Chick measurements and stable isotope signatures (Larus fuscus)
Description
Lesser Black-backed Gull (Larus fuscus) chick growth and diet measurements taken (1) in the breeding colony of the Port of Zeebrugge (Belgium) and (2) on aviary-raised chicks. Head length from the external occipital protuberance to the tip of the bill was measured to the nearest mm with a digital caliper, and body mass was measured to the nearest g with a digital scale. Measurements in the field were taken every first, third and fifth weekday, until all remaining chicks had reached 30 days after hatching. Measurements in the aviary were taken upon hatching and, from then on, every 5 days. We inferred chick diets in the field from the stable isotope profile of their innermost right primary feather, and raised chicks in captivity on an either fully marine, fully terrestrial, or 50/50 mixture unrestricted diet.
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We studied the development of free-ranging chicks in a mixed breeding colony of lesser black-backed gulls and herring gulls (Larus argentatus) at the outer port of Zeebrugge (Belgium, 51°20'53"N 3°10'20"E). The colony counted 1458 L. fuscus pairs in 2016 and 1326 pairs in 2017 (Stienen et al. 2017, 2018.). L. fuscus exhibits a limited sexual dimorphism (males being on average larger than females but with a large overlap) and shows bi-parental care. The species lays 2-3 eggs during May and June, of which the first two are largest (Verboven et al. 2005). At the onset of breeding in 2016 and 2017, 17 and 6 nests were randomly selected for monitoring and enclosed with chicken wire, respectively. Sample size was reduced in 2017 to avoid further disturbance of the colony after a year with high nest predator pressure. Each selected nest was visited every first, third and fifth weekday. At the estimate day of hatching, experimental nests received first- or second-laid eggs of equal developmental stage (= pipping eggs), each originating from a separate nest to ensure hatching synchrony, thus preventing survival differences due to hatching asynchrony within and among clutches, and ruling out effects of parental genetic quality on chick development. On the hatching day, nestlings were individually marked with coloured tape, and down feathers were collected for molecular sexing. During each visit, chick body mass (to the nearest g) and head length (to the nearest mm) were measured. At 30 days of age, the right innermost primary feather (P1) of each monitored chick was plucked and stored for isotope analysis.