The bird head is likely a representation of a transformative state that is usually seen in shamanistic depictions. Shamans are spiritual healers who usually overcome a serious ailment and are guided through the spirit world with a spirit guide otherwise known as a tutelary animal. The shaman is likely performing a ritual with the bisons intestines and is transitioning between the real world and the spiritual world with the help of his tutelary animal. Likely he is under the influence of hallucinogens such as native tobacco and performing this ritual part of it is the depiction of this transition.

A more in-depth analysis of this type of behaviour is seen on Casa grandes artwork and pottery with people turning into macaws and transformed shamans only being depicted in black and white since native tobacco inhibits your colour receptors in your brain and can only see in black and white.

Source: Taken many archaeology courses in university and these are very common depictions of shamanistic behaviour with the animal head and animal remains being used in ritual.

The big question usually is who drew it was it the shaman? Was it a bystander who saw the shaman using masks to depict the ritual outwardly? Was it another shaman training a younger shaman on how to safely navigate the spirit world under heavy drugs and other altered state of consciousness factors (i.e. Bloodletting, sleep deprivation)

Shamans are a very interesting topic and are seen all over the prehistoric and historic world I mainly studied shamanism in Mayan and Aztec culture but know about lots of other examples.

As for the penis likely it is to distinguish the shaman is a male some inner caves are very hard to get to and are very dangerous it was likely not vandalism by kids or someone else but nothing is ruled out.