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.