Sooty FalconFalco concolorreproduction and population dynamics on the islands in the Sea of Oman

Abstract

Knowledge of demographic parameters affecting population dynamics is critical to the formulation of effective conservation strategies. Sooty Falcon Falco concolor is a little‐studied, Near‐threatened species; estimates of global population size and trend for this species are uncertain. They lay eggs during mid‐summer and sometimes nest in colonies. This unusual breeding ecology suggests that demographic parameters driving their population growth rate may differ from those of most other falcons. We studied Sooty Falcon reproduction at breeding aggregations on Fahal Island and the Daymaniyat islands in the Sea of Oman during 2007–2014, modelled population growth and identified important life history parameters using elasticity analysis. The mean (± se) clutch and brood size was 2.83 ± 0.06 and 2.11 ± 0.07, respectively. Overall, 11.7% of nests failed between the egg and nestling stages, and the failure rate differed significantly between Fahal and the Daymaniyats, and across years. The mean proportion of eggs that hatched annually was 0.66 ± 0.02, and broods were significantly smaller on the Daymaniyats than on Fahal. Falcons on Fahal Island had a higher rate of hatching, a higher rate of nests that produced at least one chick, and produced more chicks per nest than on the Daymaniyats. We suggest that Fahal's proximity to the mainland gives breeding Sooty Falcons access to a more plentiful and stable source of food, especially during the period between arrival from the wintering grounds and the onset of the autumn migration of prey birds, resulting in the better reproductive rates for falcons on Fahal Island, relative to those on the Daymaniyat Islands. The annual asymptotic population growth rate (λ) was 0.87 (95% confidence interval (CI) 0.75–0.99), suggesting a declining population, although Sooty Falcons enjoyed a slightly higher population growth rate on Fahal than on the Daymaniyats. Because our study population is on the edge of the breeding range and is isolated from other breeding areas, measures to improve reproductive success of Sooty Falcons breeding on the islands in the Sea of Oman could be important for conservation of Sooty Falcons in Oman.

Island accessibility and distance from beach influence nesting success of Sooty FalconsFalco concolorin Oman

“…The decline in the presence of breeding pairs in the study area since 2007 (and probably since 1978) is consistent with indications of decline from other studies (McGrady et al . , ) and with the view that the Sooty Falcon is suffering an ongoing, slow to moderate population decline globally (BirdLife International ). Our preliminary analyses revealed that Sooty Falcon population decline became even steeper when we accounted for temporal autocorrelation, suggesting that the population decline is real and not a consequence of temporal autocorrelation.…”

“…Assuming that the difficulty of landing on some islands protected them from human interference, it may be that declines on inaccessible islands that occurred during our study reflect range‐wide population declines (BirdLife International ) and the inter‐related effects of lower survival and lower recruitment in the Oman population (McGrady et al . , ).…”

“…Fahal Island is rugged, and parts are inaccessible by foot and could not be observed directly from boats or from other parts of the island (McGrady et al . ). Because of its ruggedness, on Fahal Island in any given year we could not access or get a direct view of nest locations for 20–30% of the pairs; nest locations for these were estimated by watching Sooty Falcons, especially during food passes and pair change‐overs during incubation, and mapped.…”

“…Surveys were carried out during 5–22 August and 18 September–5 October, but the exact dates in each year and the order in which islands were surveyed varied depending on weather, tides and boat availability (McGrady et al . ). Fahal and D9 required more than 1 day for each survey but all other islands required only one visit during each period.…”

“…), information on reproductive rates and population dynamics in Oman (McGrady et al . ) and Bahrain (Kavanagh & King ) and information on mortality from tracking via satellite (http://sootyfalconoman.blogspot.co.at/; http://sootyfalcon.blogspot.co.at/) raise concerns about the status of Sooty Falcon and its conservation prospects. Currently, there exists no quantitative assessment of factors influencing Sooty Falcon nesting success, and this, in conjunction with the poor population estimates mentioned above, may have undermined Sooty Falcon conservation.…”