This is BibTeX, Version 0.99cThe top-level auxiliary file: thesis_defense.aux
The style file: plainnat.bst
Database file #1: optimal_foraging_theory.bib
You're missing a field part---line 5 of file optimal_foraging_theory.bib
 : 
 : % Upper-case A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
(Error may have been on previous line)
I'm skipping whatever remains of this command
I was expecting a `{' or a `('---line 15 of file optimal_foraging_theory.bib
 : % At @ left 
 :             bracket [ Backslash \
I'm skipping whatever remains of this entry
(There were 2 error messages)