Table 1 Hypothesised causal links and mechanisms between our causal and affected variables including parental care strategy, following three categories of hypotheses: ecological factor, social environment, and trophic interactions.

Ecological

Relative nest abundance

50% Snow cover

1. The Nesting Site-Abundance Hypothesis: Densities correlates with the availability of snow-free grounds & the phenology of snowmelt during the pre-breeding stage. The more snow-free ground available, the higher the relative nest abundance (modulates nest space available, exposure to predation & pre-breeding feeding conditions (e.g.¹⁹)

Ecological

Relative nest abundance

NAO

2. The Macroclimate-Abundance Hypothesis: Food depletion resulting from competition at stopover sites/depleted food-supplies, and adverse weather conditions during migration can lead to substantial mortality and poor body conditions reflected in lower breeding densities²⁰. NAO index in May can capture the conditions experienced by Sanderlings at the end of their migration²¹. We expect higher breeding densities when the NAO index in May is higher

Trophic interactions

Relative nest abundance

Predation

3. The Predation-Abundance Hypothesis: Predators’ distribution, abundance and activity can prevent shorebirds to breed and therefore impact their distribution and abundance, both directly (via active predation) or indirectly via clues used by preys (e.g., visual or sent clues). Indeed, birds acquire information from the environment to adjust their habitat selection, behaviours, & dispersal strategies to minimise predation risk^22,23. The higher the predation potential, the lower the densities

Ecological

Laying date

50% Snow cover

4. The Nesting Site—Breeding Phenology Hypothesis: Snow-free ground are necessary to breed, hence snowmelt is a necessary condition for nest initiation (e.g.,^24,25). We therefore expect a later snowmelt to induce delayed breeding

Ecological

Laying date

Regional temperature

5. The Energetic-Breeding Phenology Hypothesis: Low temperatures during the pre-nesting period can (a) reduce the availability of arthropods to shorebirds²⁶, and (b) increase the thermoregulatory costs for the birds²⁷. We expect that lower temperatures would delay laying dates

Ecological

Laying date

NAO

6. The Macroclimate-Breeding Phenology Hypothesis: Weather conditions during the migration period can impact the arrival date of birds^28,29, their body condition at arrival³⁰, their clutch size³¹, & their overall breeding success^31,32. We expect poor migration conditions (i.e., low NAO indexes) to be linked with delayed laying because of later arrival dates & altered body conditions

Ecological

Strategy

50% Snow cover

7. The Energetic Constraint Hypothesis: Under coldconditions during migration/on the breeding ground, extended snow cover during the pre-breeding period can lead to poor feeding conditions & increased energetic costs. Under these energetic constraints, we expect parental cooperation when raising the offspring, and an increasing level of biparental care (e.g.,^6,12)

Ecological

Strategy

Regional temperature

Ecological

Strategy

NAO

Social environment

Strategy

Relative nest abundance

8. The Re-mating Opportunities Hypothesis: Social environment shapes the mating system and parental role⁶. Availability of more potential partners may lead to an increase in re-mating opportunities, and to more uniparental nests

Social environment /intrinsic variable

Strategy

Laying date

9. The Re-mating Timing Hypothesis: Having a time constrained window for reproduction, and if desertion is linked with re-mating, then earlier laying dates could allow time for other copulation attempts, increasing reproductive success¹⁵. Earlier laying dates could lead to more uniparental nests

Trophic interactions

Strategy

Predation

10. The Predation Hypothesis: Biparental nests have a higher nest attentiveness; cryptic incubating adults might be harder to locate than unattended nests. Due to their incubation behaviour, uniparental nests have higher probabilities to be predated³³. We would expect that under high predation pressure, more nests would be biparental