Individual variation explains ageing patterns in a cooperatively breeding bird, the long-tailed tit Aegithalos caudatus.

In the case of direct fitness, this effect was marginally significant when individuals were grouped as 4+ ( β = −0.53 ± 0.30, χ 2 = 3.78, p = 0.05) but not when assigned their true age (Table S1 c).

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