Nest survival patterns in Eurasian Bittern: effect of nest age, time and habitat variables

Field data collection

I carried out the fieldwork in ten fishpond complexes in the Lublin region in eastern Poland. The fishpond complexes varied in size from 15 to 185 ha and were partially covered by vegetation stands dominated by Common Reed Phragmites australis, Reed Mace Typha angustifolia and sedges Carex sp. The water depth in the emergent vegetation varied from 0 to 120 cm. The ponds were similar in depth (from 0.7 to 1.3 m), but they differed strongly in surface area and coverage of emergent aquatic vegetation. They were filled with water from adjoining rivers and streams or by precipitation. The ponds were surrounded by arable fields, meadows and small villages and woodland areas of different ages.

I collected data on the timing of breeding, brood size and breeding success of Bitterns in 2003–2008, surveying the study fishponds from the pre-breeding period (the beginning of March) to early July at least once a week. I plotted the movements of the birds, locations of booming males and active nests on 1:5000 maps and located nests by walking along transects within the emergent vegetation. I paid special attention to searching for nests within or near booming areas of males, because in the studied population most of nesting females located nests in the male’s territories (M Polak, 2003–2008, unpublished data). I subsequently visited all 103 active nests at least once a week from the end of April to early July in order to obtain data on breeding characteristics (mean number of inspections = 4, range = 1–9). To reduce the impact of nest visits on predation risk, I kept the number of inspections to a minimum. I ringed nestlings for individual recognition with metal and coloured rings, and I defined the clutch initiation date as the day when the first egg was laid. I was able to determine laying dates by direct observation (ca. 30% of nests), or indirectly, by estimating the hatching date of the oldest nestling, assuming an incubation period of 25–26 days (Mallord et al., 2000; Demongin, Dmitrenok & Bretagnolle, 2007). I defined nests as successful if at least one young bird survived up to 15 days old. I established the number of nestlings during the first inspection of a nest after hatching.

I measured habitat variables using a modified version of the methodology applied in studies of the Bittern in the UK (Tyler, Smith & Burges, 1998; Gilbert et al., 2005). I examined eight different temporal and habitat variables (Table 1). Providing a very good description of the Bittern’s habitat use, these variables may determine the reproductive success of this species in specific semi-natural habitats like fishponds (Polak, Kasprzykowski & Kucharczyk, 2008). In particular; the ratio of open water area to emergent vegetation area is a much better habitat descriptor of a fishpond than other macrohabitat components like the length of water edge, which are important for populations living in more natural habitats. I made all the measurements between the end of April and the end of May: I measured the microvariables in within 50 x 50 cm squares placed around the nests and the macrovariables, including the areas of the reed patches in which the nests occurred, from aerial photographs (GEOPORTAL, www.geoportal.gov.pl). For a detailed description of the study area and methods, see Polak, Kasprzykowski & Kucharczyk (2008), Polak & Kasprzykowski (2010) and Polak & Kasprzykowski (2013).

