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We have:

  * 一羽 いちわ
  * 二羽 にわ

but it seems the pronunciations of the counter for 3 and 4 are irregular. In
each case, are both pronunciations acceptable, or we must use the former?

  * 三羽 さんば、さんわ
  * 四羽 よんば、よんわ

Similarly, the counts for 6, 8, and 10 also seem to be irregular.

Should we memorize these by rote? Or is there a rule/pattern that can help us
memorize these?
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question
pronunciation counters
Pronunciation of bird counters: 三羽 and 四羽
(I am posting an answer to my own question here. Please correct me if I am
wrong.)

According to this page ([羽: COUNTING BIRDS, BATS, and BUN-
BUNS](https://www.tofugu.com/japanese/japanese-counter-wa/)), both
pronunciations can be used.

>   * 3 BIRDS: さんわ VS さんば:
>     * Both さんわ and さんば are common. In the case of the idiom 三羽烏 (さんばがらす),
> meaning a "distinguished trio," the reading is always ば.
>   * 4 BIRDS: よわ VS よんわ VS よんば
>     * While よんわ is the most common pronunciation, よんば is listed as the
> preferred reading in the NHK Accent Dictionary. It's unlikely you'll run
> into よわ, which is becoming archaic.
>
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answer
As a non-Japanese-teacher, I think neither form is immediately wrong, or I can
rather say that it mostly depends on personal preferences (disclaimer: I don't
know if JLPT says otherwise :P).

Etymologically, the counter 羽【わ】 was は in its older form, obviously cognate
with 羽【はね】 "wing". Unlike most counters, this short piece of word was easily
affected by the phonological change that also impacted the pronunciation of
topic particle は. Therefore, there is a disagreement among today's speakers on
whether the base form of the counter is わ or は.

count | always わ | "weakened" は  
---|---|---  
一羽 | いちわ | いっぱ (rare)  
二羽 | にわ | にわ  
三羽 | さんわ | さんば  
四羽 | よんわ / よわ (rare) | よんわ / よんば†  
五羽 | ごわ | ごわ  
六羽 | ろくわ | ろっぱ  
七羽 | ななわ | ななわ  
八羽 | はちわ | はっぱ  
九羽 | きゅうわ | きゅうわ  
十羽 | じゅうわ | じっぱ / じゅっぱ‡  
  
Speakers can freely choose either series, or even mix them as they like, and
it does not hinder communication.

Similarly we have another counter
[把【わ】](https://dictionary.goo.ne.jp/word/%E6%8A%8A_%28%E3%82%8F%29/) "bundle
of" in the exactly same situation, where NHKアクセント新辞典 only lists ～わ except 十把
じっぱ/じゅっぱ for some reason, so this seems really a matter of discretion.

* * *

† よん + h- usually does not trigger rendaku, because よん is originally a
replacement of native よ- or on'yomi し. So this is actually irregular.  
‡ Japanese teachers may not be pleased.
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answer