ECOLOGY
OF
STEPPE BIRDS
LEL
LESSER KESTREL (Falco naumanni)
MIGRATION
Migratory movements and winter behaviour
We used geolocators to investigate the temporal and spatial patterns of Portuguese lesser kestrel migration and wintering behaviour and provide important and new information on premigratory movements, arrival and departure dates and movements in wintering grounds.
Prior to southward migration, tracked lesser kestrels showed two different behaviours, staying in the proximity of the breeding areas or moving considerable distances northward (> 700 km), possibly to benefit from later peaks in Orthoptera abundance at higher altitudes or latitudes. Autumn migration took place mostly in late September agreeing with observations at the Strait of Gibraltar; spring migration occurred mainly in the first half of February. Southward and northward migratory journeys were of similar durations (3 to 5 days) to cover a distance of about 2,500 km. Wintering grounds included Senegal, Mauritania and Mali, with individuals overlapping considerably in Senegal. Movements registered within the wintering grounds (up to hundreds of kilometres) suggest itinerant behaviour in relation to local flushes of prey.
Individual core areas (25, 50, 75 and 95% kernel contours) of lesser kestrel activity during the winter period (21 October to 31 January) and spring migration routes to breeding areas in Castro Verde, Portugal.
More info:
Catry I, Dias MP, Catry T, Afanasyev V, Fox J, Franco AMA, Sutherland WJ (2011) Individual variation in migratory movements and winter behaviour of Iberian Lesser Kestrels Falco naumanni revealed by geolocators. Ibis 153: 154-164.
FORAGING HABITAT
Assessing foraging habitat and home-ranges
Understanding foraging habitat and diet preferences provides critical information for habitat management and species conservation. Telemetry studies performed in our study area showed that lesser kestrels selected extensive agricultural habitats, such as fallows and cereal fields, and avoided scrubland and forests. Moreover, these data indicate that, in good quality habitat such as traditional extensive cereal landscapes, lesser kestrels prefer to forage close to the colony.
We also compared habitat preferences and foraging behaviour of lesser kestrels in two protected areas with different habitat surroundings. The first area is a pseudosteppe characterized by traditional extensive cereal cultivation, whereas the second area is a previous agricultural zone now abandoned or replaced by forested areas. In the second area, tracked birds from one colony travelled significantly farther distances (6.2 km ± 1.7 vs. 1.8 km ± 0.4 and 1.9 km ± 0.6) and had significant larger foraging-ranges (144 km2 vs. 18.8 and 14.8 km2) when compared to the birds of two colonies in the extensive agricultural area. Longer foraging trips were reflected in lower chick feeding rates, lower fledging success and reduced chick fitness.
These results suggest that lesser kestrels benefit from a landscape mosaic of low intensive agriculture created by traditional agricultural practices within a 3 km radius of the colonies.
Distribution of foraging locations of lesser kestrels in relation to the distance from the colony in two areas with traditional extensive cereal cultivation (Pardieiro and Belver) and one area characterized by agricultural abandonment and forestation (Mértola).
Influence of spatial and temporal dynamics of agricultural practices on breeding success
To further investigate the impact of both spatial and temporal changes in land-use patterns on food supply for kestrel nestlings, we applied a spatially explicit behaviour-based model to assess how farmland management and the pattern of events across the annual farming calendar influences the foraging decisions of lesser kestrels.
Distribution of cereal (grey squares) and fallow (white squares) patches within a 2-km radius around two lesser kestrel colonies and model predictions of daily food supply for nestlings (the solid line exemplifies one simulation run) in relation to the metabolic requirements of one to five chicks (dotted lines). The models estimated, respectively, mean breeding success at 2.88 and 3.72 chicks.
Our results show that the location of cereal and fallow patches within a 2-km radius of a kestrel colony influences the total food supply delivered to the nestlings, explaining the differences in breeding success between years and colonies. Furthermore, the particular sequence in which patches are harvested by farmers is also predicted to influence offspring survival.
Our results show that the location of cereal and fallow patches within a 2-km radius of a kestrel colony influences the total food supply delivered to the nestlings, explaining the differences in breeding success between years and colonies. Furthermore, the particular sequence in which patches are harvested by farmers is also predicted to influence offspring survival.
Scenarios of future agricultural change/Predict species response to environmental changes
During recent decades, cereal steppe has changed rapidly as a result of the Common Agricultural Policy (CAP), the main instrument behind two divergent current trends: agricultural intensification in the best agricultural areas and land abandonment in less productive areas. Many studies have shown the dramatic impact of the CAP policies on steppe bird populations. To investigate the impacts of such changes on lesser kestrel breeding success, we simulated the most likely scenarios of future agricultural changes on lesser kestrel hunting performance.
Future scenarios of agricultural intensification, simulated by increasing the proportion of cereal fields around the colonies, are predicted to negatively impact lesser kestrel breeding success. Interestingly, the particular sequence in which patches are harvested is also predicted to impact offspring: if cereal harvest starts from the farthest patches the negative impact of increasing the proportion of cereal fields can be alleviated. On the other hand, the effect of dry cereal abandonment, if replaced by low intensity grazed fallows would not be detrimental for lesser kestrels.
Predicted number of lesser kestrel fledglings along a gradient of agricultural intensification. The scenario with replacement of cereal patches by grazed fallow (100% fallow) is represented when the proportion of cereal is zero. Three possible sequences were considered in relation to the distance from the colony: starting from the closest patch (triangles), starting from the farthest (diamonds) and at random (circles). Symbols represent the mean value of 100 simulations, and error bars the maximum and minimum values amongst all simulations.
