Plural problem in Serbian

[Tijana Krstic]

Hello Seelangers,
I am an undergraduate student taking a course on comparative Slavic
linguistics at American University in Bulgaria. Since literature
concerning Slavic languages is very scarce in the university library,
I need some help with solving an internal reconstruction problem in
Serbian.
In Serbian there are two ways of making plural for some neuter gender
nouns, those designating animal offspring. Thus, there is:

a)  pile (sg.) "a chicken"--pilic'i (pl.) "chicken"
b)  pile (sg.)--pilad (pl.) "chicken"

Other examples are: mache--machic'i (machad) "kittens"
prase--prasic'i (prasad) "piglets"
kuche--kuchic'i (kuchad) "puppies"
pache--pachic'i (pachad) "ducklings"
tele-- telic'i  (telad)  "calves"
There are probably several more, but very few, because this rule for
creating plural operates only for a very limited number of neuter
gender nouns designating animal offspring.  The other form, with
suffix -ad, also represents the so-called collective nouns.
1) Could anyone tell me what is the precise difference between these
two forms of plural (difference in meaning and usage)?
2) Is one of the forms older than the other?
3) Is there a similar example of plural form ending with -ad in any
other Slavic language?
If someone could shed some light on any of the aspects of the
question, I will be very gratefull.

Thanx in advance
Tijana