It is commonly experienced at banding stations: the first couple of net rounds can produce a good number of netted birds but the numbers taper off dramatically by mid-morning. This has been the pattern for the past week. Lisa Horn (who seems to be becoming my personal research assistant) did some more searching around and came up with some interesting papers. Here are some of findings:

Some studies have found that migratory passerines make pre-dawn/morning landscape level (over many kilometers) flights which are interpreted as information gathering in order to select an appropriate area for rest and refueling. They then select sites at a finer scale. There is evidence that after landfall, birds adjust their initial selection of the stop over site if neccessary (however the process is poorly understood). This study suggests that birds make nocturnal flights for multple purposes (besides migration), including to adjust their selection of stop-over site to find better quality habitat. (On another interesting note, the study mentions that pre-migratory reed warblers engage in post-fledgling dispersal and exploration by making landscape scale movements at night). Making these movements at night could offer the birds an opportunity to gain information more safely (protection from predators under the cover of darkness) and therefore more efficiently if they need to select better habitat for refueling (Mills et al, 2011).

Further to this, another study found that food supply plays an important role in the decision of migrants to use a habitat for a stop over. If foraging conditions are unsuitable, they can either explore and try to find something better, or continue migration to another one (Ottich and Dierschke, 2003).

Another study suggests that besides ‘sampling’ the landscape to find optimum habitat, birds can also use cues and assess the habitat without having to sample. It has been documented that tape-luring can be used to capture some nocturnal migrants, and suggested that some species use acoustic cues in their decision of whether or not to make a landfall (but that it’s more common in passerines of wetlands). Another interesting point which you may or may not know is that the ‘peak’ you get in the morning of the birds caught in mist nets is mainly due to fine-tuning of habitat selection by migrants who are looking to define a temporary territory for their stay. The time each bird spends moving around and ‘sampling’ is likely both condition (how much of a fat store they have) and age-dependent. An emaciated bird would likely be less choosey and accept anything that can offer a positive fuel deposition rate, while a fat bird can be more choosey if they want. The age-related aspect I found interesting; it’s hypothesized that since for many species juveniles are subordinate to adults, they must move around more to search for their own small home area during a stop-over since adults have claimed the best habitats. They also say that juveniles are found over a wider range of habitats than adults, suggesting their selection of appropriate habitat is imperfect (combined with the potential displacement from dominant adults). Another factor that birds consider is the risk of predation, which can be a potential trade-off because what if the best habitats to refuel leave birds exposed to predators? (Chernetsov, 2006).

So based on all this, one could assume that to answer your question of how they know an area has a bad crop or a good crop, I would assume that in the case of Ruthven, they discovered the lack of food availability during their initial explorations. From above, I feel like they could have still made a decision that it looked like a good place to stop, but then there wasn’t enough fruit so they thought it might be worthwhile to look elsewhere (if they had enough fat to afford it). I’m not sure how much of a role acoustic cues could have played into it (that the birds didn’t hear a lot of other birds so they assumed it wasn’t worth stopping for) or risk of predation.

Again this morning we had good captures early (White-throats were “chipping” before we had the nets open) but when the sun got up the catching slowed quickly. It makes intuitive sense that migrants that move at night and descend in the dark would move at first light, readjusting their positioning to find optimal feeding habitat. And when they move they get caught. But it doesn’t take them long and then they hunker down to feed.

It’s not often that a young person comes to the banding lab and says she wants to be an ornithologist. So it was neat today when Alesandra made the claim….and the training began.
Banded 41:
1 Mourning Dove
1 Blue Jay
2 Black-capped Chickadees
1 House Wren
3 Ruby-crowned Kinglets
1 Gray-cheeked Thrush
6 Hermit Thrushes
2 American Robins
1 Myrtle Warbler
1 Fox Sparrow (1st of the season)
4 Song Sparrows
1 Swamp Sparrow
16 White-throated Sparrows
1 American Goldfinches

ET’s: 34 spp.
Rick