Breeding biology of the African Pygmy Falcon: long-term variation and seasonal decline in breeding performance of an arid zone raptor

Abstract

Birds of prey are apex predators and understanding their life history can serve as a valuable baseline for investigating their ecological role. Pygmy Falcons (Polihieraxsemitorquatus), Africa’s smallest diurnal raptor, have evolved to be obligate associates of Sociable Weaver (Philetairussocius) colonies throughout their southern African range. As a predator, Pygmy Falcons likely impact prey communities in this system including their host and other colony associates. However, no study has explored their breeding biology in detail using long-term data. We provide results from 10 years of data collected between 2011 and 2020. We followed 66 unique Pygmy Falcon territories and 323 breeding attempts in the Kalahari, South Africa. We explored annual variation in the population density of Pygmy Falcons and investigated between- and within-season variation in reproductive investment and output. The highest average density was recorded in 2015 and the lowest in 2019 and 2020. Our results show that the breeding occurs between early austral spring (August) and late summer (February), with a peak from September to December. The maximum and most frequent clutch size was three eggs and falcons usually initiated only one breeding attempt (but up to three) in a season. The incubation and nestling periods lasted on average 33 (±SD 4) and 37 (± 5) days, respectively. A three-egg clutch size and number of fledglings produced varied between seasons, and breeding success showed a seasonal decline across the breeding season. Falcons were 7 times less likely to lay a 3-egg clutch in 2019 than in 2011 and 2012, and breeding attempts initiated in September were 3.4 times more likely to be successful than those initiated in November. Predation accounted for 49.5% of breeding failures. Our results show significant variation in the breeding parameters of these falcons, but further studies are needed to understand what drives these variations.

Costs and benefits in extreme nesting associations: do Sociable Weavers benefit from hosting African Pygmy Falcons?

“…Our results suggest that snakes forage in colonies more often when weavers are breeding and when colonies are larger, although part of this signal might be explained by snakes spending longer in larger colonies. Interestingly, falcons also favour occupying bigger weaver colonies (Olubodun et al 2023). Nevertheless, despite falcons being able to repel snakes from weaver colonies, we found that this did not lead to an increase in reproductive output for weavers.…”

“…Biotic and abiotic conditions may also affect the outcomes of nesting associate interactions (G€ otmark 1989, Larsen & Grundetjern 1997, Quinn & Ueta 2008, Canestrari et al 2014. For example, the variable timing of breeding of the two species (Mares et al 2017, Olubodun et al 2023) holds significant consequences for the net fitness outcomes. Predation pressure will also alter the net outcomes; weaver nest predation pressure by snakes (and indeed falcons) may change due to environmental conditions, especially in arid environments where resource availability fluctuates between abundant and scarce (Hillel & Tadmor 1962, Rosenzweig 1968.…”

“…In years of high nest predation pressure by snakes, there may be benefits for weavers to coinhabit a colony with falcons, because snake visits are destructive and could predate every active nest in the colony (Covas 2002). Falcon populations also fluctuate between years (Bolopo et al 2019, Olubodun et al 2023, and an increased number of falcons in a landscape may increase competition for resources, adding to weaver predation pressure. These aspects of costs and benefits to these associates need further investigation and should be a focus of future research.…”

“…Annually, there are between 29 and 39 active falcon territories in the study area and each territory can contain two to five adult birds (Bolopo et al 2019). Falcon territories can contain multiple weaver colonies, but falcons only occupy one (rarely two) weaver colony at a time, preferring the larger colonies (Olubodun et al 2023). Falcon groups and territories have been observed since 2011, with all adults being colour-ringed from 2015 onwards (Bolopo et al 2019).…”

“…Falcon nests hatch after c. 33 days of incubation and chicks remain in the nest for a further 5 weeks (Olubodun et al 2023). From day 24, chicks are large enough to be seen by observers standing below the colonies without using the mirror, and if the chamber appeared empty, we would then scan the territory looking for fledged chicks.…”