The phrase "extant birds" refers to birds that are still alive and existing today. It means the birds that we can currently see and study, as opposed to those that have gone extinct over time. Full definition
Angst et al. conducted carbon isotope analysis on fossils of Gastornis and several contemporaneous herbivorous mammals across several European sites and compared these to isotope profiles from extant birds with known diets. (science.sciencemag.org)
The study aimed to test the hypothesis that data from extant birds could be used to predict the incubation behaviour of Theropods, the group of carnivorous dinosaurs from which birds descended. (sciencedaily.com)
A new method used to perform geochemical analysis of fossilized eggs from China has shown that oviraptorosaurs incubated their eggs with their bodies within a 35 - 40 ° C range, similar to extant birds today. (sciencedaily.com)