What style are you using? That's the key info we need. ;)

Upload it to pastebin.com

Change the following 2 macros to this:

<macro name="container-contributors">
<choose>
<if type="chapter paper-conference" match="any">
<text macro="container-prefix" prefix=". " suffix=" "/>
<group delimiter=". ">
<names variable="container-author" delimiter=", ">
<label form="short" prefix=" (" suffix=")" plural="never"/>
</names>
<names variable="editor translator" delimiter=", ">
<name and="symbol" delimiter-precedes-last="never" initialize-with=". " name-as-sort-order="all"/>
<label form="short" prefix=" (" suffix=")" plural="never"/>
</names>
</group>
</if>
</choose>
</macro>


<macro name="container-prefix">
<text term="in" text-case="capitalize-first"/>
</macro>


If this style is actually for a journal called British Birds, we'd welcome if you submitted to the repository so other users can profit from your work as well. Let us know.

1. In above code box change <text macro="container-prefix" prefix=". " suffix=" "/> to <text macro="container-prefix" prefix=". " suffix=": "/>

2. You need to set the editor label plural value from "never" to "contextual".

Do you know how I can put a comma before Birds of the World like this:

'(eds.), Birds of the World'

I have tried adding a comma to the 'suffix' in various macros, but this does not seem to be having any effect.

Many thanks for reply.

Latest code: https://pastebin.com/T9QyiV0w

For book section, the bibliography entry looks like this:

Hoyo, J. del, Collar, N. & Christie, D. A. 2020. Eurasian Three-toed Woodpecker (Picoides tridactylus), version 1.0. In: Billerman, S. M., Keeney, B. K., Rodewald, P. G. & Schulenberg, T. S. (eds.) Birds of the World. Cornell Lab of Ornithology. Ithaca, NY, USA. [Retrieved from https://birdsoftheworld.org/bow/species/ettwoo1/cur/introduction?login#conserv on 15th December 2020].

But I need it to have a comma between (eds.) Birds of the World, like this:

Hoyo, J. del, Collar, N. & Christie, D. A. 2020. Eurasian Three-toed Woodpecker (Picoides tridactylus), version 1.0. In: Billerman, S. M., Keeney, B. K., Rodewald, P. G. & Schulenberg, T. S. (eds.), Birds of the World. Cornell Lab of Ornithology. Ithaca, NY, USA. [Retrieved from https://birdsoftheworld.org/bow/species/ettwoo1/cur/introduction?login#conserv on 15th December 2020].

I added suffix=", " to the code, bit it doesn't seem to have had any effect. Where exactly do I need to add suffix=", "?

Original code:

macro name="container-contributors"
choose
if type="chapter paper-conference" match="any"
text macro="container-prefix" prefix=". " suffix=": "/
group delimiter=". "
names variable="container-author" delimiter=", "
label form="short" prefix=" (" suffix=")" plural="contextual"/
/names

With suffix=", " added in second last line:

macro name="container-contributors"
choose
if type="chapter paper-conference" match="any"
text macro="container-prefix" prefix=". " suffix=": "/
group delimiter=". "
names variable="container-author" delimiter=", "
label form="short" prefix=" (" suffix=")" plural="contextual" suffix=", "/
/names

You can add it on line 448 like so:
<text macro="container-contributors" suffix=","/>

You never answered if this style is actually for the journal British Birds. Chicago-based styles are inherently not beginner-friendly. If you are ok with it, I can go over the style and submit it to the repository for you.

Its this journal, but it calls itself a magazine: https://britishbirds.co.uk/

I think my code will definitely need refining and I don't think it can be trusted. I didn't know what I was doing at all.