Coping with uncertainty: breeding adjustments to an unpredictable environment in an opportunistic raptor

Abstract

No environment is truly constant in time. As a result, animals have evolved multiple adjustments to cope with such fluctuations. However, the allocation of effort to costly activities that imply long-term commitments, such as breeding, may be extremely challenging when future resources change constantly and unpredictably, a context that has received little investigation. To fill this gap, we studied the breeding response by a wetland-dependent raptor, the black kite Milvus migrans, to within and between-years fluctuations in resource availability (inundation levels). The breeding performance of the population was decomposed into reproductive components expressed in a sequence of successive tasks along the breeding cycle (e.g. timing of laying, clutch size, hatching success, brood reduction). Variation in each component was related to resource levels observed at different key dates of the season in order to test whether and when population-level reproduction was adjusted to available resources. Along a 22-year time-series, inundation levels fluctuated unpredictably within and among years, and mostly affected the later components of kites' reproduction, such as hatching success and the incidence of brood reduction, which were the main determinants of the population yearly breeding output. Results were consistent with multiple adjustments to cope with uncertainty. As the season progressed and resources became easier to assess, a bet-hedging waiting strategy based on a conservatively small, invariant and asynchronous clutch gave way to real-time resource-tracking mechanisms mediated by progressive adjustments to current prey availability, so that population-level breeding rates were determined and tuned to resources rather late in the season. Such adjustments were the likely outcome of the interaction between parental tactics and environmental constraints. Behavioural flexibility, such as dietary opportunism, probably promoted further resistance to resource oscillations. Given that all ecosystems show some degree of unpredictability, resource-tracking adjustments, such as the ones depicted here, are likely to be commonplace in most communities.

Long-Term Monitoring of the Flooding Regime and Hydroperiod of Doñana Marshes with Landsat Time Series (1974–2014)

“…Hydroperiod becomes a net predictor for aquatic plant species distribution, marsh primary production, and habitat characteristics. Several works have already identified the importance of the hydroperiod duration in predicting the presence, abundance, and breeding success of different waterbird species in the Doñana marshes [76][77][78][79][80][81], and also on raptors foraging in the marshes [82]. The output layers for every single date, i.e., flooding masks and hydroperiod maps per cycle, are accessible online via Web Map Services [83].…”

Habitat selection by Black kite breeders and floaters: Implications for conservation management of raptor floaters

“…In our study population, all individuals are migratory and remain in Doñana from March to August, where they mostly breed as monogamous pairs (Sergio et al, 2007). The local breeding density can be very high (from 1 to 30 pairs/km 2 , Sergio et al, , 2011b; authors' unpublished results) and most pairs could be considered to nest within a very large, loose colony. Diet composition is very heterogeneous and dominated by wetland birds and their nestlings, crayfish, rabbits (Oryctolagus cuniculus ) and carrion (Hiraldo et al, 1990;Viñuela and Veiga, 1997).…”

“…For example, large patches of forest have been recently thinned or removed, while the hydrology of the seasonal marshes that characterise the park is subjected to a recently implemented large-scale restoration program (Project ''Doñana 2005'': García Novo andMarín Cabrera, 2005a), which included the restoration of various sites totalling more than 50 km 2 of seasonal marshland which had been originally converted to agriculture (García Novo and Marín Cabrera, 2005b;Santamaría et al, 2005;Martín-López et al, 2011). The above described habitat changes and active management inside and outside the park call for more solid knowledge of the habitat preferences of key species such as Black kites, which are the most abundant large predators in the park and which depend heavily on woodland for nesting and marshland for hunting (Sergio et al, 2011b). Understanding habitat preferences of key indicator species could be fundamental to forecast future impacts of habitat management and to implement more efficient postintervention monitoring.…”

Full Issue

“…Black Kites (Milvus nigrans), a wetland-dependent species, have been found to deal with unpredictable inundation levels and subsequent effects on prey via adjustments in clutch size, hatching success, and brood reduction (Sergio et al 2011). Similar unpredictability in prey availability due to abiotic factors such as flooding may partly explain why Mississippi Kite nests in the MAV only rarely contain 2-egg clutches, why kites have not been observed to fledge more than one chick per nesting attempt (Evans 1981, Barber et al 1998, St. Pierre 2006, and why chicks express facultative siblicide and cannibalism Bednarz 2009, Chiavacci 2010).…”