= Solution
The <regression diagnostics> raise concerns about the mean and <variance> assumptions. The residual-versus-fitted smoother is curved rather than horizontal, and the scale-location plot suggests changing residual spread, contrary to <homoscedasticity>. Diagonal bands are expected when an <integer>-valued response is fitted by a continuous model, since each fixed response value gives $r=y-\widehat y$. They are not, by themselves, proof of a failed mean model. The residual <Q-Q plot> is broadly linear but has tail departures and unusual observations, so exact <normality> is questionable.
The <Cook's distance> plot identifies observations worth checking for recording errors and influence on the age estimate. A rough $4/n\simeq0.023$ screening threshold flags several points, but it is not an automatic deletion rule. Repeat the fit with and without influential observations as a sensitivity analysis and report material changes.
The bird <random intercept> <Q-Q plot> has a pronounced upper-tail departure; the year plot is closer to a line, although eleven groups give little power to assess <normality>. These plotted effects are estimated, shrunken <conditional mode of Gaussian random effects>, not independent observed draws from the latent <normal distribution>, so their appearance requires care.
The analysis is therefore provisional. Check a nonlinear age effect, the changing residual <variance>, dependence beyond the fitted <random intercepts>, and a response model appropriate for counts. If clutch size is known, a <binomial distribution> model for hatched eggs out of available eggs may be more meaningful. A bootstrap from the existing Gaussian model cannot validate the assumptions questioned by these plots. Distinguishing within-bird ageing from between-bird differences would also strengthen interpretation.
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