex crazy males. The females try to grab the shells quickly and their mates desperately try to protect them but a gang of males often overpowers the females. In fact it is almost impossible for the females to eat egg/oyster shells without being raped. But the females still occasionally manage to snatch a few egg/oyster shells pieces before being sexually assaulted and fly away as fast as they can! And often a gang of male martins is in hot pursuit!
Later in the season when the females are feeding nestlings, the gang rapes tend to end as the females are no longer receptive and aren’t ready to be fertilized and the males KNOW it. During this time large numbers of males and females will gather egg/oyster shells to feed their young with little sexual activity between the genders. However, there may be some rapes of females by single males.
I mainly place my egg/oyster shells in bare spots on my driveway near the martin housing and the martins seem to prefer this method. I do have a bed and breakfast and have placed shells in the metal pans. The martins will use them, too.
I have thought about placing egg/oyster shells much farther away from my martin housing to try to minimize the violent rapes of the females by gangs of males during the early part of the nesting season. But finding a place where females can go without a gang of males following is almost impossible! I have watched poor females land several hundred yards from my colony on the ground and almost immediately they are attacked by males!
Here are two photos showing both male and female martins collecting egg shells for their nestlings. No gang raping at this time as the males and females are feeding young. But in the second photo there is one ASY male raping a female!
Steve
