1. Field methods.
We conducted the study between the end of May – early June 2017 on a free-living, known-age breeding population of common gulls located on Kakrarahu islet in Matsalu National Park on the west coast of Estonia (58°46’ N, 23°26’ E)34. Based on studies of offspring recruitment rate, reproductive success in common gulls increases to the 10th breeding year (12-13 years of age) and decreases thereafter (Rattiste, 2004). It is known that they undergo senescence in a number of physiological functions (Rattiste et al. 2015; Sepp et al. 2017; Urvik, Rattiste, Giraudeau, Okuliarová, Hõrak & Sepp, 2018). In addition, this species is characterized by bi-parental care and a low frequency of extra-pair mating ((Bukacinska, Bukacinski, Epplen,, Sauer, & Lubjuhn, 1998), making it possible to study the effects of age of the parent of both sexes, and the age-dependent quality of parental care. All of the birds included in the study were banded as chicks and their exact age was therefore known. Common gulls typically lay clutches of three eggs.
A total of 40 clutch nests were included in the experiment. Nests and experimental groups were chosen based on the age of the mother, but the father’s age was also known. Common gulls mates assortatively with respect to age and the ages of the parents were highly positively correlated (Spearman r=0.74, p<0.0001) Half of the breeders (n = 20 females) were young, on their 1st-3rd breeding event (age exactly 5 years). Another half (n = 20 females) were middle-aged or older (15-30 years old, average age 18 ± 3.37 (SD) years). In total, 19 males were 5-7 years old (average age 5.52 ± 0.80 (SD) years) and were grouped as “young”, 21 males were 10+ years (average age 16.29 ± 5.58 (SD) years) old and were grouped as “old”. Distribution of the ages of parent birds are shown in figures S4-S7.
We cross-fostered whole clutches right after the clutch was completed both within and between maternal age classes (Table 1), so all of the chicks included in this experiment hatched in foster-parent’s nests. Half of the clutches were cross-fostered within age classes and half were cross-fostered between age classes. The 72 chicks that were successfully caught for second sampling were included in the study. Within two days from hatching, we collected the first blood sample (10-30 µL taken, from brachial vein) for telomere measurement (average age 0.61±0.09 (SE) days) and individually marked the chicks for identification. Chick head size (the distance from the tip of the bill to the back of the head) was measured with a calliper to the nearest 0.1 mm. The day of the second blood sample varied between chicks, due to difficulties in trapping precocial chicks. We captured the chicks near their nests and collected a second blood sample between the 8th and 20th day after hatching (with one exception of a 5-days old chick). Average age at second blood sampling was 10.62 days (±0.35 SE). Blood samples were kept on ice in an insulated box while on the islet, and stored at -20°C until the end of the field work period, when they were transferred to -80ºC and held there until analysed. The experimental protocol was approved by the Ministry of Rural Affairs of the Republic of Estonia (licence no. 106, issued 24.05.2017) and was performed in accordance with relevant Estonian and European guidelines and regulations.