To cite unmarked in publications use:

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 http://www.jstatsoft.org/v43/i10/.

Corresponding BibTeX entry:

  @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 = {http://www.jstatsoft.org/v43/i10/},
  }