Sentence case for titles in biblatex

The format definition

\DeclareFieldFormat{titlecase}{\MakeSentenceCase{#1}}

makes all titles in sentence case, which isn't what you want. Titles need to be printed according to both the entry and field types. For example, with the title field we need to handle @article and @book entries differently. With @inproceedings entries we need to handle the title and booktitle fields differently.

To do this we can redefine the title bibmacro to print the title field of @article and any @in* entry type in sentence case. Taking the original definition found in biblatex.def:

\DeclareFieldFormat{sentencecase}{\MakeSentenceCase{#1}}

\renewbibmacro*{title}{%
  \ifthenelse{\iffieldundef{title}\AND\iffieldundef{subtitle}}
    {}
    {\ifthenelse{\ifentrytype{article}\OR\ifentrytype{inbook}%
      \OR\ifentrytype{incollection}\OR\ifentrytype{inproceedings}%
      \OR\ifentrytype{inreference}}
      {\printtext[title]{%
        \printfield[sentencecase]{title}%
        \setunit{\subtitlepunct}%
        \printfield[sentencecase]{subtitle}}}%
      {\printtext[title]{%
        \printfield[titlecase]{title}%
        \setunit{\subtitlepunct}%
        \printfield[titlecase]{subtitle}}}%
     \newunit}%
  \printfield{titleaddon}}

Alternatively we can identify book-like entries directly and apply sentence casing to everything else. This is trickier because many more types qualify as book-like references and titles for these sources are printed by more than just one macro. In biblatex.def these include: title, booktitle, maintitle, journal, periodical and issue. To avoid redefining all of these, you can redefine the titlecase format instead.

\DeclareFieldFormat{titlecase}{\MakeTitleCase{#1}}

\newrobustcmd{\MakeTitleCase}[1]{%
  \ifthenelse{\ifcurrentfield{booktitle}\OR\ifcurrentfield{booksubtitle}%
    \OR\ifcurrentfield{maintitle}\OR\ifcurrentfield{mainsubtitle}%
    \OR\ifcurrentfield{journaltitle}\OR\ifcurrentfield{journalsubtitle}%
    \OR\ifcurrentfield{issuetitle}\OR\ifcurrentfield{issuesubtitle}%
    \OR\ifentrytype{book}\OR\ifentrytype{mvbook}\OR\ifentrytype{bookinbook}%
    \OR\ifentrytype{booklet}\OR\ifentrytype{suppbook}%
    \OR\ifentrytype{collection}\OR\ifentrytype{mvcollection}%
    \OR\ifentrytype{suppcollection}\OR\ifentrytype{manual}%
    \OR\ifentrytype{periodical}\OR\ifentrytype{suppperiodical}%
    \OR\ifentrytype{proceedings}\OR\ifentrytype{mvproceedings}%
    \OR\ifentrytype{reference}\OR\ifentrytype{mvreference}%
    \OR\ifentrytype{report}\OR\ifentrytype{thesis}}
    {#1}
    {\MakeSentenceCase{#1}}}

edit by @moewe: Note that the biblatex documentation recommends to use the starred form \MakeSentenceCase* instead of \MakeSentenceCase. The starred macro considers the language of the entry (as given in the langid field, or failing that assuming the current language) and only applies sentence case where it is appropriate (by default only for English-language publications).

biblatex-ext introduces new field formats for a finer control over title casing. Where standard biblatex has the format titlecase that applies to all title-like fields alike, biblatex-ext has

• titlecase:title
• titlecase:booktitle
• titlecase:maintitle
• titlecase:maintitle
• titlecase:journaltitle
• titlecase:issuetitle

which apply only to the specific field in their name. As usual, these field formats can be defined per type. See pp. 21-22 of the biblatex-ext documentation. (Of course biblatex-ext still supports the standard titlecase format.)

If you only want the titles of entries whose titles are in quotation marks in sentence case, then use

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[style=ext-authoryear, backend=biber]{biblatex}

\DeclareFieldFormat
  [article,inbook,incollection,inproceedings,patent,thesis,unpublished]
  {titlecase:title}{\MakeSentenceCase*{#1}}

\addbibresource{biblatex-examples.bib}

\begin{document}
\nocite{springer,weinberg,doody,maron,pines}
\printbibliography
\end{document}