Double-brooding and annual breeding success of great tits in urban and forest habitats

Urban areas differ from natural habitats in several environmental features that influence the characteristics of animals living there. For example, birds often start breeding seasonally earlier and fledge fewer offspring per brood in cities than in natural habitats. However, longer breeding seasons in cities may increase the frequency of double-brooding in urban compared to non-urban populations, thus potentially increasing urban birds’ annual reproductive output and resulting in lower habitat difference in reproductive success than estimated by studies focusing on first clutches only. In this study, we investigated two urban and two forests great tit Parus major populations from 2013 to 2019. We compared the probability of double-brooding and the total number of annually fledged chicks per female between urban and forest habitats, while controlling for the effects of potentially confounding variables. There was a trend for a higher probability of double-brooding in urban (44% of females) than in forest populations (36%), although this was not consistent between the two urban sites. Females produced significantly fewer fledglings annually in the cities than in the forest sites, and this difference was present both within single- and double-brooded females. Furthermore, double-brooded urban females produced a similar number of fledglings per season as single-brooded forest females. These results indicate that double-brooding increases the reproductive success of female great tits in both habitats, but urban females cannot effectively compensate in this way for their lower reproductive output per brood. However, other mechanisms, like increased post-fledging survival can mitigate habitat differences in reproductive success.

Breeding biology of two sympatric Nesospiza finches at Nightingale Island, Tristan da Cunha

Nesospiza finches are a classic example of a simple adaptive radiation, with two ecologically distinct forms confined to the Tristan da Cunha Archipelago, South Atlantic Ocean: an abundant, small-billed dietary generalist, and a scarce, large-billed specialist. These have segregated into two species at Nightingale Island, but there is still local introgression between the two forms at Inaccessible Island. We describe the phenology and breeding behaviour of the two sympatric species at Nightingale Island (2.6 km2): Wilkins’s Finch Nesospiza wilkinsi (Endangered) and Nightingale Island Finch N. questi (Vulnerable). The finch breeding season starts in late October-November but the onset of breeding varies by 4–5 weeks among years. The small-billed Nightingale Island Finch typically (two of three study seasons) starts breeding 1–3 weeks earlier than the large-billed Wilkins’s Finch, unlike at Inaccessible Island where the Wilkins’s Finches start breeding first. Laying of initial clutches was quite well synchronised, peaking 1–2 weeks after the first nests were found. Females constructed the nests, which were mostly (>90%) in dense Spartina arundinacea tussock grass stands and occasionally in ferns or sedge grasses. Clutches comprised one or two eggs, with no difference between Wilkins’s (1.66 ± 0.48) or Nightingale Island finch clutches (1.71 ± 0.46). Incubation periods averaged longer for Wilkins’s Finch (18.3 ± 0.5 d) than Nightingale Island Finch (17.7 ± 0.5 d), but this difference was not statistically significant. Females incubated the eggs, and were fed by the males. The difference in egg volume within two-egg clutches was 2–13% for Wilkins’s Finches (mean 5.9 ± 3.3%) and 1–19% for Nightingale Island Finches (mean 8.4 ± 5.3%). At least 31% of pairs re -laid after their first breeding attempt failed but there was no evidence of double brooding. Repeat nests were 0–20 m (mean 5.6 ± 4.9 m) from the initial nest site and inter-seasonal nest sites for 38 known pairs were 0–33 m apart (mean 12 ± 9 m). No inter-species pairs or hybrid birds were seen, but two instances of inter-species fledgling provisioning were observed.

Increased reproductive investment associated with greater survival and longevity in Cassin's auklets

Individuals increase lifetime reproductive output through a trade-off between investment in future survival and immediate reproductive success. This pattern may be obscured in certain higher quality individuals that possess greater reproductive potential. The Cassin's auklet ( Ptychoramphus aleuticus ) is a long-lived species where some individuals exhibit greater reproductive ability through a behaviour called double brooding. Here, we analyse 32 years of breeding histories from marked known-age auklets to test whether double brooding increases lifetime fitness despite the increased mortality and reduced lifespan higher reproductive effort would be expected to incur. Multistate mark–recapture modelling revealed that double brooding was strongly positively associated with higher annual survival and longevity. The mean (95% confidence interval) apparent survival was 0.69 (0.21, 0.91) for individuals that executed a single brood and 0.96 (0.84, 0.99) for those that double-brooded. Generalized linear mixed models indicated individuals that attempted multiple double broods over their lifetime were able to produce on average seven times as many chicks and live nearly 6 years longer than birds that never attempted a double brood. We found that high-quality individuals exhibited both increased reproductive effort and longevity, where heterogeneity in individual quality masked expected life-history trade-offs.