**Background:** Optimization of clutch size and timing of reproduction are key determinants of lifetime reproductive success in vertebrates. Both individual quality (age, body condition) and environmental variation (climate, food availability) may influence these life history trade-offs. In high-latitude alpine systems, harsh and unpredictable conditions impose strong selection on reproductive strategies. The authors tested competing hypotheses of common versus state-dependent optimal clutch size (following Gaillard et al. 2016), and examined how spring climate, maternal age, and body mass affect timing of breeding and reproductive output in willow ptarmigan (Lagopus l. lagopus).
**Methods:** Data were collected over 17 breeding seasons (1978–1994) in a ~30 km² study area on the eastern border of Dovrefjell-Sunndalsfjella National Park, Norway (62°17'N, 09°36'E). A total of 290 breeding females with 319 breeding attempts were monitored. Nests were located using pointing dogs and by searching for clocker droppings; females were captured with throw-nets, weighed to the nearest 5 g, and aged as juvenile (<1 year) or adult (>1 year) based on primary feather pigmentation. Clutch initiation dates were estimated via flotation tests and the assumption of one egg laid per day with a 21-day incubation period. Weather data (temperature, snow depth) came from Fokstua Meteorological Station ~30 km south; the NAO index (May–July) was also used. Individual-based weather windows (pre-oviposition, pre-incubation, incubation; each 21 days) and general seasonal windows (15, 30, 45, 60 days before median laying date of 27 May) were defined. Only initial nesting attempts were analyzed; renests and one outlier (16 eggs) were excluded. Statistical analyses used generalized linear mixed models (Conway-Maxwell Poisson for count data) and linear mixed models, with year as a random intercept. Model selection was based on AICc. Repeatability was estimated using the rptR package, controlling for significant fixed effects.
**Key Results:** The most common clutch sizes were 9 or 10 eggs (51.6% of initial clutches); mean clutch size was 9.7 (95% CI [9.25, 10.15], range 5–13). For the optimal clutch size analysis (model 1, n = 154), the null model ranked highest, indicating no clear effects of maternal weight, age, pre-incubation temperature, or NAO on clutch size. For hatchling number (model 2, n = 87), the top models included a quadratic effect of eggs, with number of hatchlings increasing with clutch size up to a peak, but clutch sizes remained below the estimated optimum. An interaction between eggs and maternal weight was supported (β_eggs×weight = 0.019, CI [0.001, 0.037]), where heavier females produced more hatchlings from large clutches. For timing (model 3, n = 155), higher spring temperature advanced egg-laying: a 1°C increase over 15 days advanced laying by 0.62 days (CI [−1.136, −0.097]). Juveniles started laying earlier than adults (β_age.juv = −1.04, CI [−2.152, 0.086]), contrary to prediction. No effect of maternal weight on timing was found. Earlier laying was strongly associated with larger clutch size (model 4, n = 182): a reduction of 0.20 eggs per day delay (β = −0.083, CI [−0.107, −0.058]). Earlier laying also increased hatchling number (model 5, n = 108): a reduction of 0.18 hatchlings per day delay (β = −0.083, CI [−0.119, −0.047]). Spring temperature (11–20 days before median laying date) positively predicted female body mass (model 6, n = 164): a 1°C increase was associated with a 5.1 g increase (β = 9.773, CI [2.524, 16.759]). Repeatability was high for timing of first egg (R = 0.59, CI [0.31, 0.86]) and very high for clutch size (R = 0.74, CI [0.56, 0.89]).
**Clinical Implications:** While this is a non-clinical ecological study, the findings have implications for understanding how alpine and Arctic species may respond to climate change. Warmer springs advance breeding and increase female body mass, which could initially boost reproductive output. However, earlier breeding may expose chicks to stochastic late-spring weather events, potentially reducing juvenile survival. The high individual repeatability in reproductive strategies suggests that phenotypic composition of populations, rather than age structure alone, will influence population-level responses to climate change. The lack of state-dependent clutch size adjustment under unpredictable conditions may represent a bet-hedging strategy that could be maladaptive if climate change alters the predictability or timing of resource availability.