Red-necked Grebes are normally a rare sight in Ohio, or anywhere else in the interior U.S. south of the Great Lakes and away from the Atlantic Ocean. Not this March, however - these chunky grebes have staged an invasion of epic proportions; a movement previously unrivaled in scope. I wrote about this earlier, with tentative thoughts, RIGHT HERE.
The map depicts a staggering invasion of Red-necked Grebes. Fifty-six counties, and a total of 302 birds by my reckoning. This far eclipses the previous record flights, which took place in February and March of 1994, when 111+ grebes were tallied in Ohio, and 2003 when around 200 birds were reported. This year's irruption is not confined to Ohio; many eastern states are experiencing a similar incursion.
The map above is courtesy of the National Oceanic and Atmospheric Administration's National Ice Center. It shows the maximum extent of ice cover for the three most recent greatest ice-overs of the Great Lakes. Interestingly, there was no recorded irruption of Red-necked Grebes in winter 1979, but 1994 was the aforementioned previous record for Red-necked Grebes in Ohio ( and elsewhere).
Note that this winter's ice cover peaked in early March - about the time that Red-necked Grebes really began making themselves known in interior locales. Prior to March, reports were almost non-existent from areas where the invasion later became pronounced. Pay special notice to Lake Ontario, the easternmost of the Great Lakes. This is the lake in which most Red-necked Grebes that overwinter on the Great Lakes supposedly occur. And Lake Ontario, which averages 283 feet in depth but is the smallest of the lakes by surface area, also has the most open water (white on the map indicates open water). The other four lakes are nearly frozen solid.
Red-necked Grebes feed primarily on fish at any season, and their diet is likely completely piscivorous or nearly so in winter, especially on big lakes. These birds are also champion divers, and are known to descend to depths of 150 feet, or deeper. But probably not much deeper. Remember, Lake Ontario is far deeper than that, and is generally the deepest towards the middle. Much of the unfrozen portion of the lake is beyond the reach of the diving ability of the grebes, at least if they want to get to or near the bottom.
So, we should look at the general habits of fish that the grebes feed on to further our investigation. In the dead of winter, lake water temperatures stratify the opposite of what they do in summer, when water at the surface is warmest, and gets progressively cooler as one descends. In winter, the water at the lake bottom is actually slightly warmer than at the surface, perhaps by a few degrees F. So, it may be that fishes that the grebes would forage on tend to descend deeper and into the warmer waters and beyond the reach of the grebes' diving abilities.
A more likely explanation, I think, is that Red-necked Grebes overwintering on Lake Ontario actually remain in shallower waters - 100 feet in depth or less - closer to the shoreline. It's likely that lake bottom structure and topographic variation would be greater in near-shore zones than out in the middle, and such variation probably increases fish abundance and diversity. And fishes in shallower waters are more readily available to foraging grebes. As this winter's prolonged brutal temperatures increased Lake Ontario's shoreline ice shelves much further out into the lake than in most winters, the prime grebe feeding zones may have eventually iced over. The maximum icing of their foraging habitat occurred about the time that we inlanders began to see large numbers of Red-necked Grebes appearing.
Of course, migrant Red-necked Grebes moving west and across inland areas from Atlantic wintering ground can begin to appear by this time, and it may be that winter refugees from Lake Ontario are being augmented by migrants by now, too. Still, the more I puzzle over this unprecedented phenomenon, the more I think it likely that the exceptionally frigid weather of this winter and its impact on Lake Ontario probably is the major reason for the extraordinary grebe flight. The difficulty of detecting and studying wintering Red-necked Grebes in their largely inaccessible wintering haunts means that large pieces of their life history remains unknown, or at least poorly known.
Thanks to fisheries and limnology experts Brian, John, and Ray for sharing their insights with me.