To cite unmarked in publications use:

  Kellner KF, Smith AD, Royle JA, Kery M, Belant JL, Chandler RB
  (2023). The unmarked R package: Twelve years of advances in
  occurrence and abundance modelling in ecology. Methods in Ecology and
  Evolution, 14(6), 1408-1415. URL
  https://doi.org/10.1111/2041-210X.14123

A BibTeX entry for LaTeX users is

  @Article{,
    title = {The {unmarked} {R} package: Twelve years of advances in occurrence and abundance modelling in ecology},
    author = {Kenneth F. Kellner and Adam D. Smith and J. Andrew Royle and Marc Kery and Jerrold L. Belant and Richard B. Chandler},
    journal = {Methods in Ecology and Evolution},
    year = {2023},
    volume = {14},
    number = {6},
    pages = {1408--1415},
    url = {https://www.jstatsoft.org/v43/i10/},
  }

  Ian Fiske, Richard Chandler (2011). unmarked: An R Package for
  Fitting Hierarchical Models of Wildlife Occurrence and Abundance.
  Journal of Statistical Software, 43(10), 1-23. URL
  https://www.jstatsoft.org/v43/i10/.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{unmarked}: An {R} Package for Fitting Hierarchical Models of Wildlife Occurrence and Abundance},
    author = {Ian Fiske and Richard Chandler},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {43},
    number = {10},
    pages = {1--23},
    url = {https://www.jstatsoft.org/v43/i10/},
  }