Abstract
Colonial breeding in birds provides protection from predators and may be particularly important when birds have to cope with an invasive predator. The probability of nest predation in a colony can vary depending on several factors, such as the nest’s location in the colony and the level of aggregation of nests. We studied the nesting success of colonial great crested grebes and monitored the occurrence of the non-native invasive American mink in the colony. From among 92 grebe nests, 54.3% were successful. The daily survival rate (DSR) of grebe nests was positively affected by the increasing distance between the nest and lake shoreline, and negatively affected by the increasing distance between the nest and the five nearest grebe nests. The probability of mink occurrence in the colony increased with consecutive days of the breeding season and decreased with increasing distance from the lake shoreline. The DSR of grebe nests decreased with the increasing probability of mink occurrence along the shoreline distance gradient and the day of the breeding season. The results of the study confirm the impact of the American mink on waterbirds during the breeding season but also indicate that large breeding colonies are partially safe from mink predation, and that nest accessibility and the dilution effect influence the probability of nest survival. Our data suggest that the limited access to safe breeding sites on large lakes that can supply adult grebes and their chicks with food may affect bird productivity and population numbers at the landscape level.
Highlights
Colonial breeding in birds and other animals is an evolutionary and ecological phenomenon and is always a tradeoff between costs and benefits (Danchin and Wagner 1997; Sachs et al 2007; Brown 2016)
The results of the study showed that the daily survival rate (DSR) of great crested grebe nests depended on the nest’s location in the colony: it was higher for nests that were (1) located further from the lake shoreline and (2) more aggregated
Mink activity in the grebe colony increased with time, and the probability of mink occurrence in the colony was negatively related to the distance from the lake shoreline; the DSR was negatively related to the probability of mink occurrence
Summary
Colonial breeding in birds and other animals is an evolutionary and ecological phenomenon and is always a tradeoff between costs and benefits (Danchin and Wagner 1997; Sachs et al 2007; Brown 2016). Colonies are rarely completely inaccessible, and their isolation, which can be achieved by locating colony in a habitat hardly accessible to predators or by increased distance from shoreline, affects predator activity in the colony. This activity may change over time according to the duration of the breeding season and availability of prey in a colony. If the probability of predator occurrence increases rapidly at the beginning of the breeding season (i.e. the occurrence probability curve has a concave shape), we can expect that a large number of nests will be lost due to predation. The nesting success of colonial waterbirds is related to the degree of colony isolation (it affects predator access), and the way in which predators respond to colonies (how fast they cue on prey)
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