Starting September 29, we spent 16 days in Cape May, NJ, a peninsula famous for its fall bird migration. It is a very interesting area because the migration is exceptionally diverse (involving songbirds, raptors, and seabirds), complex (in terms of the effects of weather and geography), and well-studied (with several long-running counts organized by the Cape May Bird Observatory).
As shown in the map below, New Jersey (bordered in red) is separated from Pennsylvania and Delaware by the Delaware River, which expands into Delaware Bay. The southernmost bridge spanning the river is just below Wilmington, DE. A vehicle and passenger ferry runs from Cape May across the bay to Lewes, DE (shown as a dashed line on the map), a distance of 17 miles, taking about an hour and a half, which is roughly half the driving time.
Although some landbirds fly impressive distances over water during migration, most will avoid a significant water crossing if possible, or at least will make the crossing under favorable conditions. Consequently, peninsulas often concentrate migrating landbirds, especially if birds are “funneled” into them from a wide area. Cape May is exemplary in this respect, collecting birds from a roughly 60-mile wide corridor into a peninsula about one-tenth as wide.
Although geographical factors are important, meterological conditions are also critical in determining how good the migration will be on any given day. Wind direction is especially important. A wind with a significant northerly component will provide a good tailwind for birds heading south, which will cause more birds to choose to migrate. A wind with a significant westerly component will cause landbirds to drift east while they are flying, bringing them nearer to the coast, where they are more likely to get funneled to Cape May. Thus a northwest wind is ideal for landbirds, both increasing migration volume and concentration. For seabirds, an easterly wind component helps bring birds closer to shore, so a northeast wind would be propitious.
Temperature is also a factor, with cooler temperatures stimulating migration. Consequently, the best of all possible landbird scenarios is the passage of a cold front with winds coming from the northwest. Historically, such favorable conditions occurred a number of times during autumn, leading to quite a few spectacular migration days each fall season. But over the last five years or so, it has stayed warmer later, with fewer and weaker cold fronts, and particularly the September and early October songbird flights have been somewhat less dramatic than in the past.
Most songbirds migrate primarily at night, and under ordinary circumstances, they land soon after sunrise in habitat providing food and shelter, where they spend the day recovering. However, potentially the greatest songbird migration spectacle is called “morning flight”, and it can occur when the terrain revealed at dawn is unfavorable, requiring adjustment and additional flying time by the songbirds, allowing the migrants to be seen in flight in the daylight, often close to the ground (whereas their nighttime cruising altitudes are typically a few thousand feet).
If a nocturnal migrant finds itself over water at dawn, it will typically reorient to fly towards the most easily reached land, accounting for wind direction. Once over land, it may continue flying if suitable vegetation is not present, as in agricultural or urban areas. Morning flight can be sensational at Cape May, as birds over Delaware Bay reorient northward and return to the peninsula, then commence to search for well-vegetated areas, which are patchily distributed. Ironically, but as expected from the above explanation, the vast majority of songbirds seen during morning flight in Cape May in fall are actually heading north!
Cape May is probably even more famous for its raptor migration than its songbird migration. This is a diurnal movement, so there is no morning flight phenomenon; the birds can see where they are going while flying, and so do not need to reorient. But otherwise, most of the comments above are applicable to the raptor migration, though the intensity of the migration is probably somewhat less strongly influenced by wind direction. The term “raptor” generally refers to birds in any of four groups that are now known to be only distantly related to one another: hawks and eagles; falcons and caracaras; New World vultures; and owls. Cape May is especially noted for large numbers of three species of falcons (Peregrine Falcon, Merlin, and American Kestrel) and two species in the genus Accipiter (Sharp-shinned and Cooper’s Hawks), but a decent mid-fall day would yield a dozen raptor species or so.
The accipiters present an interesting case regarding age distribution. In round numbers, we probably saw about a thousand accipiters during our stay, but did not notice a single adult bird. The primary reason was that immature accipiters migrate earlier than adults, and the latter only just began to show up near the end of our stay. A secondary factor is that a lot of birds are fledged each breeding season, and so fall migration can be dominated by immatures, which as a class have not yet experienced a great deal of mortality (this is true for many birds, not just raptors). But the third factor is especially intriguing; there just do not seem to be enough adults later in the season to account for all the immatures early in the season, suggesting that many adults avoid Cape May, perhaps as a result of being forced to cross Delaware Bay, or to backtrack, earlier in their lives, and learning their lesson.
In addition to songbird and raptor migration, there is a notable fall passage of seabirds off the New Jersey coast. Though we were a bit early in the season for this, we spent most of a day at the Avalon Seawatch, which was very interesting. Avalon is a prime location for a fall seawatch because the coastline abruptly juts out to the east by about two-thirds of a mile, so birds flying parallel with the beach suddenly find themselves squeezed out (if they are close to shore) or just a lot closer to shore than they were. Thus, the birds effectively are concentrated by the shoreline jog, and some more oceanic species may be closer to shore than is typical. Both factors contribute to making this a fine seawatch, and the Cape May Bird Observatory has been running counts here for about 15 years, so a great deal is known regarding timing and volume of migration.
Our two-plus weeks in Cape May were a lot of fun. Each night before going to bed we’d check the weather forecast and particularly the predicted winds overnight at 2500 feet altitude, and unless they were dismal, we’d then wake up about two hours before sunrise, check what actually happened, and after breakfast drive over to Higbee Beach, arriving half an hour before sunrise. Some days we’d stand on a raised dike with the naturalist conducting the official morning flight count, which was great for getting an overview of the flight that day, and learning more about migration and in-flight identification of songbirds. Other days we’d start walking the trails first thing to get a chance to see the birds as they first landed in the trees and fields. Once songbirds had finished setting down and dispersing, we’d typically head over to the hawkwatch at the lighthouse, to see some hawks in the late morning, before the heat of the day might create thermal updrafts that would take many of the hawks high into the sky. The morning flight, hawkwatch, and seawatch counts were updated every 30 minutes on-line, and it was fun to be able to see how the counts progressed during the day.
During our 16 days in Cape May, we saw two really good songbird flights, one outstanding falcon flight, and several days that were generally good for raptors. One day, over 500 Red-breasted Nuthatches were seen, breaking the previous morning flight record. In winter, this common boreal species feeds mostly on conifer seeds, and in years when the seed crop is poor, substantial numbers travel south, as we observed. During the day of the falcon flight, over 5000 American Kestrels were observed, barely breaking the previous hawkwatch record. We saw this flight from the observation platform at the end of Coral Street, which is an excellent vantage point for both seabirds and raptors.
Other avian highlights during our stay included Razorbill, Lapland Longspur, Clay-colored Sparrow, Saltmarsh Sparrow, Lesser Black-backed Gull, Gull-billed Tern, Piping Plover, Stilt Sandpiper, Red Knot, and 15 Yellow-billed Cuckoos in one day. We found a total of nine new plants, mostly dune and swale inhabitants, and also added one turtle, three dragonfly, and three crab species to our life list. We spent one windless day doing a lovely 8-mile paddle on Cedar Swamp Creek in Cape May National Wildlife Refuge, on which trip we broke the 2000-mile mark in Headwind, our present canoe.
With nearly a week until winds were forecast to be favorable again, and with the warbler diversity on the wane due to the lateness of the season, we finally headed west to begin a four-week ramble back to El Paso.
Nomadic Naturalists 