Many bird species have very little sexual dimorphism. It takes an expert breeder to tell male and female peach-faced lovebirds apart, for example, and they have to do it by palpating the pelvic bone and judging from the width of a pelvis which sex the bird is. Even that has a degree of subjectivity and guesswork, and they’re not always right. Mockingbirds, which I’ve studied, are another example. In the spring-summer, the only way to tell a male from a female is that only the male sings, but in the fall-winter, both sexes sing equally and even that means of distinguishing them is lost.
Generally speaking, the more socially monogamous a bird species is, the less sexual dimorphism there is. In highly polygynous species (one male mating with multiple females), such as mallards, peafowl, and birds of paradise, there is much more sexual dimorphism.
Which is male and which is female in these lovebird and mockingbird pairs; can you tell? I can’t. Nor can an ornithologist, without either dissecting them or watching for behavioral differences.