Childrens series about Hal and another teenage naturalist [SOLVED] [s]

[Mercia]

This is a series of books that I read from the classroom library circa 1974, but I suspect that they were from the 1960s or earlier. Two teenage boys are amateur naturalists, one called Hal, who get into adventures, possibly solving crimes. I think these might be books written by a male author more famous for something else. One book cover had the two teenagers looking out from behind bulrushes at night.

I have done numerous internet searches down the years, but never found out the author or name of the series.

[Tab]

There is the Hal and Roger Hunt Adventure series.
https://www.goodreads.com/series/5772...

If fits the time period.

[Mercia]

Thanks Tab that looks like that is the series. None of the covers match the one that I remember, but I reckon that was a UK cover I am remembering. I guessed right that the books were from the 60s, they looked old and tatty on the teacher's shelves even in the early 70s.