Modelling

I used the nest survival module in version 8.0 of the MARK program (White & Burnham, 1999; Rotella, Dinsmore & Shaffer, 2004) to compare nest survival models and to obtain estimates of daily nest survival. All the analyses performed in MARK included broods with success and nests lost only as a result of predation. I scaled the dates such that 1 was the date when I found the first nest. I thus defined a 79-day nesting season beginning on 19 April and ending on 6 July. Consequently, the season consisted of 78 daily intervals for which I estimated the daily survival rate. I selected the best predictive models using Akaike’s information criterion corrected for a small sample size (AIC_c; Burnham & Anderson, 2002). The list of candidate models was based on combinations of factors that I assumed a priori might affect Bittern nest survival. I compared the model support using AIC_c and evaluated the strength of evidence for each model using normalized weights w_i. I followed the convention that the model with the lowest AIC_c represented the best compromise between goodness-of-fit and model complexity (Whittingham et al., 2006). Because the candidate model set contained a mix of models with linear and quadratic terms, as well as interactions, I was unable to use model averaging in the interpretation of variable estimates (Wilson, Martin & Hannon, 2007). Firstly, I considered the influence of habitat variables. I constructed models of nest survival that incorporated combinations of individual variables and compared them to the null model of constant survival rate S (.). I expected that the better concealed nests (denser vegetation) would have a greater chance of survival than the more exposed ones (Goławski & Mitrus, 2008). The decision when to initiate breeding is of crucial significance where the adaptation of individuals is concerned (Lewis et al., 2012). Due to severe winter weather, in eastern Europe Bitterns usually have only a short time window for start of breeding, and these individuals are under strong pressure from the passage of time (Polak & Kasprzykowski, 2010). The daily survival rate of many avian species varies in relation to nest age and may decline as a result of the greater frequency of visits by parents when feeding nestlings and increase as a result of intensive nest defence (Martin, Scott & Menge, 2000). Recent studies have indicated, however, that during the nestling period female Bitterns do not defend their broods or behave aggressively towards predators (White, Purps & Alsbury, 2006). Nest age and day of season can be difficult to separate when nests are initiated synchronously, but as most nests were initiated across the season, I included both variables in the models. As suggested by Dinsmore, White & Knopf (2002), all were unstandardized, because the unstandardized variables did not affect numerical optimization. Since a goodness-of-fit test is not yet available in MARK for nest survival models, I did not adjust for overdispersion. The logit link function was adopted. Unless otherwise indicated, means are expressed ±SD and all tests are two-tailed. The study fulfilled the current Polish Law and was permitted by the 1st Local Ethic Commission in Lublin (approval number: 514/2005). The study was permitted by Ministry of the Environment (approval number: DOPog–4201–03–48/05/al.). Provincial Nature Conservator in Lublin allowed for this research project by the letter (approval number: ŚR.IV.6631–ZW/70/2003).

Effect of season

Numerous studies have shown that early nesting during the breeding period is advantageous to many species of birds (Wesołowski, 1998), including herons (Jakubas, 2011), ducks (Elmberg et al., 2009) and other aquatic birds (Lewis et al., 2012). On arrival from their wintering grounds, early breeders are the first to occupy the best breeding territories, which offer abundant food resources and safe breeding sites (Forsman et al., 1998). A gradual decline in the values of reproductive characteristics (clutch size, egg volume, breeding success) during the breeding season has been demonstrated in many bird species (Borgmann, Conway & Morrison, 2013), including herons (Moser, 1986; Jakubas, 2011). This may be due to the fact that after their return from the wintering grounds, the first to begin nesting are birds in good genetic and phenotypic condition, which outcompete younger and less experienced individuals (Lewis et al., 2012). The quickest possible start to breeding is therefore of the utmost importance, especially in species with prolonged parental care (Tobółka, Żołnierowicz & Reeve, 2015). On the other hand, some studies have shown that early nesting can have negative consequences for individuals choosing this strategy (Janiszewski, Minias & Wojciechowski, 2013). Previous work (Polak & Kasprzykowski, 2013; Kasprzykowski, Polak & Chylarecki, 2014) indicates that an early start to breeding by female Bitterns is not a successful strategy in this study area. In the temperate climate zone the beginning of the season is often a time of inclement weather with below-zero temperatures, strong winds and precipitation, which can significantly affect the reproductive characteristics of nesting birds (Jakubas, 2011; Tobółka, Żołnierowicz & Reeve, 2015). In the Bittern population I studied here, intensive rainfall during the pre-breeding period adversely affected the size of eggs laid by the females (Polak & Kasprzykowski, 2013). Previous studies had also shown that nestlings from later nests gained weight more quickly in comparison with young birds from early nests (Kasprzykowski, Polak & Chylarecki, 2014). The present study showed that the risk of nest predation in this population was greatest at the beginning of the season and that clutches laid early were more vulnerable to depredation. On their return from the wintering grounds female Bitterns have difficulty in finding suitable sites for concealing their nests, because in the reedbeds around the fishponds reed and bulrush shoots have yet to start growing; they are therefore forced to choose nesting sites in the previous year’s vegetation. This accords with earlier studies, which showed that female Bitterns preferred reedbeds with a high density of last year’s stems within which to conceal their nests (Polak, Kasprzykowski & Kucharczyk, 2008).

