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

Abstract

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.

Keywords: Cassin's auklet; double brooding; heterogeneity; individual quality; mark–recapture; survival.

Figure 1.

Distribution of total lifetime double brooding attempts (nDB) for individual Cassin's auklets that only attempted single broods (grey), had double-brooded once (orange) or double-brooded two or more times (blue) in their lifetime. Data are not censored for LRS of birds that were still alive in 2015.

Figure 2.

Distribution of LRS for individual Cassin's auklets that only attempted single broods (grey), had double-brooded once (orange), or double-brooded two or more times (blue) in their lifetime. Data shown are not censored for birds that were still alive in 2015.

Figure 3.

Response curves of the top generalized linear mixed model for LRS as a function of nBY and double brooding attempts (0, 1 and 2 or more lifetime double broods) for male and female Cassin's auklets. Raw data are plotted with a jitter to show clustered points.

Figure 4.

Response curves for modelling apparent survival as a function of the interaction between lifetime double brooding attempts (0, 1 and 2 or more) and age for pooled and unknown sexes, while holding time constant at 2008 (an average year). Shaded ribbons indicated 95% confidence intervals. Dashed lines represent survival of birds following a year where they executed a single brood (state A), dotted lines for survival of birds following double broods (state B). Confidence intervals for estimates of a double-brooded state overlap with those in a single-brooded state, and are not shown.

Figure 5.

Annual survival estimates for Cassin's auklets from SEFI in relation to annual upwelling conditions. (a) Mean (red line) spring upwelling strength (UI) anomalies for the waters surrounding the Farallon Islands. Shaded ribbon indicates annual maximum and minimum values for the months of April–August. (b) Response curves for modelling apparent survival as a function of time, for birds that had never double-brooded in a single-brooded state (grey), and repeat double brooders in a double-brooded state (blue), while holding age constant at 5 years. Shaded ribbons indicated 95% confidence intervals.

Figure 6.

Age structure of the single-brooded (grey) and double-brooded (blue) known-age breeding population in followed nest-boxes, where the width of each density curve indicates the relative number of individuals (n) of a specific age group in a given year.