It has long been recognized that the minimum number of individuals (MNI) and the number of taxa identified (NTAXA) are both often tightly related to the number of identified specimens (NISP) in a collection. The relationship between NISP and NTAXA has been suggested to vary between bird remains and mammal remains for three reasons that concern the rate at which identifiable skeletal parts of each are input to the zooarchaeological record. Rigorous testing of the relationship using 59 pairs of assemblages of bird and mammal remains confirms that the NTAXA of birds increases more rapidly per NISP than does the NTAXA of mammals per NISP. Data also indicate that two of the three proposed reasons (bird taxa outnumber mammal taxa on the landscape; each mammalian individual contributes more NISP than each avian individual) are the likely causes for intertaxonomic variability in the relationship between NISP and NTAXA. The third reason (fragmentation reduces identifiability of bird remains more rapidly than it does mammal remains) has yet to be empirically evaluated but is logical.