Table 3. The effect of parental age on yearly breeding success of sons and daughters.

The study was performed on Blackbirds (Turdus merula) in the Żeromski Park (Szczecin, NW Poland) in 2006, 2007, 2016 and 2017.

nind sons = 75, nbroods = 202	Estimate	se/sd	Chi2	P
(Intercept)	−0.126	0.883
Father’s age	0.687	0.339	4.126	0.042
Father’s age2	−0.062	0.033	3.598	0.058
Mother’s age	−0.415	0.448	0.863	0.353
Mother’s age2	0.036	0.050	0.533	0.465
r(ID)	0.124	0.351

Notes.

Estimated parameters of a generalised linear mixed model with negative binomial error distribution; yearly breeding success - calculated as the total number of fledglings produced in a given breeding season.

ID

individual number of nestling

r

random effect

n_ind

number of birds (sons/daughters) which bred in the study area

n_broods

total number of records used in the analyses; individual birds repeated their breeding attempts in different breeding years