Symbiotic associations are found throughout the tree of life where different organisms have evolved interdependent interactions with consequences for their fitness. Like in other vertebrate groups, birds (Class: Aves) have evolved many symbiotic associations with taxonomically diverse microbes (e.g., bacteria and protists) that can play important roles contributing to the ecology, behaviours and diversity of their avian hosts. Thus, untangling the evolutionary stability and drivers of these associations is important to thoroughly understand the host biology. In my PhD, I have investigated the dynamics of the factors that influence these avian-microbial symbioses, specifically focusing on gut and uropygial gland microbial communities and haemosporidian blood parasites of tropical (Papua New Guinea) and temperate (Europe) avian hosts. Overall, the results in my PhD chapters show that: 1) Passerine gut microbiomes are highly variable with small core microbiomes. Different host species harbour specific consortia of gut bacteria, but host phylogeny does not influence the composition of the microbiomes. However, the host digestive tract length and individual diets play important roles for the stability and diversity of these bacterial communities. 2) Uropygial gland microbiomes harbour multiple potential defensive bacterial symbionts that can together act against an array of feather degrading bacteria. 3) The relative importance of different environmental and anthropogenic factors governing avian-haemosporidian associations vary between different host species, and avian hosts can escape from parasite pressures when colonizing new areas. Of the factors influencing these avian-microbe (both gut microbiomes and blood parasites) associations, host species tend to be the main driver, indicating species-specific interactions modelling these symbioses. The influence of gut physiology and diet on gut microbiomes suggests that long-term associations between avian hosts and gut bacteria are determined by the stability of gut environments and host diets. The species-specific effects of environmental and anthropogenic variables on haemosporidian prevalences, indicate the potential importance of host ecology on shaping these host-parasite associations. Overall, the results of my PhD thesis show that the examined avian-microbe associations are driven by a multitude of abiotic and biotic factors and that their relative importance of shaping these symbioses can differ with host physiology and ecology.