Failed Breeders (1 Viewer)

At one point do birds become failed breeders in the sense that they give up? How far into the summer do birds, not including the usual multi-brooded birds, lay upto?

Himalaya said:

At one point do birds become failed breeders in the sense that they give up? How far into the summer do birds, not including the usual multi-brooded birds, lay upto?

Click to expand...

In northern latitudes, the ratio of daylight-length to darkness probably triggers endogenous body chemistry changes that reduce reproductive fitness and (for migrants in particular) increase muscle and build-up fat resources to fuel departure from the breeding grounds.

Migrants also will begin to display the migratory urge (Zugunruhe is the technical term, from German), where behaviour is often characterised by restless interruptions that become more frequent until suddenly the birds depart - the Common Swift is one example where late chicks are abandoned abruptly.

Non-migratory species in semi-tropical and tropical zones quite often do not have such constrained breeding seasons, but peaks may coincide with insect and fruit population peaks; also, where rainy seasons are predictable, bird breeding may be aligned to take advantage.
MJB

MJB said:

In northern latitudes, the ratio of daylight-length to darkness probably triggers endogenous body chemistry changes that reduce reproductive fitness and (for migrants in particular) increase muscle and build-up fat resources to fuel departure from the breeding grounds.

Migrants also will begin to display the migratory urge (Zugunruhe is the technical term, from German), where behaviour is often characterised by restless interruptions that become more frequent until suddenly the birds depart - the Common Swift is one example where late chicks are abandoned abruptly.

Non-migratory species in semi-tropical and tropical zones quite often do not have such constrained breeding seasons, but peaks may coincide with insect and fruit population peaks; also, where rainy seasons are predictable, bird breeding may be aligned to take advantage.
MJB

Click to expand...

MJB,
I am trying to get a better understanding of the term you used, Zugunruhe. According to my ornithological dictionary, it is used for the unrest displayed by a bird already actively migrating, while your description to me sounds like it is used (also?) before the start of migration. Again according to the dictionary, Zugstimmung would be the term for behavioral changes before migration starts. Could you please comment on how well the dictionary fits with actual usage?

thanks
Niels

njlarsen said:

MJB, I am trying to get a better understanding of the term you used, Zugunruhe. According to my ornithological dictionary, it is used for the unrest displayed by a bird already actively migrating, while your description to me sounds like it is used (also?) before the start of migration. Again according to the dictionary, Zugstimmung would be the term for behavioral changes before migration starts. Could you please comment on how well the dictionary fits with actual usage? thanks Niels

Click to expand...

Niels,
I was citing Peter Berthold's use of Zugunruhe in his book (in English) Bird Migration (OUP). I understand he used it in a broad sense to describe migratory restlessness. However, there is a caveat - I'm away from home and my references, and so I may be relying on memory too much!
MJB

Thanks :t:

Niels

That is quite interesting. I thought they would finish chick rearing.

How often and how late would say a ground nesting wader lay until before they decide enough is enough? When i watch programmes like Springwatch i am surprise how many are on eggs even until mid-June. The you have birds like Hobby who in some cases may not even lay until early July it appears.

MJB said:

In northern latitudes, the ratio of daylight-length to darkness probably triggers endogenous body chemistry changes that reduce reproductive fitness and (for migrants in particular) increase muscle and build-up fat resources to fuel departure from the breeding grounds.

Migrants also will begin to display the migratory urge (Zugunruhe is the technical term, from German), where behaviour is often characterised by restless interruptions that become more frequent until suddenly the birds depart - the Common Swift is one example where late chicks are abandoned abruptly.

Non-migratory species in semi-tropical and tropical zones quite often do not have such constrained breeding seasons, but peaks may coincide with insect and fruit population peaks; also, where rainy seasons are predictable, bird breeding may be aligned to take advantage.
MJB

Click to expand...

Himalaya said:

That is quite interesting. I thought they would finish chick rearing. How often and how late would say a ground nesting wader lay until before they decide enough is enough? When i watch programmes like Springwatch i am surprise how many are on eggs even until mid-June. The you have birds like Hobby who in some cases may not even lay until early July it appears.

Click to expand...

With ground-nesting waders, much depends on the latitude - high-Arctic breeders have a very short breeding season that can be severely disrupted by late-onset spring weather or early-onset autumn/winter weather.

At present, I'm in eastern Austria, where Black-tailed Godwit breeding has been affected adversely by the spring drought that followed a lack of winter snow, but there appears to have been some compensatory breeding activity, because I recorded competing aerial-displaying males on 2 June. Normally in the flat grasslands here, there are many water-filled small lakes that feed grass growth, leading to insect and arthropod abundance on the ground, easy food for the chicks; the grass grows tall enough to give good cover from aerial predators from the lake edges outwards. This year, the grass is mostly long enough, but not at the lake margins; most of the lakes are dry.

Some bird species at temperate latitudes will have their second broods by late June, of course.

The Hobby, like several migrant falcons, depends on the abundance of newly-fledged passerines as food for its brood, In the cases of Eleonora's and Sooty Falcons of Mediterranean latitudes, they depend on passerines on outward migration in late summer and autumn; the falcon broods may not fledge until late September.
MJB

Himalaya said:

How often and how late would say a ground nesting wader lay until before they decide enough is enough? When i watch programmes like Springwatch i am surprise how many are on eggs even until mid-June. The you have birds like Hobby who in some cases may not even lay until early July it appears.

Click to expand...

Apart from the daylight length mentioned by MJB, there is the basic mathematics of food intake and what the energy is used for. Egg production is directly related to how much food is available even if there are subtle influences such as population density vs stress. Incredibly, the cuckoo can produce enough eggs to be on the equivalent of a three or four brood strategy simply because the female does not to give over any of her food to feed young and can produce eggs continually until she leaves for Africa. Swifts and most birds of prey are single-brooded because of the energy taken to obtain food, especially when young are present. Blue tits are single-brooded because they are too small to regain condition after raising a large brood although they will sometimes lay a smaller clutch of eggs if the firsat attempt fails before hatching. Most waders are similar to the blue tit in this respect but it is also because the development phase of the young after hatching is much longer. Waders will lay a new clutch if the first fails prior to hatching but they hardly ever nest again if the young are killed even if this occurs soon after the young have hatched. The reason is that the annual moult kicks in and this is where the energy is required late in the summer (for most birds, of course). Waders that fail to breed or lose nests early will often set off away from the breeding grounds and make a leisurely return to wintering grounds, moulting on the way or when they arrive at a place that is suitable. Interestingly, a number of our garden birds adapt their strategies during late broods to undergo the moult although many species can delay the moult if food is marginal.

Great subject BTW.

I at one time read that some tropical birds that are resident species have a very narrow breeding season, lay smaller clutches, and that if they do not succeed they will wait another year. These birds generally live more years than similar size migratory birds so can afford a different strategy.

Niels

Nightranger said:

Great subject BTW.

Click to expand...

Yes, lots of interesting facts here! Thanks all.