You didn’t mention what species of birds you have. Since you mentioned their cere color – the skin around their nostrils – I am going to assume these are parakeets since that is a way to tell gender in many parakeets. With females, the cere is white to brown – brown & crusty when they are in breeding condition. In males it is lavender to bright blue. However, in some mutations, mainly the pastel colors, the cere may remain lavender in both male and females.
Based on their behavior, you may have a male and female. Although sometimes same sex birds can behave like a bonded pair. Obviously one bird – probably the one you call Shirley – is a female since there is an egg, Shirley is the one spending time in the nest, and she has a white cere. However, just because she laid an egg does not mean it is fertile or that she is ready to breed, especially since her cere did not turn brown and crusty. Since the other bird is mounting Shirley and either mating or attempting to, this is most likely a male. A female bird probably would not mount another female bird. Eggs are generally laid one day apart and each egg needs to be fertilized, thus the mating when Shirley leaves the nest box.
If you don’t want to deal with breeding birds and baby parakeets, you need to remove the nest box and discard the egg or eggs immediately, before they have a chance to start developing. Then split the birds up or the mating and egg laying will continue with or without a nest.
If you want to see this through, you need to start researching parakeet breeding. The parents need to be on a fortified diet – a loose seed mix is not adequate, especially for breeding birds. A pelleted diet or a diet like our foraging diets – Nutri-Berries or Avi-Cakes – should be the main diet. You need to also offer dark leafy greens, chopped veggies and fruits, cooked eggs & multi grain bread. If they do hatch chicks and do feed them, the parents will need a LOT of food daily, replenished throughout the day, as well as fresh water. Both parents help raise the chicks, so both birds need to stay together.
If you don’t want them to breed, do as I mentioned above with removing the nest and splitting the pair up. Birds do not need a nest unless they are breeding, and then only during breeding season. Adult birds do not sleep in a nest in the wild. a nest is specifically for eggs and chicks and is abandoned once the chicks have left the nest. The rest of the time the adults roost in trees mainly. So never give a bird a nest unless you have a bonded pair and intend to breed them. Otherwise you end up with the situation you have. And even if you only have a female, giving her a nest is a bad idea because it will encourage egg laying. Laying eggs is hard on a bird and a single bird can become a chronic egg layer if they have a nest.
Regardless of what you decide about letting them breed, they need a fortified diet. Here is a feeding guide to help you.
Bird Food Guide
Thank you for asking Lafeber,