Easy but ephemeral food: the impacts of harvesting on foraging decisions of lesser kestrels
Vegetation structure has a major effect on food abundance and accessibility and it is very dependent on agricultural management practices such as grazing and mowing. These practices may have a dual role on bird populations. On one hand, mechanical ploughing, mowing and harvesting have been suggested as one main driver of the decline of ground-nesting farmland birds through mowing-induced nest destruction and mortality. However, at the same time, grass and cereal cutting are known to provide good foraging opportunities for many species through increased prey detection.
Each year, during the lesser kestrel nestling period when food demand is highest, cereal fields are harvested and subsequently transformed into stubble. Vegetation structure is dramatically transformed and the tall vegetation of cereal crops is replaced by the short vegetation landscape of stubble fields, likely impacting prey availability. Cereal fields are either selected or avoided by lesser kestrels while cereal stubbles, often not included in habitat selection analyses, seem to be a preferred habitat for foraging individuals Nonetheless, these studies did not consider the harvest period and the potential decay in the quality of stubble fields within the season. Therefore, we aimed to investigate how the potential trade-offs of cereal harvesting affects foraging decisions of lesser kestrels and how long these effects may last.
Shift of directions in departures from the colony before and after the beginning of harvesting. The harvest machine was located north to the colony.
Harvesting impacted prey accessibility due to a temporary flush of prey, which resulted in a significant reduction in foraging time and an increase in the intake rate of kestrels. Accordingly, patches being harvested became the most preferred habitat. Nonetheless, harvesting likely caused high orthopteran mortality and dispersal leading to a gradual decline in prey abundance in stubbles. Lower prey abundance increased foraging time and reduced intake rate, and stubbles became avoided by foraging individuals.
Although harvesting significantly increases foraging opportunities for lesser kestrels through intake rate maximization, patches being harvested represent an ephemeral high-quality habitat and its benefits are relatively short-lived as cereals are converted into low-quality stubbles.
(A) Expected intake rate in the main habitats according to the distance of foraging patches from the colony: (a) patches being harvested, (b) fallow, (c) cereal and (d) stubbles and (B) mean time to obtain a prey (± se) and prey biomass (min-max).
DIET AND PREY
The importance of mole crickets in the diet of lesser kestrels
Preferred foraging habitats will often be those with the highest prey availability. Therefore, understanding the ecological requirements of preferred prey can help develop management measures to improve food supply for target species. For the colonial lesser kestrel, mole crickets Gryllotalpa spp., are one of most important prey items during the mate feeding period. We showed that lesser kestrel colonies with higher mole cricket consumption have earlier laying dates and larger clutches. Thus, the impact of environmental variables and land use on mole crickets is likely to be relevant to lesser kestrel conservation.
Ecological habitat requirements of mole crickets, as revealed by habitat selection analysis, indicated their preference for non-cultivated, soft and damp soils, where they can dig tunnels and galleries. Among variables that might be the target of management, the presence of riparian vegetation positively influenced the occurrence of mole crickets, whilst tillage and sowing of streambeds were revealed as the most important threats. Thus, the maintenance of native vegetation in the margins of watercourses could improve soil resilience to erosion, increase water retention, soil penetrability and fertility, and provide food supply and shelter for mole crickets.
Effect of mole cricket consumption (mean number of individuals per pellet) during mate-feeding on Lesser Kestrel laying date in 2007 (P<0.05) and 2008 (P<0.01). Each circle corresponds to one colony; laying dates refer to day of the year (day 1 = 1st January).
Probability of occurence of mole crickets according to soil penetrability.
Sexual and parent-offspring dietary segregation in Lesser Kestrels
Diet analyses are central to the study of foraging ecology, but intra-specific dietary segregation studies are largely biased towards a few groups, namely seabirds and shorebirds. Among raptors, sex and age have only occasionally been incorporated into general studies of diet, despite clear evidence of inter-sexual and age-related habitat segregation and differences in foraging behaviour. Most studies with raptors fail to focus on intra-population variation, which may jeopardize the implementation of effective management and conservation plans.
Stable isotopes: an effective tool to unveil within-species dietary segregation
Failure to account for within-species differences in diet are due to limitations of the sampling techniques used, namely analysis of faecal droppings and regurgitated pellets, which in most cases prevent drawing the link between dietary remains and particular individuals. More recently, the use of stable isotopes analysis (SIA), in particular of carbon (δ13C) and nitrogen (δ15N), allowed a relevant advance in studies of avian trophic ecology, both overtaking previous methodological bias and allowing identification of intra-population variation in diet. Isotopic ratios in bird tissues reflect its diet during the period of tissue synthesis. Blood, being a metabolic active tissue, typically yields an isotopic record of some days to few weeks prior to collection date, which might be ideal for studies of trophic ecology during the breeding season, with different sampling events allowing diet characterization throughout the whole season.
Diet segregation between sexes
During courtship, female lesser kestrels showed significantly more depleted d13C signatures than males, likely due to a higher consumption of mole crickets, while males showed a wider isotopic niche. Dietary sexual segregation seemed to dissipate with the progress of the season; during incubation, diet preferences were broadly alike in male and female kestrels, as revealed by similar d13C and d15N signatures and high overlap in niche width. The main driving force of observed sexual segregation appears to be the different energetic requirements of males and females before laying, when females need a higher allocation of resources to egg production.
Diet segregation by age
δ15N isotopic signatures differed significantly between adults and chicks and niche overlap between these age classes was low. Stable isotopic mixing models showed that, compared to adults, the diet of chicks was less diverse and mainly dominated by grasshoppers. Different resource allocation between chicks and adults might also result from different energy requirements, as rapidly growing chicks require more energy than adults, ultimately leading to a parent–offspring dietary segregation.
