Diagnosing the timing of demographic bottlenecks: sub-adult survival in red-billed choughs

P>1. Determining the precise timing and location of major demographic bottlenecks, such as periods of low survival, is key to identifying ecological causes of variation in population growth rate. Such understanding is key to designing efficient and effective mitigation.

2. In a protected population of red-billed chough Pyrrhocorax pyrrhocorax on Islay, Scotland, variation in population growth rate largely reflects among-year variation in first-year survival. First-year survival was unprecedentedly low during 2007-2010, threatening population viability.

3. We used colour-ring resightings to estimate monthly survival probabilities (Theta(m)) throughout the first year from fledging for eight chough cohorts (totalling 519 individuals) representing the full observed range of variation in first-year survival. We thereby identify the time and location of recent low survival.

4. On average across all cohorts, Theta(m) varied among months, being low during the first month after ringing (May-June, accounting for c. 24% of all first-year mortality) and high during the last 4 months of the first year (January-May, accounting for c. 6% of all first-year mortality). Most mortality (c. 70%) occurred after fledglings dispersed from natal territories.

5. The 2007-2009 cohorts experienced low Theta(m) during July-December. This represents an additional low survival period compared to previous cohorts rather than decreased Theta(m) across all months or further decreases through periods when Theta(m) was low across all cohorts.

6. Synthesis and applications. These data have general relevance in showing that dramatically low annual survival, which needs to spark rapid management action, can reflect different and unanticipated periods of low survival rather than exaggeration of typical variation. With specific regard to conserving Islay's chough population, our data show that sub-adult survival has recently been low during July-December, probably reflecting conditions on key grassland foraging areas. Managers aiming to increase population viability should increase invertebrate diversity, abundance and availability at these times and locations, thereby increasing foraging options available to choughs.