Previously, I wrote about regularly visiting a nest of pied flycatchers in order to see how the little chicks were rapidly developing: from tiny pink monsters to complete birds in just two weeks time. Last summer, I decided to do the same for our two other box-breeding birds: blue tits (Cyanistes caeruleus) and their larger cousins, the great tits (Parus major). Having these picture series also happens to be very practical as reference material for determining the age of newly encountered birds: their development is so rapid, that clear differences can be seen in for example the emergence and growth of particular feathers from one day to the next.
These two species don’t develop quite as quickly as the pied flycatchers, leaving the nest a few days later (when about 18 days old). I only visited them until the age at which we measure and ring them (13 days for blue tits, 14 days for great tits) as additional disturbances in the later days might have led to them trying to fledge early. In these last few days in the nest, it’s especially their wing and tail feathers that would have grown some more — they can’t quite fly with what you see in the last pictures here, which is a good thing too: when handling them, the energetic little things are enough of a flight risk even without being capable of actually flying!
Aside from their size, the blue tits (above) and great tits (below) are largely similar for the first few days, with their distinctive colours only becoming apparent in the second week.
Although, more accurately, they don’t actually really have their distinctive colours yet. While their plumage as youngsters isn’t as dissimilar to that of adults as it is in flycatchers, it still is markedly different. Compared to an adult great tit, the plumage of the young one looks like it had a yellow filter put over it: the white areas are yellow, the glossy black parts a dull brown. For the blue tit, the characteristic colour from which it gets its name is still entirely absent, instead appearing some shade of green!
For adult birds, their colours are known to play an important role in attracting a partner: some of these pigments cannot actually be produced by the birds themselves, and instead originate from their food, which means that only those birds who are capable of finding a lot of the right type of food can produce the right shade. Demonstrating an ability to efficiently find food makes you a pretty attractive suitor, as the female birds largely rely on their partners to provide them with food when they themselves are incubating eggs, and it will take the combined efforts of both parents to feed all those hungry mouths of their offspring. For the chicks, signalling to prospective partners is not something they have to worry about until next year, so the more subdued colours are more practical by offering some camouflage. Looking different from the adults may also prevent getting into trouble with other birds of the same species, as they won’t be regarded as potential competitors when entering these birds’ territories.
-  Norris KJ (1990) Female choice and the evolution of the conspicuous plumage coloration of monogamous male great tits. Behavioral Ecology and Sociobiology 6: 129–138.
-  Fitze PS, Kolliker M, Richner H (2003) Effects of common origin and common environment on nestling plumage coloration in the great tit (Parus major). Evolution 57(1): 144–150.