The avian physiology section is now five years old. This special e-book is to commemorate this event. For this highlights issue celebrating the first five years of the Avian Physiology section, it was decided to focus on the top papers/reviews published. Table 1 lists the top fifteen papers/reviews based on either views or down-loads as a pdf. There is some agreement between the two lists. What is compelling is that of the top papers, all except one encompasses research conducted in domesticated birds, predominantly with chickens with one focused on turkeys. It is perhaps not unexpected that research on chickens dominates the top papers because of the following: - Chickens are commercially important being the number one meat animal globally. - Chickens are used as the model species for birds. Other possibilities that could be the case include the following: - There are more researchers working on chickens compared to other commercial or wild species. - There are other avenues to publish wild bird research. - Lack of funding for open access article processing charges. The exceptions to the chicken papers/reviews is a paper on a passerine bird, the migratory blackheaded bunting (Emberiza melanocephala) (Singh et al., 2018) and a review paper focused on turkey satellite cell proliferation and differentiation (Velleman and Song, 2017). Of the top papers/reviews, six were related to muscle physiology (Bottje et al., 2017; Greene et al., 2019; Lassiter et al., 2019; Lin et al., 2017; Piekarski et al., 2020; Velleman and Song, 2017). These papers address different aspects of the physiology of muscle functioning and development in chickens or turkeys, specifically the following: - Small RNAs from the transcribed from mitochondrial genome in muscle (Bottje et al., 2017) - Leptin induced autophagy of muscle cells (Piekarski et al., 2020) - Proliferation and differentiation of myoblasts: influence of syndecan-4 and glypican-1 (Velleman and Song, 2017) - Myostatin signaling and muscle growth (Lassiter et al., 2019) The role of growth factors and signal transduction pathways in myoblast proliferation (Lin et al., 2017) - Muscle pathology, specifically, woody breast myopathy (Greene et al., 2019) There are also among the top papers/reviews on multiple diverse other organs and systems namely the following: adipose tissue (Abdalla et al., 2018), bone (Adhikari et al., 2019), feathering (Chen et al., 2019), gastro-intestinal functioning/microbiome (Li et al., 2018; Kraimi et al., 2018; Rodrigues et al., 2020), immune functioning (Bi et al., 2018; Wei et al., 2018), kidneys (Li et al., 2018), liver (Flees et al., 2017; Singh et al., 2018), endogenous opioids (Scanes and Pierzchala-Koziec, 2018), photoperiodism (Hanlon et al., 2020), pituitary gland (Zhang et al., 2021) and reproduction (Eusemann et al., 2018; Li et al., 2018). The geographical distribution of the top papers is interesting as summarized in Table 2. The United States of America and the People’s Republic of China were the country of origin of most of the papers. In addition, there were papers/reviews from Europe (France, Germany and a collaborative paper between researchers in the USA and Poland) and India. Of the top papers/reviews from the USA, six come from a single department, namely, the Department of Poultry Science/Center of Excellence for Poultry Science, University of Arkansas (Bottje et al., 2017; Flees et al., 2017; Green et al., 2019; Lassiter et al., 2019; Piekarski et al., 2020; Rodrigues
