During this breeding season we started a new project on Jackdaws in southern Sweden. Jackdaws are medium-sized birds that most people have probably seen already. They breed in holes and when one hangs up appropriate nestboxes, they can even breed in dense colonies. The group of Jan-Åke Nilsson and Dennis Hasselquist has established such a colony years ago. This year all 57 nestboxes are occupied. At the end of April the Jackdaws started laying eggs. As most birds, they lay one egg per day and they start incubating when the clutch is (almost) complete. Most clutches contained 4-6 eggs and after 16-18 days of incubation the nestlings hatched.
|The left nestling hatched one day later than the rest.|
When females start to incubate only after they laid the last egg, all nestlings hatch on the same day. If birds start to incubate from the first egg (for example common in owls) but continue laying one egg per day for several days, this means that the first egg will hatch earlier than the last egg. In a clutch of 7, this will be 7 days difference between first and last. All young will be of different age then. This is what you might have seen in pictures of young owls when several brothers and sisters are standing next to each other, but everyone has a different size (=age). Jackdaws sometimes also start incubating when the egg before last is laid. Thus the last nestling will hatch one day later than the others.
|A female Jackdaw with a brood patch.|
The gender that incubates the eggs loses feathers on the belly to ensure an optimal transfer of the body warmth to the eggs. They develop a so-called “brood patch”. Normally the feathers are there to insulate the body, so they would make incubating eggs very difficult. Besides losing the feathers, the blood transfer in the skin is increased to facilitate the perfect regulation of the temperature. The skin looks than almost like burned skin. If the male is the sex that incubates the eggs, the male develops such a brood patch. If both parents share incubating, both have a brood patch. In Jackdaws only females incubate. Thus the brood patch also provides an easy way to distinguish the sexes. But this characteristic is only available during the breeding season. Soon the females will grow feathers on the belly again.
|Naked, blind and hungry.|
When young Jackdaws hatch, they are naked and blind. They open their eyes when they are 11-14 days old. That long they only react to noises by their mother and open their bills straight up to get food. The feathers develop in the course of the next weeks. As long as they have no feathers, the mother needs to keep them warm (a behaviour called brooding) because the nestlings can’t keep up their body temperature alone. Only when they are about 12 days old, they can stay warm without the help of their mother if the ambient temperature is reasonable. If small nestlings stay for too long without their mother, they will get cold and eventually die. In Jackdaws only the female incubates the eggs and broods the young. The male helps with nest building and feeds the female while she is incubating. He also helps feeding the nestlings. In the first 12 days of their life, when the female needs to stay with the young to keep them warm, the male is responsible for bringing food for his family. This role allocation can be very different in other species. In some other species, males and females share incubating the eggs (e.g. pigeons), in some species only the males incubates (some waders). Nest building can be the responsibility of only the male or only the female (e.g. Skylarks).
Are all nestlings naked when they hatch? No, young of some species have feathers when they hatch. Not real feathers, but down feathers. Basically all birds that stay in a nest after they hatch are more or less naked (except some tiny feathers). These are called altricial birds. But there are species that do not stay in a nest, but start running around as soon as they hatch. These birds are called presocial birds. You may wonder what kind of birds these are, but you all know at least one example: chicken! There are more species, for example all ducks and geese. Young birds of these species have feathers when they hatch to give them some protection against wind and rain. Still they need to be kept warm by their mother in very regular time intervals (a behaviour called brooding). For these species it is very important that all young hatch on the same day.
|12-14 days old nestlings.|
When the nestlings were 12-14 days old, we caught the parents to take a blood sample and to attach GPS loggers to females (se below). As Jackdaws within a colony are very synchronized, there are many birds to catch within a few days. For the field worker this means extremely busy days and working for 15-18 hours per day is the rule rather than the exception. The GPS loggers provide us with information on the movement behaviour of the Jackdaws during a complete annual cycle. We will combine these data with data we collected on their breeding success. Furthermore we took a blood sample from all individuals to quantify different components of their physiology.
|A female wtih a GPS-logger.|