Young and female-biased irruptions in pygmy owls Glaucidium passerinum in southern Finland

Abstract

Irruptive migrants often show biased sex and age ratio, and typically young females are the most abundant class participating in irruptions. Several hypotheses have been proposed to explain differential migration in birds. We examined these hypotheses in a sexually size‐dimorphic species (females being larger than males), the pygmy owl Glaucidium passerinum, whose migration behaviour has also been poorly documented. Migration data were collected at the Hanko Bird Observatory, SW Finland during 1979–2010. Pygmy owls showed large fluctuations in migration numbers between years. Most of the migrants were young birds and females migrated in larger numbers than males. Results support the ‘territory defence hypothesis’ since males showed weaker tendency to migrate despite the smaller body size, but the ‘dominance hypothesis’ was also supported (subordinate young outnumber dominant adults in migration numbers). Findings support the idea that in owl species, which are dependent on nesting cavities, males show lower migration behaviour than females, whereas the pattern is opposite in species which normally breed outside cavities. In addition, adults migrated later than young, likely because they need to moult remiges before departure. Early hatched young also migrated earlier than late hatched young, suggesting that more dominant young migrate first.

The effects of hatching date on timing of autumn migration in partial migrants – an individual approach

“…Our study indicates that the age of migrating birds, or if interpreted slightly differently, the breeding area residence time is far from constant and becomes shorter with progressing hatching date. Similar results of late hatched individuals starting their migratory restlessness at a young age has been found in three species in aviary studies (Pulido et al 2001, Styrsky et al 2004, Helm et al 2005), but two individual‐based studies done in the wild found a constant migration age regardless of hatching date (Donaghy et al 2011, Lehikoinen et al 2011). Population level studies have also shown that several long‐ and short‐distance migrants migrate at younger age in years with late breeding (Bojarinova et al 2002, Sokolov 2006).…”

“…It is also noteworthy that, like in the tit species, our early‐hatched hawks had tendency to migrate early. Nevertheless, it seems that the connection between hatching and migration dates is much weaker than in single‐brooded irruptive pygmy owls (Lehikoinen et al 2011). Pygmy owls tend to migrate only during poor food conditions and thus early migrating can be beneficial, since early birds may get the best wintering territories in this hoarding species (Lehikoinen et al 2011).…”

“…To our knowledge, only two single‐species studies have linked hatching dates to autumn migration dates using ringing recoveries of individually marked nestlings (Vilbaste 1975; according to Bojarinova et al 2002, Lehikoinen et al 2011). According to the first study, individuals from great tit Parus major second broods migrate 15–20 d later than birds from the first broods (Vilbaste 1975).…”

“…In this case, hatching date affects the timing of autumn migration, and birds from second broods migrate at a younger age (Vilbaste 1975). The other study found that in the irruptive single‐brooded pygmy owl Glaucidium passerinum the time interval between hatching and migration is constant (Lehikoinen et al 2011).…”

An overview of monitoring for raptors in Finland

“…Since the mid-1990s, the data reflect the real changes in the population and indicate how the Pygmy Owl population has increased during favourable circumstances to a very high level and after that crashed owing to the strong autumn invasion in 2003 and 2009 detected at coastal bird observatories (e.g. Lehikoinen et al 2011). The present data show that Pygmy Owls, which participated in the mass invasion, disappeared from the Finnish population.…”