Nest age

Most work done to date has shown that the older the nest, the greater the chances of the brood fledging (Dinsmore, White & Knopf, 2002; Segura & Reboreda, 2012). The degree of aggression on the part of the parents towards potential predators usually varies during the breeding season (Caro, 2005). The intensity with which they defend their nest grows as the investment they make in their brood increases (Lima, 2009). Moreover, the probability of fledging of young birds from replacement clutches gradually decreases as the season progresses (Janiszewski, Minias & Wojciechowski, 2013). On the other hand, some researchers have observed the opposite trend, namely, that the predation level increases during the chick-feeding period, since the adults’ frequent foraging flights and feeding their chicks heightens the risk of their brood being discovered by predators (Martin, Scott & Menge, 2000). Distinguishing the nest age factor from the effect of season, which simultaneously alters the level of nest survival, is particularly difficult in the case of birds that have a short, synchronized pattern of breeding onset phenology (Dinsmore, White & Knopf, 2002). Earlier observations showed, however, that in this study area Bitterns have quite a prolonged reproductive period (Polak & Kasprzykowski, 2010) and this enables the influence of these two factors to be better differentiated in the final analysis. Unlike other more aggressive species, female Bitterns do not exhibit intensive anti-predator behaviour near the nest (White, Purps & Alsbury, 2006). They are cryptically coloured and sit motionless on the nest when danger threatens, in this way reducing their chance of being detected by a predator. In this case intensive brood defence in the nestling period was not a significant factor enhancing brood survival in Bitterns in the later stages of their reproduction. It is possible that on these fishponds the most conspicuous nests were quickly discovered by predators during the egg-laying and incubation periods, while well-concealed nests were less subject to nest predation (Martin, Scott & Menge, 2000; Dinsmore, White & Knopf, 2002).

Water depth

Many studies have shown that deep water in wetlands is an effective barrier to terrestrial nest predators (Sanchez-Lafuente, Alcántra & Romero, 1998; Hoover, 2006). In my study I was able to confirm that daily survival rate of Bittern broods was greater, the deeper the water. This suggests that in this population the main predators were probably mammals, which unlike raptors, may have limited access to nests in reedbeds growing in deep water. Similar relationships have been found for other Bittern populations (Adamo, Puglisi & Baldaccini, 2004; Gilbert et al., 2005) and for other aquatic birds (Sanchez-Lafuente, Alcántra & Romero, 1998). This, however, is not a rule for all reedbed species. Jedlikowski, Brzeziński & Chibowski (2015) demonstrated that water depth did not significantly affect breeding success in the Water Rail Rallus aquaticus and Little Crake Porzana parva, since the main predator of their nests was a raptor, the Marsh Harrier Circus aeruginosus.

Vegetation structure

Building the nest in tall, dense vegetation can be a successful strategy for alleviating predation pressure on broods (Martin, 1993; Lima, 2009): this has been demonstrated in many bird species, both shrub-nesting passerines (Goławski & Mitrus, 2008) and aquatic birds inhabiting wetland reedbeds (Kristiansen, 1998). I have shown here that where predation levels are high (Polak & Kasprzykowski, 2010), nests well concealed in dense vegetation are more likely to survive. This analysis did not indicate, however, that vegetation height was a variable significantly improving brood success: this factor is significant for populations of birds depredated by avian raptors (Jedlikowski, Brzeziński & Chibowski, 2015).

Fragmentation of habitat patches

Ample evidence is available to show that habitat fragmentation significantly modifies the pattern of nest predation in many aquatic bird species: nests located in smaller habitat patches are more exposed to depredation (Hoover, 2006). A mosaic of diverse habitats with an extensive ecotone may make it easier for predators to gain access to nests (Caro, 2005). However, I did not observe this to be the case in this Bittern population. All nests were in relatively small reedbeds and very probably the habitat patches situated around fishponds in eastern Poland were too small to be safe nesting sites for Bitterns (see Báldi & Batáry, 2005).