Breeding success of birds can be affected by different factors such as food availability (Cavé 1968, Village 1982, Wiehn and Korpimäki 1997), competition (Nilsson 1984, Korpimäki 1987, Hakkarainen and Korpimäki 1996), predation (Newton 1979, Sergio et al. 2003), risk of predation (Skutch 1949, Snow 1962, 1978, Slagsvold 1982), territoriality (Village 1983), human disturbance (Gutzwiller et al. 2002), laying date (Daan et al. 1988, Korpimäki and Wiehn 1998, Aparicio 1994), and abiotic factors such as temperature and rainfall (Kostrzewa and Kostrzewa 1991, Avilés et al. 2000, Salvati 2002).
The Eurasian Kestrel (Falco tinnunculus; hereafter “kestrel”), is a small open-country raptor that (like most falcons) does not build its own nest but, unlike most raptors, breeds in both open-type nests (e.g., cliff ledges, corvid nests) and closed-type nests (e.g., cavities; Village 1990). In previous studies, breeding success of kestrels nesting in cavities and nest boxes was higher (Kostrzewa and Kostrzewa 1997), and such nest types were more favored (Korpimäki 1983), compared to open-type stick nests of other species (Village 1998, Valkama and Korpmäki 1999), possibly because of decreased risk of predation. Reproductive rates of kestrels in other studies are only rarely reported for the different nest types separately (as in Kostrzewa and Kostrzewa 1997, Valkama and Korpmäki 1999, Fargallo et al. 2001), but commonly grouped together (Pikula et al. 1984, Gil-Delgado et al. 1995, Van Zyl 1997). Consequently, there is a need to verify whether differences in reproductive rates may also be due to the different nest types, in order to allow comparison within and between studies.
Three kestrel nest types were used in this study: two different types of nest boxes (i.e., artificial cavities), the larger of which also represents a limiting resource for the larger Barn Owls (Tyto alba) in the study area, and natural open-type nests in date palms (Phoenix dactylifera). By studying three different nest types, each influenced by different factors and all used by kestrels within the same region, we were able to compare how nest type can influence kestrel breeding success. Additionally, although the biology of kestrels is well known in Europe (Village 1990), it has not been studied in detail in the Middle East. We also investigated the effects of temperature, rain, and diet on reproductive rate, to further our understanding of how different factors affect breeding success.
The study site consisted of agricultural fields, orchards, and plantations in Kibbutz Sde Eliyahu, located in the Jordan Rift Valley, Israel (32°30′N, 35°30′E), 7 km southwest of the city of Beit Shean and 150–250 m below sea level. The climate is arid with maximum and minimum mean daily temperatures (during March and July 1999) of 32.3°C and 16.7°C, respectively, and average yearly rainfall of 267 mm (for 2001–2006; M. Hyman pers. comm.).
The majority of the study site (combined area of 989 acres) is used for organic agriculture, primarily crop fields and date plantations. The crop fields include fodder (wheat, sweet corn, alfalfa, clover, vetch and oats), grain crops and seeds (wheat and sweet corn) and spices and herbs (oregano, hyssop, basil, and dill). The date palm plantation has ten different varieties, the oldest planted in the early 1950s.
Kestrels in the study site breed in three different nest types, two of which are human-made (small and large nest boxes), and one natural (date palms). Sixty large nest boxes (50 cm wide × 75 cm long × 50 cm high; entrance hole 25 cm high × 15 cm, mounted 2.5–3 m aboveground) were placed in the fields and date palm plantations from 1993–1997 with the intention that Barn Owls would use them (Aviel et al. 2003). Although these nests were the lowest in height of the three nest types in this study, kestrels have been shown to nest successfully at similar heights in Europe (Cavé 1968) and in other locations in Israel (M. Charter unpubl. data.). In addition, unlike Europe (Village 1990, Valkama and Korpmäki 1999), our study site does not contain arboreal mammals; thus, clutch and brood predation by mammals is not a factor. Eleven small nest boxes were specifically built for kestrels in 1998 (50 cm wide × 30 cm long × 30 cm high; entrance hole 22 cm high × 15 cm, attached to date palms at 5–6 m). Because of the small size of these boxes, Barn Owls do not use them as nest sites but do occasionally use them as roosts. The third type of nests, natural nests on date palms, are open sites in which kestrels nest in the offshoots (clipped palm branches). The number of potential nest sites in the date plantations is estimated at up to 13 nests per year and changes from year to year are mainly due to the partial destruction of nests by the kestrels during the breeding season and the effects of weather during the winter.
Active nests in the date palms were located by direct observation of the kestrels' breeding behavior, mainly copulating close to the nests (Village 1990). From 1999–2006, for each breeding attempt (defined as a nest in which eggs were laid; Steenhof 1987), we recorded the date of egg-laying, hatching, and fledging of young when possible, along with clutch size, brood size (number of young observed in nest during first visit, <1 wk after hatching) and the number of young fledged (23–27 d old). When laying date was unknown, it was determined by backcalculating using an incubation period of 28 d (Cramp 1985). During years when nests were visited late, only breeding parameters that were known were included in the analysis. We calculated the following: (a) hatching success as the percentage of eggs that hatched within each clutch; (b) the percentage of young that fledged from each brood for all pairs that hatched at least one egg. (c) egg productivity as the percentage of eggs per nest that hatched and fledged young (d) brood size per breeding attempt (e) the number of young fledged per breeding attempt; and (f) the number of young per successful nest, where a successful nest was defined as one that produced at least one chick that fledged. Breeding data were recorded for each breeding attempt unless stated otherwise.
A permanent weather station located at Kibbutz Sde Eliyahu provided data on precipitation and daily temperatures (mean, minimum and maximum) from 2001–2006. Weather data were divided into two periods: Winter (1 November–28 February) and breeding season (1 March–31 July). The daily mean temperatures, maximum temperatures, minimum temperatures, and humidity for the winter and breeding season were calculated by averaging daily values. Rainfall was recorded as the total amount of rainfall for each period. Weather data for winter and breeding season were tested for correlations with annual kestrel clutch size, number of young fledged, and percentage hatching success of the three nest types combined and separately (160 Spearman rank correlations; N = 6 yr for all correlations). Only significant (P < 0.05) correlations are presented.