Fledging success and the factors influencing fledging success

Chick fate (survived/died) could be determined for 90.1% of the chicks known to hatch (n = 1301).

Of the 1172 chicks whose fate was known, 309 (26.4%) fledged. A significantly higher proportion of chicks fledged in 1999 and 2000 than in 1998 (Table 7.). Similarly, the number, and the proportion, of chicks fledged per brood was lowest in 1998 and higher in 1999 and 2000 (Table 7.). The proportion of broods fledging chicks was also higher in 1999 and 2000 than in 1998, whereas the number and proportion of fledged chicks in successful broods did not differ among the years (Table 7.).

The mortality of chicks was highest during the first week post-hatch, when more than half of the chicks disappeared in each study year (Figure 9.). The percentage of chicks surviving until one week of age per brood was 37.5%, whereas 29.9% survived until two weeks and 25.6%

survived until three weeks of age. Mortality after three weeks post-hatch was rare (Figure 9.).

The order in which chicks hatched did not influence whether or not chicks fledged (G = 4.347, df = 3, n = 426, p = 0.2264). The proportion of fledged young varied between a high of 26.3% for first-hatched chicks and a low of 16.8% for third-hatched chicks. The residual body size (which was independent of the brood mean) of newly hatched chicks did not influence whether or not they fledged (logistic regression, R² = 0.001, n = 576, p = 0.3774). The effect of residual chick body condition on fledging success also was non-significant (logistic regression, R² = 0.0002, n = 572, p = 0.7010).

2.3.3.4.2. Fledging success of broods

The fate of broods was known for 340 broods (92.6% of 367 broods total). Nine broods ceased to function because chicks died in the nest (4 cases), were adopted by other broods (4 cases) or were

abandoned by the parents (1 case). At least one chick fledged from 154 (46.5%) of the remaining broods (n = 331), whereas 53.5% of the broods failed to fledge any young. In eight broods only adopted chicks fledged. Four chicks fledged in only seven broods (4.8% of the broods fledging at least one natal chick, n = 146), whereas three chicks fledged in 23 broods (15.8%), two chicks fledged in 53 broods (36.3%) and one chick fledged in 63 broods (43.2%).

The number of young fledged per brood differed both by year (Table 7., two-way ANOVA on log-transformed data, F2,166 = 6.127, p = 0.0027) and brood-rearing location (F14,166 = 2.662, p

= 0.0016), whereas the interaction between year and location was not significant (F4,166 = 1.462, p

= 0.2162). The proportion of chicks fledged per brood also differed among years (Table 7., F2,162 = 10.671, p = 0.0001) and locations (F14,162 = 3.992, p = 0.0001), whereas the interaction of year and location was not significant (F4,162 = 0.251, p = 0.9088). The cause of the among-year variation was that fledging success was low in 1998 and high in 1999 and 2000 (Table 7.). Fledging success varied among brood-rearing locations between 7% to 70% of the chicks fledging per brood. The spatial variation in fledging success was caused mainly by a difference between natural and artificial habitats. In 1998 and 1999, when nesting occurred in both natural and artificial habitats, 68.2% of the broods in natural habitats (n = 66) fledged chicks, whereas only 30.2% of the broods in artificial habitats (n = 126) were successful (Fisher’s p < 0.0001). Pairs in natural habitats (n = 66) fledged 1.2  1.15 or 47.6  40.56% of their chicks, whereas pairs in artificial habitats (n = 126) fledged 0.5  0.94 or 16.1  28.41% of their chicks (Mann-Whitney-test, number of fledged young: U = 9731.5, p < 0.0001, proportion of fledged young: U = 9430.0, p < 0.0001). The reason for this difference was that pairs nesting in artificial habitats moved their broods to natural habitats (alkaline lakes) by traveling several kilometers on land, which caused high mortality among young chicks for two reasons. First, chicks were exposed to both aerial and terrestrial predators. Second,

single pairs could not defend their chicks from predators effectively, because avocets rely on mobbing by many individuals to deter predators.

Pairs that laid their eggs earlier were more likely to fledge chicks than pairs laying later when differences in laying dates caused by year and colony were controlled for (logistic regression, R² = 0.02, ² = 6.949, n = 315, p = 0.0084).

Brood size was also important in the probability of producing fledglings. More than half of the broods with three or more chicks fledged young, whereas less than half of the broods with one or two chicks did so, both in the low predation years (1999 and 2000) (Table 8., G = 11.516, df = 4, p = 0.0213) and in all years combined (G = 15.493, df = 4, p = 0.0038). There were not enough data to test the high predation year of 1998 separately (Table 8.). However, brood success did not differ by brood size when data from 1998 were grouped by brood size because 14.3% of the broods with one or two chicks (n = 14 broods) fledged young, and 32.1% of the broods with three or more chicks (n = 28) fledged young (Table 8., Fisher’s p = 0.2826).

Brood size also significantly influenced the number of chicks fledged per brood both when data from all years were combined (Figure 10., Kruskal-Wallis H = 21.501, df = 4, p = 0.0003) and when only the years of low predation (1999 and 2000) were included (Figure 10., Kruskal-Wallis H = 18.030, df = 4, p = 0.0012). There was no difference in the number of young fledged by brood size in the high predation year of 1998 (Kruskal-Wallis H = 5.496, df = 4, p = 0.2401).

After controlling for annual differences in the abundance of aquatic macroinvertebrates (B.

Kiss & Sz. Lengyel, unpubl. data), prey density positively influenced whether the brood occupying the territory was successful or not (logistic regression, R² = 0.124, n = 63, p = 0.0141). The number of fledged young was positively correlated with prey density (r = 0.461, n = 63, p = 0.0001), in part because larger broods occupied territories with higher prey density (r = 0.355, n = 63, p = 0.0040) and had a higher fledging success than smaller broods (Figure 10.).

Broods from solitary and colonial nests were equally likely to fledge chicks (solitary: 50%, n = 10 broods, colonial: 45.0%, n = 313, Fisher’s exact p = 0.7594) and there was no difference in the average number of chicks fledged between solitary and colonial broods (solitary: 1.4  1.65 chicks, n = 10, colonial: 0.8  1.06 chicks, n = 313) (Mann-Whitney U = 1296.0, p = 0.3078).

There was no correlation between the number of broods at a given brood-rearing site and the average proportion of young fledged per brood (r = 0.145, n = 12, p = 0.6624).

The average body size of chicks in a brood did not influence whether the brood produced fledglings or not (logistic regression, R² = 0.002, ² = 0.501, n = 242 broods, p = 0.4790).

Similarly, the body condition of chicks did not affect brood success (logistic regression, R² = 0.003, ² = 0.866, n = 242, p = 0.3522). These results were similar (p > 0.48) in both the high predation year (1998) and the low predation years (1999-2000).

In summary, fledging success was lower in 1998 than it was in 1999 and 2000, and it was influenced by both season and habitat. Early broods were more likely fledge young than were late broods and more chicks survived in natural habitats than in artificial habitats where broods traveled on land to feeding areas. Larger broods occupied better territories than smaller broods, and brood size positively influenced the probability of fledging and the number of fledglings per brood. Within broods, neither hatching order nor chick body size influenced fledging success.