Age of experience
MOST British passerines breed in the year following hatching. Although less than one year old, they are able to display, pair up, build a nest, lay eggs (if female) and raise young. With little preparation and next to no experience, it may be their one and only chance to reproduce, and some don’t survive the exercise. Those which do become more successful the older they get.
These first breeders are in direct competition with established, adult birds which may already have a mate and a territory which they can easily retain and defend from allcomers. An experienced old-timer is likely to win a mate more quickly, will be in better condition, can feed more effectively, will find it easier to keep a territory and will be better equipped to build and defend a nest.
Resident species benefit from being familiar with the area, while new summer visitors arriving in their first spring have to face their elders (and definitely betters), who will beat the youngsters back and already be established on territory.
Where there are dominant, territorial adults there will be less chance to find suitable habitat vacant, with the best nest sites already occupied and no spare mates of the right quality. A young, inexperienced bird will probably find a poor mate in a sub-optimal breeding area and so have little chance of success. Better to wait a year.
Most passerines have fairly short lives, though, so can’t afford to delay breeding. The only British passerines which regularly do so are members of the crow family. Jackdaw, Rook, Magpie and Chough breed when two, Carrion Crow when three and Raven when four.
Bird families with a longer lifespan have no rush to breed in their first year, and so can take their time to mature and gain valuable experience. They can delay breeding for one or more years, watching and learning in their first year, or staying away completely in the case of many seabirds, to become fit and strong before returning to their natal area.
Great Cormorant and European Shag breed when three, as do European Storm Petrel and Black Guillemot. Northern Gannet,
Manx Shearwater, Razorbill and Guillemot all take five years, while Puffin takes six. The most delayed of all is Northern Fulmar which takes seven years.
Gulls are well known not to reach maturity for several years, resulting in what many birders find to be a confusing array of immature plumages. Great Blackbacked Gull may take four years to become mature, while Lesser Black-backed and European Herring take four, Common Gull takes three and Black-headed can breed when only two. The most pelagic of our gulls, Kittiwake, takes four years. Most terns take four years, as does Arctic Skua, while the larger Great Skua takes six.
Most ducks breed in their first year, but Greater Scaup, Common Goldeneye and mergansers take two. Geese may take two or three years and swans take up to four. Large herons take two years, while smaller egrets take only one. Waders mainly take two years, with Eurasian Oystercatcher, Red Knot and Sanderling taking three.
Among birds of prey, the smaller species like falcons and Eurasian Sparrowhawk breed after one year, while Peregrine Falcon and Northern Goshawk take two. Larger raptors such as Common Buzzard and harriers also take two, while Western Osprey and Red Kite take three and the two large eagles take five years.
Black Grouse form leks where the males compete for breeding rights. Birds may not breed until three or more years old as young birds can’t stand up to the older males.
There may also be variation within a species, with some delaying breeding while others start to breed in their first year. A study of Northern Lapwing showed that two-thirds bred when one year old, while a quarter waited for two years and the remainder took three years.
By having an immature plumage before being old enough to breed, young birds avoid being seen as rivals by adults. Several hypotheses have been put forward as to why some birds delay breeding even though they are mature, and maintain a subadult plumage as one-year-olds. These seemingly immature males often resemble females, and it has been suggested that this means they will not be seen as competitors by experienced adult males, which might enable them to sneak in an early opportunity to breed. Chris Harbard