Which kind of stretchers is it. Passive infinitive + verb third form. As it is written here (may have been decimated). May have been is passive infinitive and decimated is verb third form.
may have been decimated
May have been is not by itself a passive, but a modal perfect. Followed by a past participle it becomes a passive: in full, a present-tense (or non-past-tense) modal perfect passive construction.
- may is the present-tense verb in the construction, a modal. Modals possess only two forms, both tensed.
- have is the perfect auxiliary, cast in the infinitive form because it is the complement of a modal verb
- been is the passive auxiliary, cast in the perfect participle form because it is the complement of the perfect auxiliary
- decimated is the lexical verb, cast in the perfect participle form because it is the complement of the passive auxiliary.
Building the construction backwards, in the present tense:
PRESENT: [SUBJ] decimates tensed lexical verb
PASSIVE: [SUBJ] is decimated tensed BE + PaPpl of lexical verb
PERFECT: [SUBJ] has been decimated tensed HAVE + PaPpl of BE + PaPpl of lexical verb
MODAL: [SUBJ] may have been decimated tensed MAY + Inf of HAVE + PaPpl of BE + PaPpl
of lexical verb