Risk and Benefits of Early Migration
Here on my nature blog here you will constantly see me noting my first of the year sightings! Normally those are the birds trying to be the first on their breeding grounds. These early birds are the experienced adults taking advantage of the many benefits of arriving early. Those males who arrive first will have their first choice in the most suitable nesting habitat and provide a better chance for the males to be pair up with a female. These earlier nesting pairs will then have a longer nesting period with a better fledgling success rate. Those individuals that show up later will then result in a less suitable habitat where they could have less available food for their young. Obviously this will reduce their chances of producing successful fledglings and open up the opportunity for predators finding their nest.
These long-distance migrants have many hazardous obstacles that the birds will need to avoid during their migration. My Ornithology text book by Frank Gill states “More than half the small land birds of the Northern Hemisphere never return from their southbound migration”. This is so hard to believe but there are so many physical risk that they encounter like: exhaustion, predators or even the weather. It was my post yesterdays where I discussed temperatures being in the ~70sF (Tuesday’s 1/2 day off). There were a few species including a dragonfly which I had listed as my first for the year find. Could they be risking their lives for the possible benefits of arriving early?
I said in that same post yesterday “I noticed that the Eastern Bluebirds and Tree Swallows had already started fighting over some of the bluebird boxes”. This comment was stating that local Tree Swallows have finally arrived and they should start nesting soon. I have been seeing these Tree Swallows since last Saturday and their main food supply includes flying insects which they catch while in flight. Last evening the temperatures got low enough that we had mixed freezing rain and snow. No doubt temperatures will reduce the numbers of flying insects and limit the opportunities for Tree Swallows to obtain food. I am currently looking outside with 2-3 inches of snow on the ground and there is no weather changes predicted until after Easter Sunday (it will be in the 40’s then). Doing some reading on Tree Swallows and when the weather is bad they have been known to feed on berries. But will these berries be enough food to survive a snowstorm? After work I had observed hundreds of Tree Swallows flying adamantly searching for food along the Red House Lake (lucky the lake is warm enough to provide a few insects and not need to rely entirely on berries).
Last evening as we were getting the freezing rain and snow I happened to passed a few of the nest boxes with numerous Tree Swallows sitting on top of them. I continued watching the swallows through my binoculars and there were 3, 4 and then 5 swallows entering one of the boxes in a matter of seconds. I jumped out of the truck and tried to approach the box for a photo for the blog! This one bird didn’t seem at the least bit concerned that I was there (see photo above). Eventually it flew away and I then peered into the box with this cute little swallow peaking out at me. How many were inside the box is hard to guess but there was no way that I would disturb them any more than I had already. These birds were apparently stressed and I probably shouldn’t have disrupted them like I had already.
But, where does a field biologist draw the lines in protecting the birds vs. letting nature take its course? From past experience I know those birds on the bottom of the bird packed box will end up suffocating! No matter what I do there will always be the birds on the bottom that will end up dieing! Will those on the bottom give a needed advantage to other individuals on top to survive this 5 day winter storm? Is arriving early too much of a risk or will those that survive greatly benefit from being the first on site? Or will the early Tree Swallows be too weak to protect the prime nest boxes and the stronger later migrants get the best pickings? So many questions and I do apologize for my rambling. This is all I could think about last night.