Win-stay/lose-switch, prospecting-based settlement strategy may not be adaptive under rapid environmental change

Understanding animal responses to environmental change is crucial for management of ecological traps. Between-year habitat selection was investigated in red‐necked grebes (Podiceps grisegena) breeding on semi-natural fish ponds, where differential stocking of fish created contrasting yet poorly predictable brood-stage food availabilities. Grebes lured to low-quality ponds were more likely to shift territories than birds nesting on high-quality ponds, and tended to move to ponds whose habitat quality had been high in the previous year, irrespective of the current quality of the new and old territories. The territory switchers typically visited their future breeding ponds during or immediately after the brood-rearing period. However, owing to rotation of fish stocks, the habitat quality of many ponds changed in the following year, and then switchers from low-quality ponds and stayers on previously high-quality ponds were ecologically trapped. Thus, although breeders were making an informed choice, their settlement decisions, based on the win–stay/lose–switch rule and prospecting a year in advance, were inappropriate in conditions of year-to-year habitat fluctuations. Effective adaptation to rapid environmental change may necessitate both learning to correctly evaluate uncertain environmental cues and abandonment of previously adaptive decision-making algorithms (here prioritizing past-year information and assuming temporal autocorrelation of habitat quality).

Common Terns Sterna hirundo incubating a Red-necked Grebe Podiceps grisegena egg

During a study of Red-necked Grebes Podiceps grisegena in Lake Slagsmyren, Sweden, a pair of Common Terns Sterna hirundo were observed to have placed their two eggs on a deserted, floating nest of a Red-necked Grebe. Prior to this, the grebes had laid one egg of their own in the nest and this egg was included by the terns in their clutch and incubated by them. The species assignment of the odd egg was confirmed with DNA sequencing. A lack of natural nest sites for the terns to use at the lake could have caused this somewhat unusual choice of a nest site. I discuss possible explanations for adopting a foreign egg, including the adaptive behavioural response to roll an egg into the nest bowl to salvage lost eggs, the incubation stimulus that foreign eggs or egg-like objects potentially provide, and the limited egg discrimination abilities of Common Terns. The incubation of a foreign egg may reasonably be assumed to cost energy but to be of little benefit, if any, to the incubator.

Changes to the Population Status of Horned Grebes (Podiceps auritus) and Red-necked Grebes (Podiceps grisegena) in Southwestern Manitoba, Canada

Continental trend data for North America suggest that Horned Grebe (Podiceps auritus) breeding populations are declining and Red-necked Grebe (P. grisegena) populations are increasing. However, data reliability is low due to lack of survey routes in the northern boreal and taiga ecozones, areas encompassing much of the breeding range of both species. Locally in the southern Manitoba prairie ecozone, reliability of long-term trend data is also considered low and these data suggest that Horned Grebe populations are declining faster than the continental trend and that Red-necked Grebe populations are increasing rapidly. The lack of current quantitative information on population densities of these two species in southern Manitoba prompted me to compare 1970s historical data from two sites to recent data collected at the same locations in 2008–2016. I surveyed 42 (1970–1972) and 38 (2008–2016), and 144 (2009–2015) Class III-V wetlands at Erickson and Minnedosa, Manitoba, respectively. Historical Minnedosa data were available from previous field studies. At both locations, Horned Grebe breeding populations have fallen significantly, and Red-necked Grebe populations have risen significantly since the 1970s. The results of this study corroborate the Breeding Bird Survey’s trend data for Horned and Red-necked Grebes in southwestern Manitoba pothole habitat.