Q. Do I put a capital letter at the start of a subtitle in AMA?

In AMA style, you don't automatically capitalise the word following a colon in titles. Whether you capitalise the first word of a subtitle or not depends on where it appears.

In your reference list

Journal articles, book chapters and other parts of larger works

Titles of parts of items, such as journal articles or a book chapter in an edited book are formatted in sentence case. In this case you would not capitalise the first word of the subtitle unless it was a name or acronym:

Wisdom at the end of life: hospice patients' reflections on the meaning of life and death.

Reflections on wisdom at the end of life: qualitative study of hospice patients aged 58–97 years.

Major works (e.g. books, journals, documents, websites, etc.)

Whole works, such as books or journal titles should be formatted in title case (the first word and all of the main words should have a capital letter). When using title case, you would capitalise the first word of the subtitle, even if it is a minor word like a, an or the:

Counseling Clients Near the End of Life: A Practical Guide for Mental Health Professionals.

New Techniques of Grief Therapy: Bereavement and Beyond.

Full reference examples:

The following examples show a journal article, a complete book and a chapter of an edited book. You can see the titles of the parts (the article and chapter) are in sentence case, while the title of the main work (the journal and books) are in title case (and italics).

1. Wright ST, Breier JM, Depner RM, Grant PC, Lodi-Smith J. Wisdom at the end of life: hospice patients' reflections on the meaning of life and death. Couns Psychol Q. 2018;31(2):162-185. doi:10.1080/09515070.2016.1274253.
2. Smid GE, Boelen PA. Culturally sensitive approaches to finding meaning in traumatic bereavement In: Neimeyer RA, ed. New Techniques of Grief Therapy: Bereavement and Beyond. Routledge; 2022:46-54. doi:10.4324/9781351069120-4.
3. Neimeyer RA, Burke LA. Complicated grief and the end of life: risk factors and treatment considerations. In: Werth JJL, ed. Counseling Clients Near the End of Life: A Practical Guide for Mental Health Professionals. Springer Publishing Company; 2012:205-228.

In text

If you are mentioning the title of a work in your text, you should render all titles in title case, including the titles of articles and chapters (and would therefore start the subtitle with a capital letter):

In the article "Characteristics of Resilient Hospice Workers From Their Administrators’ Perspectives: A Qualitative Study,"² Havlin and Keys noted that...