A POPULATION STUDY OF SKYLARKS ALAUDA ARVENSIS

SUMMARY A population of colour‐ringed Skylarks Alauda arvensis was studied intensely over two and less thoroughly over another five breeding seasons in Kavenglass, Cumberland. The breeding season extends from mid‐April to early July, but the onset, which is well synchronized, varies from year to year, probably depending on the spring temperature. One‐year‐olds lay the first clutch about a week later than older birds. Within a group of females there is a constancy in the laying order of first clutches from year to year. The rank of any given female seems to depend on an individual temperature threshold, which is reached four to seven days before laying, but other factors must play a role. The interval between the disappearance of a clutch or brood and the laying of the next clutch varies from four to six days depending on the stage at which the previous brood was lost. Beyond the 17th or 18th day of life the chicks of one brood no longer prevent the laying of a new clutch. No factors influencing the end of the season could be found. On average, Skylarks lay 2.7 clutches per year (range 1–5). Three successful broods seems to be the maximum. Average c/size is 3.69 eggs (range 3–5). The c/size varies with successive clutches, from 3.3 for first, to 4.0 for third clutches. Individual females tend to lay clutches of a characteristic size. No difference could be found between the clutches of one‐year and older birds, Females which retain the same territory from year to year show more consistency in clutch size than those which change territories. Data from different geographical locations in Europe suggest a cline from northwest to southeast of increasing clutch size. Clutch size does not seem to be affected by temperature. The variability of egg size and shape is discussed. 68% of the eggs laid hatched (incubation period 11 days) and 67% of the young hatched left the nest on the 8th day. About 90% of the eggs and young lost were taken by predators. No consistent seasonal differences in nest mortality could be found, nor between c/3 and c/4. An incidental experiment suggests that predation of Skylark nests is density dependent. The success of one‐year‐old females in rearing nest‐leaving young is lower than that of older birds (15% compared with 51%). From observations of the feeding behaviour of the parents it is estimated that about 25% of the young survived to flying age (16th day of life), and about 10% reached independence (30th day). Skylarks, males more than females, exhibit a strong site‐tenacity. One‐year‐old birds tend to return to the birthplace area. Associated with site‐tenacity the Skylarks show mate‐faithfulness. Of 36 pairs 17 remained faithful from one year to the next. During a breeding season mate‐faithfulness is practically absolute. There is a correlation between the breeding success of a pair and the likelihood of the pair remaining together the following year. Mean annual adult mortality was 33.5%, with little variation except in 1962/63 (47%‐the result of a hard winter). After reaching independence, the first‐year mortality of juveniles was estimated to be 38%, on the assumption that the population remains constant in numbers. The average “life‐expectancy” of a newly laid egg is about 3 1 / 2 months and that of a newly hatched young about 5 months. The mortality sources for adult Skylarks are briefly discussed. Skylarks have a well‐developed territorial behaviour which is an important mechanism in regulating the maximum breeding density. In conjunction with site‐tenacity, habitat preferences and social attraction, it is capable of regulating the reproductive rate of the total population by excluding a proportion of birds from breeding when the population numbers are high.